Oracle SQL

CONTENTS CONTENTS.........................................................................................................................................................2 INTRODUCTORY CONCEPTS .....................................................................................................................3

THE DATABASE OBJECTS .................................................................................................................................. 3 DATABASE OBJECT NAMING RULES .................................................................................................................4 DATATYPES........................................................................................................................................................4

THE DATA DEFINITION COMMANDS FOR TABLES ......................................................................8 CREATING A TABLE ...........................................................................................................................................8 CONSTRAINTS ON TABLES ................................................................................................................................. 9 DROPPING A TABLE.........................................................................................................................................12 ALTERING A TABLE .........................................................................................................................................13 SAMPLE TABLES ...............................................................................................................................................14

QUERYING THE TABLES............................................................................................................................15 CONDITIONAL RETRIEVAL OF ROWS ..............................................................................................................15 SQL FUNCTIONS..............................................................................................................................................17 GROUP (OR AGGREGATE) FUNCTIONS ...........................................................................................................17 SINGLE ROW ( OR SCALAR) FUNCTIONS..........................................................................................................20 JOINS TO QUERY MULTIPLE TABLES...............................................................................................................27 SET OPERATORS ..............................................................................................................................................29 NESTED QUERIES ............................................................................................................................................30

DATA MANIPULATION COMMANDS .....................................................................................................33 THE INSERT COMMAND ................................................................................................................................33 THE DELETE COMMAND ...............................................................................................................................33 THE UPDATE COMMAND ...............................................................................................................................33

THE TRANSACTION CONTROL COMMANDS ....................................................................................35 THE COMMIT COMMAND ...............................................................................................................................35 THE ROLLBACK COMMAND ..........................................................................................................................35

WORKING WITH OTHER SCHEMA OBJECTS .....................................................................................37 SEQUENCES......................................................................................................................................................37 SYNONYMS .......................................................................................................................................................38 VIEWS ...............................................................................................................................................................39 INDEXES...........................................................................................................................................................41 CLUSTERS .........................................................................................................................................................42

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IInnttrroodduuccttoorryy CCoonncceeppttss ? SQL (Structured Query Language) is a standard RDBMS language. ? Oracle SQL is the set of commands that all pr ograms and users must use to access

and modify the data within the Oracle Database. ? For executing SQL queries in the user schema, he/she must be granted some type of

system privi leges by DBA. ? The SQL statements can be broadly classified into three categ ories:

o Data Definition Language (DDL) - Used to create, alter, or drop objects (e.g., ta-bles, indexes, synonyms, views, clusters, etc.) in the database

o Data Manipulation Language (DML) - Used to query, insert, delete and update of data stored in the database.

o Data Control Language (DCL) - Used for controlling data stored in the dat abase.

The Database Objects ? The database objects are entities stored in the database. These objects may be either

schema objects , or non-schema objects . ? Schema objects are objects belonging to a particular schema. A schema is made up of

a collection of logical structures of data, or schema objects. A schema is owned by a database user and has the same na me as that user (e.g., scott, trng01, etc.). Each user owns a single schema.

? SQL is used to create and manipulate the schema objects. ? Some schema objects are listed:

o Clusters o Database Links o Database Triggers o Indexes o Packages o Sequences o Stored Functions o Stored Procedures o Synonyms o Tables o Views

? Other types of objects are also stored in the database and can be created and manip u-lated with SQL, but are not contained in a schema. These are non-schema objects, and some of them are listed blow: o directories o profiles o roles o rollback segments o tablespaces

o users

Database Object Naming Rules ? Maximum character length can be 30 (Except for database names which are li mited to

8 characters). ? Must not contain a question mark. ? Case insensitive. ? Must begin with an alphabet. ? May only contain the characters a -z, A-Z, 0-9, _, $ and #. ? Must not duplicate an Oracle Reserved word. ? Must not duplicate another database object belonging to that user.

Object Name Valid/Invalid Reason employee valid 1_test invalid Begins with a number check invalid ORACLE reserved word Spp&prts invalid Use of “&” not allowed Pass/fail invalid Use of “/” not allowed

Datatypes

Internal Datatype Description VARCHAR2(size)

Variable length character string having maximum length size bytes. Maximum size is 4000, and minimum is 1. You must specify size for a VARCHAR2.

NUMBER(p,s) Number having precision p and scale s. LONG Character data of variable length up to 2 gigabytes, or

231 -1 bytes. DATE Valid date range from January 1, 4712 BC to Dec ember 31,

4712 AD. Stores time too. RAW(size)

Raw binary data of length size bytes. Maximum size is 2000 bytes. You must specify size for a RAW value. This datatype is used to store binary data.

LONG RAW Raw binary data of variable length up to 2 giga bytes.

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ROWID

Hexadecimal string representing the unique address of a row in its table. This datatype is primarily for values r e-turned by the ROWID pseudocolumn.

CHAR(size)

Fixed length character data of length size bytes. Maximum size is 2000 by tes. Default and minimum size is 1 byte.

CLOB

A character large object containing single byte characters. Maximum size is 4 gigabytes.

BLOB A binary large object. Maximum size is 4 gigabytes. BFILE Contains a locator to a large binary file stored ou tside the

database. Enables byte stream I/O access to external LOBs residing on the database server.

Character Datatypes ? CHAR and VARCHAR2 are the character datatypes that can be used to store and ma-

nipulate text data in a table column. ? CHAR specifies a fixed length character string (e.g., CHAR(10)) for a defined col-

umn. In case you store a string that is smaller than the column length, Oracle blank -pads the value to the column length.

? The VARCHAR2 specifies the maximum number of bytes of data that it can hold in a column. Oracle stores each value in the column exactly as specified (without any blank-padding).

Number Datatype ? The NUMBER datatype is used to store zero, positive and negative f ixed and floating

point numbers. ? You can specify a fixed poi nt number with: NUMBER(p,s) ? You specify an integer with: NUMBER (p) ? You specify a floating point number with:

NUMBER LONG Datatype ? LONG is used to store large text strings, up to 2GB in size. ? There are certain restrictions when you use a LONG column:

o A table cannot contain more than one LONG column. o LONG columns cannot appear in integrity constraints (except for NULL and NOT

NULL constraints). o LONG columns cannot be indexed. o A stored function cannot return a LONG value.

o LONG columns cannot appear in certain parts of SQL statements: WHERE, GROUP BY, ORDER BY, in SELECT statements, SQL functions such as SUBSTR or INSTR, expressions or conditions

? Due to such restrictions, it is recommended that you use CLOB datatype to store large character data instead of the LONG datatype.

DATE Datatype ? The DATE datatype is used to store the date and time information. For each DATE

value the following information is stored: o century o year o month o day o hour o minute o second

? The function TO_DATE can be used to convert a string (or numeric) value to date for-mat. This function is discusses later.

? You can get the current date and time using the function SYSDATE (e.g., SELECT SYSDATE from DUAL;)

RAW and LONG RAW Datatypes ? The RAW and LONG RAW datatypes are used for raw binary data that is not to be

interpreted by Oracle. Examples of such data include pictures, sound, documents, etc. ? LONG RAW data cannot be indexed, but RAW data can be indexed. ? It is recommended that you use BLOB datatype for binary data ins tead of RAW

datatypes. Large Object (LOB) Datatypes ? The Large Object (LOB) datatypes can store up to 4 GB of unstructured data such as

text, image, video, documents, etc. ? BLOB, CLOB , and BFILE are the three types of LOBs. ? BLOB and CLOB are called inter nal LOBs as they store data within the database.

BFILE is called external LOB as the corresponding data is stored external to the dat a-base, in the OS files.

? An Oracle-supplied package, DBMS_LOB, is used to work with the LOB data. ? Any number of columns in t able can be of LOB type (unlike the LONG or LONG

RAW types) ROWID Datatype

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? ROWID (represented as a hex string) represents the address of a row in a table in the database. You can view this address by querying the pseudocolumn ROWID in a table (e.g., SELECT ROWID FROM emp;).

? An Oracle-supplied package DBMS_ROWID let you interpret the meaning of the hex address string of the ROWID.

TThhee DDaattaa DDeeffiinniittiioonn CCoommmmaannddss ffoorr TTaabblleess

? Here we look at the DDL commands to work with one of the database objects – tables.

? Table can be created with the CREATE TABLE command. ? Various integrity constraints let you impose restrictions on the data that gets into

the tables, according to the business rules. ? The structure of the table can be altered using the ALTER TABLE comm and. The

same command can be used to add or drop the constraints on the table. ? You can remove a table from the database with the DROP TABLE command.

Creating a Table Tables are created using the CREATE TABLE command, and owned by the user who cr e-ates them. ? Syntax

o DEFAULT specifies a value to be assigned to the column if a row is inserted wit h-

out a value for this column. o CLUSTER includes this table in the named cluster (this is discusses later in a

separate section) Example:

CREATE TABLE table ({column datatype [DEFAULT expr] [constraint]} [,{column datatype [DEFAULT expr] [constraint]}]...) [CLUSTER cluster (column [, column] ...)] [AS query]

CREATE TABLE products ( productno NUMBER(4), productname VARCHAR2(20), quantityonhand NUMBER (4));

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Constraints on Tables ? Constraints limit the values entered into the columns of the table , thereby ensuring the

integrity of the data in the database. ? The business rules are translated as constraints on the tables (e.g., each employee

number should be unique, the valid product categories can be A, B, or C only, etc.) ? Depending on the placement of the constraint, a constraint can be termed a column

constraint, or a table constraint: o a column constraint: Constraint specified as part of the column defin ition: o a table constraint: Constraint specified towards the end of the CREATE TABLE

statement

Types of Constraints Constraint Description NULL | NOT NULL Specifies if a column may or may not support NULL values . UNIQUE Forces values of a column (or com bination of columns) to be

unique. The unique key made up of a single column can contain nulls.

PRIMARY KEY Identifies a column (or combination of columns) as a primary key. A primary key value cannot appear in more than one row in the table, and it canno t be null either (it is equivalent to UNIQUE NOT NULL). A table can have only one primary key.

FOREIGN KEY Also called a referential integrity constraint, designates a co l-umn (or combination of columns) as a foreign key to a specified primary or unique ke y, called the referenced key. In this rel a-tionship, the table contai ning the foreign key is called the child table and the table co ntaining the referenced key is called the parent table.

CHECK Specifies a condition that the column (or combination of co l-umns) must satisfy for the row to exist in the table.

Example 1: NOT NULL constraints Named and unnamed constraints:

? In the example above , the first NOT NULL constraint an unnamed constraint, where as the second NOT NULL constraint is given a nam e NNL_pname. The

CREATE TABLE products ( productno NUMBER(4) NOT NULL, productname VARCHAR2(20) CONSTRAINT NNL_pname NOT NULL, quantityonhand NUMBER (4));

unnamed constraints will automatically be assigned system generated names (of the format SYS_Cxxx where xxx is a unique number) by Oracle.

? Naming the constraints will help manage the constraints better. ? The information about the constrain ts will be stored in the data dictionary table

user_constraints. To view the constraints, issue a query like the one given below:

Example 2: UNIQUE constraints

Example 3: PRIMARY KEY constraint

SELECT TABLE_NAME, CONSTRAINT_NAME, CONSTRAINT_TYPE FROM USER_CONSTRAINTS WHERE TABLE_NAME = 'PRODUCTS';

CREATE TABLE dept (deptno NUMBER(2), dname VARCHAR2(10) CONSTRAINT unq_dname UNIQUE, loc VARCHAR2(10) ); /* column constraint */

CREATE TABLE dept (deptno NUMBER(2), dname VARCHAR2(10), loc VARCHAR2(10), CONSTRAINT unq_dname UNIQUE (dname) ); /* table constraint */

CREATE TABLE dept (deptno NUMBER(2) CONSTRAINT pk_dept PRIMARY KEY, dname VARCHAR2(10) CONSTRAINT unq_dname UNIQUE, loc VARCHAR2(10) ); /* column constraints */

CREATE TABLE dept (deptno NUMBER(2), dname VARCHAR2(10), loc VARCHAR2(10),

CONSTRAINT pk_dept PRIMARY KEY (deptno), CONSTRAINT unq_dname UNIQUE (dname) ); /* table constraints */

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Example 4: FOREIGN KEY constraint Note:

? Before you define a referential integrity constraint in the child table, the refe r-enced UNIQUE or PRIMARY KEY constraint on the parent table must already be defined . For the example above, dept table should have deptno designated as PRIMARY KEY already.

? Having a foreign key allows you to maintain referential integrity of your data. For example, having declared a foreign key deptno in emp table, if you try to insert a row in table emp with a deptno value 50, this deptno should be present in the parent table dept already; else the insert fails.

? The ON DELETE CASCADE option with foreign key: You cannot delete the pa r-ent table row in case there are referencing child table rows. With ON DELETE CASCADE option, Oracle permits deletions of referenced key values in the pa rent table and automatically deletes dependent rows in the child table to maintain re f-erential integrity:

CREATE TABLE emp (empno NUMBER(4), ename VARCHAR2(10), job VARCHAR2(9), mgr NUMBER(4), hiredate DATE DEFAULT SYSDATE, sal NUMBER(7,2), comm NUMBER(7,2), deptno NUMBER(2) CONSTRAINT fk_deptno REFERENCES dept(deptno), grade CHAR(1) ); /* column constraint */

CREATE TABLE emp (empno NUMBER(4), ename VARCHAR2(10), job VARCHAR2(9), mgr NUMBER(4), hiredate DATE, sal NUMBER(7,2), comm NUMBER(7,2), deptno NUMBER(2), grade CHAR(1), CONSTRAINT fk_deptno FOREIGN KEY (deptno) REFERENCES dept(deptno) ); /* table constraint */

Example 5: CHECK constraints

Dropping a Table

? DROP TABLE is used to remove a table and all its data from the database . ? When you drop a table , Oracle also drops all the indexes on the table, and invalidates

other dependent objects like views, stored procedures, stored functions, packages, etc., and these invalid objects can not be used.

? Syntax:

CREATE TABLE emp (empno NUMBER(4), ename VARCHAR2(10), job VARCHAR2(9), mgr NUMBER(4), hiredate DATE, sal NUMBER(7,2), comm NUMBER(7,2), deptno NUMBER(2) CONSTRAINT fk_deptno REFERENCES dept(deptno) ON DELETE CASCADE, grade CHAR(1) );

CREATE TABLE emp (empno NUMBER(4), ename VARCHAR2(10), job VARCHAR2(9) CONSTRAINT chk_job

CHECK (job IN ('CLERK', 'SALESMAN','ANALYST', 'PRESIDENT')), mgr NUMBER(4), hiredate DATE, sal NUMBER(7,2), comm NUMBER(7,2) CONSTRAINT chk_comm CHECK (comm BETWEEN 100 AND 1000), deptno NUMBER(2) CONSTRAINT fk_deptno REFERENCES dept(deptno) ON DELETE CASCADE, grade CHAR(1) CONSTRAINT chk_totsal CHECK (sal + comm <= 6500) );

DROP TABLE table_name CASCADE CONSTRAINTS;

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Examples: Note: CASCADE CONSTRAINTS allows you to drop a parent table that has referenc-ing child table. Without this option, if such referential integrity constraints exist, Oracle returns an error message and does not drop the table.

Altering a Table ? You can alter the definition of a table for:

o adding a column o dropping a column o adding an integrity constraint o dropping an integrity constraint or trigger o modifying/ redefining a column (datatype, size, default value)

? Syntax: Note: ? ADD allows you to add a new column to table, or add a constraint to the table’s defin i-

tion. ? DROP allows you to remove a column . ? The drop clause is used to remove an integrity constraint or trigger from the dat a-

base. ? You cannot use MODIFY option:

o To reduce the width of a column unless all the columns are empty. o To change the data type unless the column is empty. o To make it NOT NULL unless all rows have values in that column.

? The only type of integrity constraint that you can add to a n existing column using the MODIFY clause with the column constraint syntax is a NOT NULL constraint. For all other constraints use ADD clause to add the constraint.

DROP TABLE emp; DROP TABLE dept CASCADE CONSTRAINTS;

ALTER TABLE table { [ADD ( { column_element | constraint } [, column_element | constraint }] ... )] [ DROP COLUMN column ] [ MODIFY (column_element | constraint [, column_element | constraint] ... )] [ DROP drop]...

Examples:

Sample Tables The emp and dept tables that we have used in many examples are sample tables from Oracle, and look like: EMP Table EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 7369 SMITH CLERK 7902 17-DEC-80 800 20 7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30 7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30 7566 JONES MANAGER 7839 02-APR-81 2975 20 7654 MARTIN SALESMAN 7698 28-SEP- 81 1250 1400 30 7698 BLAKE MANAGER 7839 01-MAY-81 2850 30 7782 CLARK MANAGER 7839 09-JUN-81 2450 10 7788 SCOTT ANALYST 7566 09-DEC-82 3000 20 7839 KING PRESIDENT 17-NOV-81 5000 10 7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30 7876 ADAMS CLERK 7788 12-JAN-83 1100 20 7900 JAMES CLERK 7698 03-DEC-81 950 30 7902 FORD ANALYST 7566 03-DEC-81 3000 20 7934 MILLER CLERK 7782 23-JAN-82 1300 10 DEPT Table DEPTNO DNAME LOC 10 ACCOUNTING NEW YORK 20 RESEARCH DALLAS 30 SALES CHICAGO 40 OPERATIONS BOSTON

ALTER TABLE emp DROP COLUMN grade; ALTER TABLE emp ADD (grade CHAR(1)); ALTER TABLE emp MODIFY (job NOT NULL); ALTER TABLE emp MODIFY (sal NUMBER(8,2)); ALTER TABLE emp ADD CONSTRAINT pk_emp PRIMARY KEY (empno); ALTER TABLE emp ADD CONSTRAINT fk_emp FOREIGN KEY (deptno) REFERENCES dept(deptno); ALTER TABLE emp1 DROP CONSTRAINT pk_emp; ALTER TABLE emp1 MODIFY (hiredate DEFAULT SYSDATE);

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QQuueerryyiinngg tthhee TTaabblleess ? The SELECT DML command is used to query the contents of a table. ? Syntax: ? The WHERE clause is used to conditionally retrieve the rows from the table(s) ? The ORDER BY clause is used to get the data in the sorted order ? The GROUP BY and HAVING clauses are used to group together rows and get the

aggregate results Examples

To display the information of all employees SELECT * FROM emp; To list employee names and their departments SELECT ename, deptno FROM emp; To get the unique values of department numbers in the emp table SELECT DISTINCT deptno FROM emp; To List different job titles available in emp table SELECT DISTINCT job FROM emp;

Conditional Retrieval of Rows

? The WHERE clause in the query is used for conditional retrieval of rows ? Operators used to specify the conditions in WHERE clause:

SELECT * | [DISTINCT] column [, column]... FROM table [, table]... [WHERE condition] [GROUP BY column [, column]...] [HAVING condition] [ORDEY BY expr];

Relational (Comparison) oper ators =, >, <, >=, <=, <> , != Logical operators AND , OR , NOT Special operators IS [NOT] NULL, IN, BETWEEN, LIKE ? The IS NULL and IS NOT NULL operators are used in comparing nulls. ? The LIKE operator is used for checking partial equality conditions with

LIKE 'pattern' – the pattern can contain the wildcard characters % (indicates any sequence of zero or more characters), and _ (underscore, for single character) .

? You can select expressions from a table . ? You can provide aliases for viewing data. Alias names are displayed in place of actual

column nam es. Alias names are given to the right of a column name, enclosed within quotes. (e.g., SELECT ename "Employee Name", sal + 100 "Increased Salary" FROM emp;)

Examples

To list the employees belonging to the department 10 SELECT * FROM emp WHERE deptno = 30; To list the name and salary of the employees whose salary is more than 1500 SELECT ename, sal FROM emp WHERE sal > 1500 ORDER BY ename; To list all the managers in department 20 SELECT * FROM emp WHERE job='MANAGER' AND deptno=20; To list emp loyee details whose salary is between 1000 and 2000 SELECT * FROM emp WHERE sal >= 1000 AND sal <=2000 ORDER BY sal DESC; SELECT * FROM emp WHERE sal BETWEEN 1000 AND 2000 ORDER BY sal DESC; To list all the clerks in the departments 10 or 20 SELECT * FROM emp WHERE job='CLERK' AND (deptno = 10 OR deptno = 20) SELECT * FROM emp WHERE job='CLERK' AND deptno IN (10, 20)

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To list names of employees who are not salesmen SELECT ename FROM emp WHERE job <> ‘SALESMAN’; To list employees who are not gett ing any commission SELECT ename FROM emp WHERE comm IS NULL; To list employees belonging to some dept SELECT ename FROM emp WHERE deptno IS NOT NULL; To list the employees whose names start with ‘A’ SELECT ename FROM emp WHERE ename LIKE ‘A%’; To list employee names having ‘I’ as the second character, and end character as 'S' SELECT ename FROM emp WHERE ename LIKE ‘_I%A’;

SQL Functions ? There are several standard functions that Oracle provides to manipulate the data and

return a result. ? There are two types of SQL functions:

o Scalar (single row) functions – act on a single row at a time, returns a single result row for every row of a queried table or view ; used for ma-nipulating data values

o Group (aggregate) functions – acts on a group of rows, returns a single result row for a group of queried rows; used for summarizing data

Group (or Aggregate) Functions ? Group functions are utilized for summarizing data from a set of rows or from the en-

tire table. ? The group functions produce a single value for an entire group or table. In all group

functions NULLs are ignored. ? The group functions supported by standard SQL are:

Group Function Description SUM([DISTINCT|ALL] n) returns the sum of values of n MAX([DISTINCT|ALL] n) returns the maximum value of n MIN([DISTINCT|ALL] n) returns the min imum value of n AVG([DISTINCT|ALL] n) returns the average value of n COUNT({* | [DISTINCT |ALL] n}) returns the number of rows in a query

Examples

To list the total number of employees in the emp table SELECT COUNT(*) FROM emp; COUNT(*) ---------- 9 To get the total salary paid to the employees SELECT SUM(sal) as Total FROM emp; TOTAL ---------- 23625 To list the number of jobs available in the emp table SELECT COUNT(DISTINCT job) "tot jobs" FROM emp; tot jobs ---------- 4 To select the maximum salary paid to all the employees SELECT MAX(sal) Maximum FROM emp; MAXIMUM ---------- 5000 To list the maximum salary of employee working as a clerk SELECT MAX(sal) FROM emp WHERE job = ‘CLERK’; MAX(SAL) ---------- 1400

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To list the average salary and number of employees working in dept 20 SELECT AVG(sal), COUNT(*) FROM emp WHERE deptno = 20; AVG(SAL) COUNT(*) ---------- ---------- 3518.75 4

GROUP BY Clause ? GROUP BY clause groups the rows using a specified column. ? This column induces a co ntrol break using the specified field as the control field. ? The group functions can be used on the individual groups to summarize the data. ? Conditional retrieval of rows from a grouped result is possible with the HAVING

clause. The HAVING clause filters the rows returned by the GROUP BY clause. Examples

To list the minimum salaries in each of the departments SELECT deptno "Department", min(sal) "Min Sal" FROM emp GROUP BY deptno; Department Min Sal ---------- ---------- 10 1400 20 1100 30 1250 To list the number of employees working with each job title SELECT job "Job", COUNT(*) "Employees" FROM emp GROUP BY job; Job Employees --------- ---------- ANALYST 2 CLERK 2 MANAGER 3 SALESMAN 2 To list the departments and the total salaries payable in each department SELECT deptno "Department", SUM(sal) "Total Salary" FROM emp GROUP by deptno; Department Total Salary ---------- ------------ 10 3950

20 14075 30 5600 To list those departments which have more than 2 employees SELECT deptno "Dept", COUNT(*) "Employees" FROM emp GROUP BY deptno HAVING COUNT(*) >2; Dept Employees ---------- ---------- 20 4 30 3

Note: The selected columns in the query (involving a GROUP BY clause) must form the list of columns spe cified in the GROUP BY clause or should be group functions.

Single Row ( or Scalar) Functions

Number Functions

ABS(value) POWER(value, exponent) CEIL(value) ROUND(value, precision) COS(value) SIGN(value) COSH(value) SIN(value) EXP(value) SINH(value) FLOOR(value) SQRT(value) LN(value) TAN(value) LOG(value) TANH(value) MOD(value) TRUNC(value, precision) Examples: Number Functions SELECT ABS(-15) “Absolute” FROM DUAL; Absolute ---------- 15

SELECT CEIL(15.7) “Ceiling” FROM DUAL; Ceiling ---------- 16

SELECT FLOOR(15.7) “Floor” FROM DUAL; Floor ---------- 15

SELECT MOD(11,4) FROM DUAL; MOD(11,4) ---------- 3

SELECT POWER(3,2) “Raised” FROM DUAL; Raised ---------- 9

SELECT ROUND(15.193,1) “Round” FROM DUAL; Round ---------- 15.2

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SELECT ROUND(15.193,-1) “Round” FROM DUAL; Round ---------- 20

SELECT SIGN(-20) “Sign” FROM DUAL; Sign ---------- -1

SELECT TRUNC(15.79,1) “Truncate” FROM DUAL; Truncate ---------- 15.7

SELECT TRUNC(15.79,-1) “Truncate” FROM DUAL; Truncate ---------- 10

String Functions

Function Description String || String Concatenates two strings ASCII(string) Returns ASCII value of the character CHR(integer) Returns character corresponding to the ASCII

integer value CONCAT(string1, string2) Concatenates two strings INITCAP(string) Capitalizes the first letter of the string INSTR(string, set[,start[,occurrence]])

Returns an integer indicating the pos ition of the character in string that is the first character of this occurrence

LENGTH(string) Returns the length of the string LOWER(string) Converts all characters to lower case LPAD(string, n[,’set’]) Returns string right-padded to length n with the

sequence of characters in set LTRIM(string[,’set’]) Removes characters from the left of sring, with

all the leftmost characters that appear in set re-moved

RPAD(string, n[,’set’]) Returns string left-padded to length n with the sequence of characters in set

RTRIM(string[,’set’]) Removes characters from the right of sring, with all the leftmost characters that appear in set removed

SOUNDEX(string) Returns a string containing the phonetic repr e-sentation of string. This function allows you to compare words that are spelled d ifferently, but sound alike in English.

SUBSTR(string, start[, count]) Returns the count characters starting from start in the string

UPPER(string) Converts all characters to upper case

Examples : String Functions SELECT ename || ', ' || job FROM emp WHERE deptno=10; ENAME||','||JOB --------------------- CLARK, MANAGER MILLER, CLERK

SELECT ASCII(‘A’), ASCII(‘B’) FROM DUAL; ASCII('A') ASCII('B') ---------- ---------- 65 66

SELECT LOWER(ename)"Lower", UPPER(ename) "Upper", INITCAP(LOWER(ename)) "Initcap" FROM emp WHERE deptno=20; Lower Upper Initcap ---------- ---------- ---------- jones JONES Jones scott SCOTT Scott adams ADAMS Adams ford FORD Ford

SELECT ename, LENGTH(ename) FROM emp WHERE deptno=20; ENAME LENGTH(ENAME) ---------- ------------- JONES 5 SCOTT 5 ADAMS 5 FORD 4

SELECT RPAD(ename, 20, '.'), deptno FROM emp WHERE deptno=20; RPAD(ENAME,20,'.') DEPTNO -------------------- ---------- JONES............... 20 SCOTT............... 20 ADAMS............... 20 FORD................ 20

SELECT LPAD(ename, 15), sal FROM emp WHERE deptno=20; LPAD(ENAME,15) SAL --------------- ---------- JONES 2975 SCOTT 5000 ADAMS 1100 FORD 5000

SELECT SUBSTR(hiredate, 4) FROM emp WHERE deptno=10; SUBSTR ------ JUN-81 JAN-82

SELECT ename, INSTR(ename, 'I') "I Posn." FROM emp WHERE deptno=10; ENAME I Posn. ---------- ---------- CLARK 0 MILLER 2

SELECT loc FROM dept WHERE SOUNDEX(loc)=SOUNDEX('Sidney'); --both Sidney and Sydney would match

General Functions

Function Description GREATEST (expr [,expr]…) Returns the greatest of the list of expre ssions. The

expression can be numeric, character, or date. LEAST (expr [,expr]…) Returns the least of the list of expressions . The ex-

pression can be numeric, character, or date. NVL (expr1, expr2) If expr1 is null, returns expr2; if expr1 is not null,

returns expr1 DECODE (value, if1, then1 [, if2, then2]... else)

If the value is if1 then the result of the DECODE is then1; if it is if2 then the result is then2, and so on; if the value equals none of the ifs, then the result is else

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Examples: General Functions

SELECT LEAST ('HARRY', 'HARRIOT', 'HAROLD') "Least" FROM DUAL; Least ------ HAROLD SELECT GREATEST (23, 645, 123) "Greatest" FROM DUAL; Greatest ---------- 645 SELECT ename, sal + comm "totsal", sal + NVL(comm, 0) "totsal" FROM emp WHERE deptno=30; ENAME totsal totsal ---------- ---------- ---------- WARD 1750 1750 BLAKE 2850 TURNER 1500 1500 SELECT ename, NVL(TO_CHAR(comm), 'No Commission') "COMMISSION" FROM emp WHERE deptno = 30; ENAME COMMISSION ---------- --------------------- WARD 500 BLAKE No Commission TURNER 0 SELECT ename, DECODE(comm, null, 'NOT Eligible', 0, 'Eligible, but 0', comm) "Commission" FROM emp WHERE deptno=30; ENAME Commission ---------- ------------------------ WARD 500 BLAKE NOT Eligible TURNER Eligible, but 0 SELECT ename, DECODE(deptno, 10, 'ACCOUNTING', 20, 'RESEARCH', 30, 'SALES', 40, 'OPERATION', 'NONE') "Department" FROM emp WHERE job='CLERK'; ENAME Department ---------- ---------- ADAMS RESEARCH MILLER ACCOUNTING

Date Functions

Function Description ADD_MONTHS(date,n) Adds n months to the date LAST_DAY(date) Returns the date of the last day of the month that

contains date MONTHS_BETWEEN(date1, date2) Returns number of months between dates date1

and date2 NEXT_DAY(date, ‘day’) Returns the date of the first weekday named by

day that is later than the date date TO_CHAR(date, ‘format’) Converts date of DATE datatype to a value of

VARCHAR2 datatype in the format specified by the date format format. If you omit format, date is converted to a VARCHAR2 value in the default date format.

TO_DATE(string, ‘format’) Converts string of CHAR or VARCHAR2 datatype to a value of DATE datatype, in the date format specified by format. If you omit format, string must be in the default date format.

Date Formats (With both TO_CHAR and TO_DATE) Format Example/Description MM 12 RM XII MON DEC MONTH DECEMBER DDD 354 DD 23 D 6 (day of the week, where Sunday is 1) DY FRI DAY FRIDAY YYYY 1982 SYYYY -1000 YYY 982 YY 82 Y 2 YEAR NINETEEN-FORTY-SIX Q 3 (Number of quarter) WW 46 W 3 J Julian days since Dec 31, 4713 BC HH 11 HH12 Same as HH HH24 17 MI 58 SS 43

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SSSS 43000 (0-83699) / , - : . Punctuation for TO_CHAR A.M P.M AM or PM B.C A.D BC or AD Date Formats (Only with TO_CHAR) Format Example/Description “string” “is” fm Suppresses padding of Month or Day in format TH DdTH or DDTH gives: 24th or 24TH SP DDSP, DdSP, or ddSP gives: THREE, Three, or three SPTH DdSPTH gives: Third THSP Same as SPTH Examples : Date Functions

To get current date SELECT SYSDATE FROM Dual; SYSDATE --------- 12-DEC-02 To get a date 5 months back SELECT SYSDATE "Today", ADD_MONTHS(SYSDATE, -5) "Result" FROM DUAL; Today Result --------- --------- 12-DEC-02 12-JUL-02 To get the date of next Friday SELECT SYSDATE, NEXT_DAY(SYSDATE, 'THURSDAY') FROM DUAL; SYSDATE NEXT_DAY --------- --------- 12-DEC-02 19-DEC-02 To get the last of the month SELECT SYSDATE "Today", LAST_DAY(SYSDATE) "Month End" FROM DUAL; Today Month End --------- --------- 12-DEC-02 31-DEC-02 To get a date that is 1st of the month 6 months from now

SELECT SYSDATE TODAY, LAST_DAY(ADD_MONTHS(SYSDATE, 6))+1 "REVIEW ON" FROM DUAL; TODAY REVIEW ON --------- --------- 12-DEC-02 01-JUL-03 To get years of experience of employees in department 20 SELECT ename "Name", hiredate "Joined", ROUND(MONTHS_BETWEEN(SYSDATE, hiredate)/12) "Experience" FROM emp WHERE deptno=20; Name Joined Experience ---------- --------- ---------- JONES 02-APR-81 22 SCOTT 19-APR-87 16 ADAMS 23-MAY-87 16 FORD 03-DEC-81 21 To get the dates in words SELECT hiredate, TO_CHAR(hiredate,'DD, Month, Year' ) "In Words" FROM emp WHERE deptno=10; HIREDATE In Words --------- ---------------------------------- 09-JUN-81 09, June , Nineteen Eighty-One 23-JAN-82 23, January , Nineteen Eighty-Two SELECT hiredate, TO_CHAR(hiredate, 'fmMonth, ddTH, YYYY') "In Words" FROM emp WHERE deptno=10; HIREDATE In Words --------- --------------------- 09-JUN-81 June, 9th, 1981 23-JAN-82 January, 23rd, 1982 To get the current time SELECT TO_CHAR(SYSDATE, 'HH:MI:SS') "Time Now" FROM DUAL; Time Now -------- 12:20:08 To get the day name of a particular date SELECT TO_DATE('29-Aug-68') "Date", TO_CHAR(TO_DATE('29-Aug-68'), 'DAY') "Day" FROM DUAL; Date Day --------- --------- 29-AUG-68 THURSDAY

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Joins to Query Multiple Tables ? Joins are powerful relational operators that combine data from multiple tables into a

single result table. ? There are different types of joins :

o Cartesian Joins o Equi Joins o Self Joins o Outer Joins

Cartesian Joins ? In this join the tables are joined without any WHERE clause ; each row of one table

matches every row of the other table. ? It is also known as Natural Join. ? Examples: Equi Joins ? When two tables are joined together using equality of values in one or more columns

they make an equi join. ? Table prefixes are used to prevent ambiguity and the where clause is used to specify

which columns are to be joined. ? Example:

SELECT supp_name, po_status FROM suppliertab, orderfile; /* List all the information about the departments and courses offered by them */ SELECT * FROM department, course;

/* List the lecturers' name and their department name */ SELECT lecturer.c_Lect_Name, department.c_Dept_Name FROM lecturer, department WHERE lecturer.i_Dept_No = department.i_Dept_No ORDER BY lecturer.i_Dept_No;

Outer Joins ? When two tables are joined, only those that satisfy the given criteria are retrieved by

the query. ? If the user wants to check which were the rows that did not have a corresponding

match and still view the data with N ULL values outer joins are made use of. ? Examples:

Note: ? In the above example , the (+) is placed on the side of the table whose co lumns

do not have a corresponding match in the other table. ? Also note that you cannot have (+) operator for both the joined tables.

Self Joins ? When one row of a table is joined with another of the same table you perform a SELF

JOIN. ? Example : In the above example, since both the employee and his/her manager are part of the same table a self joi n is made use of.

/* List suppliers and their order status (also list the supplier who have not placed the orders */ SELECT supp_name, po_num, po_status FROM suppliertab, orderfile WHERE orderfile.supp_code(+)= suppliertab.supp_code;

/* List the lecturers name and their department’s name (list those d e-partment also which does not have any employee */ SELECT lecturer.c_Lect_Name, department.c_Dept_Name FROM lecturer, department WHERE lecturer.i_Dept_No =(+) department.i_Dept_No ORDER BY lecturer.i_Dept_No;

/* List the name of each employee along with his/her manager from emp table */ SELECT worker.ename, manager.ename FROM emp worker, emp manager WHERE worker.mgr = manager.empno;

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Note: To distinguish the columns of the copy of the same table , table aliases are used. In JOINS when the table name has to be written to specify columns more than once then it is better to use aliases in place of writing big table name s.

Set Operators

? Set operators combine 2 or more queries into a single result. The data type of the corresponding columns must be the same.

? The different set operators: o UNION o INTERSECT o MINUS

UNION ? This operator is useful when it is required to draw info rmation from more than one

table that have the same structure. ? It returns rows of the first query plus the rows of the second query after eliminating

duplicate rows. ? Example:

INTERSECT ? INTERSECT returns only those rows that are common to bo th queries. ? Example:

/* List the employees from account and research tables whose salary is greater than 2000 */ SELECT ename, sal FROM account WHERE sal > 2000 UNION SELECT ename , sal FROM research WHERE sal > 2000;

MINUS ? MINUS returns rows that are unique to the first query. ? Example: .

Nested Queries ? In this type of query the result of one query is dynamically substituted in the cond i-

tion (WHERE clause) of another query. ? In nested queries, the sub query within the WHERE clause is evaluated first. The r eturn

value is then substituted in the condition of the outer query. ? Oracle supports infinite level of nesting. ? The general syntax for a nested or sub query is :

/* List the jobs that are common in research, accounts and sales d e-partment */ SELECT job FROM research INTERSECT SELECT job FROM accounts INTERSECT SELECT job FROM sales;

/* List the jobs which are done in accounts only and not in sales */ SELECT job FROM account MINUS SELECT job FROM sales;

SELECT col1 [, col2]... FROM table1 WHERE column operator (SELECT column FROM table2 WHERE condition);

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? Examples: Note: When using relational operators with subqueries, ensure that the sub query returns a single row output. For comparison with a set of rows returned by the subquery, use the IN operator.

Correlated Subqueries ? SQL sub queries that perform the subquery over and over again, once for each row of

the outer query are called correlated subqueries. ? Example :

/* To list the names of employees drawing the highest salary */ SELECT ename, sal FROM emp WHERE sal = (SELECT MAX (sal) FROM emp); ENAME SAL ---------- ---------- SCOTT 5000 FORD 5000

/* List all items whos e quantity is greater than the average transaction qua ntity for that item */ SELECT item_code, trans_qty FROM trans_file tfile WHERE trans_qty > ( SELECT AVG(trans_qty) FROM trans_file WHERE tfile.item_code = trans_file.item_code) ORDER BY item_code;

/* To list all the managers in the emp table who have more than 1 person reporting to him/her */ SELECT empno FROM emp WHERE empno IN (SELECT mgr FROM emp GROUP BY mgr HAVING COUNT(*) > 1); EMPNO ---------- 7566 7698

Note: While referring to the outer query , alias is m andatory in the inner query. Using the EXISTS operator in subqueries ? EXISTS is a special operator that returns TRUE if a subquery returns at least one

row. ? Examples: /* To list the employee details only if more than 3 employees are present in

dept 20 */ SELECT * FROM emp WHERE deptno = 20 AND EXISTS (SELECT COUNT(*) FROM emp WHERE deptno = 20 GROUP BY deptno HAVING COUNT (*) > 3 );

/* To list out the department details in which there are any employees */ SELECT dname, deptno FROM dept WHERE EXISTS (SELECT * from emp WHERE dept.deptno = emp.deptno); DNAME DEPTNO -------------------- ---------- Accounting 10 Research 20 Sales 30

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DDaattaa MMaanniippuullaattiioonn CCoommmmaannddss The DML commands INSERT, UPDATE, and DELETE are discussed in this section.

The INSERT command ? INSERT statement is used to insert values into the table. ? Syntax: ? The INSERT with a VALUES clause inserts a single row into the table ? The INSERT with a query is used to add all the rows returned by a query at once. ? All data types to be included in quote s(' ') , except numeric data types. ? Examples:

The DELETE Command ? Using the DELETE statement row(s) can be deleted from a table. ? Syntax: ? Examples:

The UPDATE Command ? The UPDATE statement is used to change existing values in a specified table.

INSERT INTO table [ ( column [, column ]… ) ] { VALUES ( expression [, expression ]… ) | query }

INSERT INTO dept VALUES (60, 'Education', 'Boston'); INSERT INTO emp (empno, ename, deptno) VALUES (8000, 'John', 20); --rest of the columns will take a null or default value

DELETE FROM table [WHERE condition];

DELETE FROM emp; -- deletes all rows DELETE FROM emp WHERE empno=8000;

? Syntax: Note: Update without a WHERE clause will update all the rows in a table. ? Example:

UPDATE table SET {column = expression [, column = expression]... | ( column [, column]…) = (subquery ) } [WHERE condition];

To modify the salary and dep artment number of SMITH UPDATE emp SET sal = sal + 100, deptno = 30 WHERE ename = 'SMITH';

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TThhee TTrraannssaaccttiioonn CCoonnttrrooll CCoommmmaannddss ? A transaction (or a logical unit of work) is a sequence of SQL statements that Oracle

treats as a single unit. ? The Transaction Control commands manage changes made by insert, update or d e-

lete commands. ? The transaction control commands are:

o COMMIT o ROLLBACK

? A transaction begins with the first executable SQL statement after a COMMIT, ROLLBACK or connection to the database. A transaction ends with a COMMIT, ROLLBACK or disconnection (intentional or unintentional) from the d atabase.

The COMMIT Command ? COMMIT is used to end the current transaction and make permanent all changes done

in the transaction. ? This comma nd also releases the transaction's locks. ? Example: Upon commit, the inserted row resides permanently in the database .

The ROLLBACK Command ? A ROLLBACK undoes all the changes in the current transaction, and ends the transa c-

tion. It also releases the transaction's locks. ? Example: ROLLBACK with SAVEPOINT ? Using the ROLLBACK command with the TO SAVEPOINT clause performs the fo l-

lowing operations: o Rolls back just the portion of the transaction after the savepoint. o Erases all savepoints created after that savepoint. o Releases all the locks obtained since the savepoint.

INSERT INTO dept VALUES (60, 'Education', 'London'); COMMIT;

DELETE FROM emp; ROLLBACK;

? Example : In the above example, only the DELETE statement will be rolled back. The UPDATE is neither committed nor rolled back at this point.

UPDATE emp SET sal = sal + 100 WHERE deptno = 10; SAVEPOINT s1; DELETE FROM emp WHERE empno = 8000; ROLLBACK TO SAVEPOINT s1;

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WWoorrkkiinngg wwiitthh OOtthheerr SScchheemmaa OObbjjeeccttss ? Some of the database schema objects that we will work with are:

o Sequences o Synonyms o Views o Indexes o Clusters

Sequences ? Sequences are used to generate sequential numbers, helpful in multi -user environ-

ments to generate and return unique se quential numbers, and to generate unique int e-gers for use as primary keys.

? Sequence generators are objects that are stored as part of the database. ? Syntax: Note: ? The default INCREMENT BY is 1. A positive number will cause incrementing up and

a negative number incrementing down. ? The default for MINVALUE for ascending sequences is 1. ? The default for MAXVALUE for descending sequences is -1. ? The default START WITH is MAXVALUE for descending sequences and MINVALUE

for ascending; use START WITH to override this default. ? To restart a sequence, specify CYCLE. ? CACHE allows a preallocated set of sequence numbers to be kept in memory. The d e-

fault is 20. ? To use the contents of a sequence use seq_name.CURRVAL or

seq_name.NEXTVAL. ? seqname.CURRVAL returns the current value of the sequence. ? seqname.NEXTVAL returns the next value of the sequence. Also increments the

value.

CREATE SEQUENCE seqname [INCREMENT BY n] [START WITH m] [MAXVALUE integer | NOMAXVALUE ] [MINVALUE integer | NOMINVALUE ] [CYCLE | NOCYCLE ] [CACHE integer | NOCACHE ]

Examples:

To use the sequence:

DUAL is Oracle supplied table that is made up of one column and one row. This table is available to all users by default.

Synonyms ? A synonym is just another name for a database object. ? Synonyms do not require additional storage other than its definition in the data di c-

tionary. ? Synonyms are used:

o For security reasons to mask the owner and name of the object o To simplify SQL statements. o To provide transparency to user from remote databases.

? Whenever a user execute the query for example,

SELECT * FROM table_name; Oracle interprets it as SELECT * FROM owner.table_name;

CREATE SEQUENCE dept_seq START WITH 1 INCREMENT BY 1;

INSERT INTO dept (deptno, dname, loc) VALUES (dept_seq.NEXTVAL, 'Education', 'Fremont');

SELECT dept_seq.CURRVAL FROM DUAL; SELECT dept_seq.NEXTVAL FROM DUAL;

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If the user is not the owner of the table then he has to use owner.table_name. In order to hide the actual owner as well as ta-ble_name synonyms can be used as fo llows: CREATE PUBLIC SYNONYM syn_name FOR owner.table_name;

Example: Creation and use of Synonyms

The PUBLIC option indicates that any user can access the synonym.

Views ? A view is a logical table. It does not contain any data itself; it draws data from other

tables and views. ? The tables on which a view is based are called base tables. ? Views are constructed with queries. A view takes the output of a query and treats it as

a table; hence, a view can also be thought of as a “stored query” or a “virtual table”. ? You can use the normal querying on the views as though views were tables the m-

selves . ? Insert, update, and delete to the base tables through the view is subject to certain r e-

strictions. ? All operations performed on a view actually affect data in the base tables of the view. ? Views can used to for various reasons like:

o Store complex queries o Provide security to base tables by exposing only certain portions of table

data through views o Isolate applications from changes in definitions of base tables

Creating Views The CREATE VIEW DDL command is used to create a view, that mentions a query based on which the view is created (in the absence of aliases, the view column names are same as that mentioned in the query) :

CREATE PUBLIC SYNONYM pucust FOR scott.customer; SELECT * FROM pucust; INSERT INTO pucust(...) VALUES(...)

CREATE [OR REPLACE] VIEW view [ ( alias [, alias ]… ) ] AS query [WITH CHECK OPTION] [WITH READ ONLY]

WITH READ ONLY option specifies that no deletes, inserts, or updates to the base table can be performed through the view. WITH CHECK OPTION specifies that inserts and updates performed through the view must result in rows that the view query can select Examples of Views Suppose you would like to expose only part of the emp table. You can do so by creating a view that selects certain columns from the table, and using (or granting privileges on, to other users) this view, as shown below. Table EMP EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO 7329 SMITH CLERK 7902 17-DEC-87 300.00 80 20 7499 ALLEN SALESMAN 7698 20-FEB-88 300.00 600 30 7521 WARD SALESMAN 7698 22-FEB-88 5.00 250 30 7566 JONES MANAGER 7839 02-APR-88 975 20

EMPNO ENAME JOB MGR DEPTNO 7329 SMITH CLERK 7902 20 7499 ALLEN SALESMAN 7698 30 7521 WARD SALESMAN 7698 30 7566 JONES MANAGER 7839 20

The above view is created with the DDL:

You can then use the view as a normal table:

If you did not want any insert or update to the base table through this view , create the view with WITH READ ONLY option:

View STAFF

CREATE VIEW staff AS SELECT empno, ename, job, manager, deptno FROM emp;

CREATE VIEW staff AS SELECT empno, ename, job, manager, deptno FROM emp WITH READ ONLY;

SELECT * FROM Staff; INSERT INTO staff VALUES (8000, 'John', 'SALESMAN', 7839, 20);

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If you want to create a view that exposes only the clerks in the emp table, create a view as follows:

The WITH CHECK OPTION specifies that inserts and updates performed through the view must result in rows that the view query can select. In this example any row other than the one having job='CLERK' cannot be inserted into the table. Note: You cannot perform inserts, updates or deletes for a view if the view query co n-tains:

o joins o set operators o group functions o GROUP BY clause o the DISTINCT operator

Indexes ? The data in the tables are not sto red in any particular order of any column. Hence,

when you access the table data, it is sequential access. This can pose a performance problem if the table is huge.

? You can use Indexes to provide random access to data, there by enhancing perfor m-ance due to faster access to the data.

? Indexes are stored separately from the actual data. The index is made up of the co l-umn on which you are indexing, and the physical address ( ROWID) of the row with that column value.

? Indexes are referred to whenever the indexed c olumns are referenced in the WHERE clause.

o E.g., if the query SELECT ename, sal FROM emp WHERE ename = 'SMITH' is issued, and if there is an index on ename already, the index will be first searched to find the location of the row with specified ename value, and then the row is fetched from the table.

? With every data manipulation, the appropriate indexes are automatically updated. o E.g., if there is an insert into the table, the corresponding indexes are also

updated to reflect the new rows. ? Indexes are created using the CREATE INDEX DDL command:

CREATE VIEW clerk (id_number, person, department, position) AS SELECT empno, ename, deptno,job FROM emp WHERE job = 'CLERK' WITH CHECK OPTION;

CREATE INDEX index_name ON table_name(column_name);

Example When you create an index: ? Oracle gets and sorts the columns to be indexed, ? Stores the ROWID along with the index value for each row , and ? Loads the index from the bottom up. Note: ? Oracle, by default, creates indexes for columns defined as PRIMARY key or UNIQUE

key, at the time of table creation . ? Internally, Oracle uses B*Trees to manage the indexes. Composite Indexes A Composite index is one that is created on multiple columns in a table. It is also known as concatenated index . Example

Clusters ? A Cluster is a group of tables that share the same data blocks, because they share

the same columns and are often used together. ? If multiple tables are clustered together, they should have commo n column(s),

called the cluster key. E.g., emp and dept tables can be clustered together based on the cluster key deptno that is common to both the tables.

? Advantages of using clusters: o Disk I/O is reduced and access time improves for joins of clustered ta bles. o Each cluster key is stored only once in the cluster and the cluster index, no

matter how many rows of different tables contain the value. Creating a Cluster ? To create a cluster:

o First create the cluster using the CREATE CLUSTER command

CREATE INDEX idx_ename ON emp(ename);

CREATE INDEX index_name ON table_name( col1, col2, col3);

CREATE INDEX ordcust ON orders(custcode,ordno)

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o Then create the tables within the cluster. This is done by including the CLUSTER clause in the CREATE TABLE command

o Before you can insert any rows into the clustered tables, you must cr e-ate a cluster index

Example: Creating tables emp and dept clustered around department number.

CREATE CLUSTER PERSONNEL(DEPT_NO NUMBER(2));

CREATE TABLE EMP( EMPNO NUMBER(4) PRIMARY KEY, DEPTNO NUMBER(2) NOT NULL, ENAME VARCHAR2(15), SAL NUMBER(10,2), COMM NUMBER(5)) CLUSTER PERSONNEL(DEPTNO);

CREATE TABLE DEPT( DEPTNO NUMBER(2) PRIMARY KEY, DNAME VARCHAR2(20), LOC VARCHAR2(15)) CLUSTER PERSONNEL(DEPTNO);

CREATE INDEX IDX_PERSONNEL ON CLUSTER PERSONNEL;

INSERT INTO dept VALUES (10,'SALES', 'NY'); ...