Oracle8 - The Complete Reference. Koch & Loney1 Chapter 17 Creating, Dropping, and Altering Tables and Views Presented by Victor Matos

Oracle8 - The Complete Reference. Koch & Loney 1

Chapter 17

Creating, Dropping, and

Altering Tables and Views

Presented by Victor Matos


Goals• Create tables containing integrity rules. • Use different datatypes for the columns of a table.• Populate tables using the “insert into table” command.• Populate tables using the subquery option.• Create/drop views.


Data Structures in Oracle8

• A database can include one or many:– Table Stores data– View Virtual tables facilitating process– Sequence Automatic serial numbers.– Index Improve performance.– Constraint Define data behavior.


Tables

• Can be created at any time using SQL-DML.

• Do not need to have a pre-allocated storage.

• Can be modify on real-time.

• All data in a relational database is stored in tables.


Tables• Each column is given a

– column name, – a datatype (defining characteristics of the data to be entered in the column), and a – width (quantity of space to allocate for data to be entered into the column).


Tables

• Usually in a relational database, some of the columns in different tables contain the same information. In this way, the tables can refer to one another.


Create Table Command

• Syntax

create table [schema.]tableName

(column1 datatype [DEFAULT expr]

[column_constraint],

. . .

[table_Constraints] );



• The components of the command are:– schema same as the owner’s name.– tableName is the name of the table.– column is the name of the column.– datatype Oracle’s datatype and size.– column_contraint field validation rule.– table_constraint integrity rule on the table.



• Example-1: Make the BADWEATHER table.

drop table BADWEATHER/create table BADWEATHER ( CITY VARCHAR2(13) NOT NULL , SAMPLEDATE DATE NOT NULL , NOON NUMBER(4,1), MIDNIGHT NUMBER(4,1), PRECIPITATION NUMBER )/

remove previous table



• Example-2: Create an EMPLOYEE table.CREATE TABLE scott.emp (empno NUMBER CONSTRAINT pk_emp PRIMARY KEY, ename VARCHAR2(10) CONSTRAINT nn_ename NOT NULL CONSTRAINT upper_ename CHECK (ename = UPPER(ename)), job VARCHAR2(9), mgr NUMBER CONSTRAINT fk_mgr REFERENCES scott.emp(empno), hiredate DATE DEFAULT SYSDATE, sal NUMBER(10,2) CONSTRAINT ck_sal CHECK (sal > 500), comm NUMBER(9,0) DEFAULT NULL, deptno NUMBER(2) CONSTRAINT nn_deptno NOT NULL CONSTRAINT fk_deptno REFERENCES scott.dept(deptno) ) PCTFREE 5 PCTUSED 75;


Referencing Another User’s Tables

• Constraints should reference objects in the current database.

• If the referred table does not belong to the user making the constraint, the owner’s name must be used to prefix the table name... references JANE.CUSTOMERS ...


DEFAULT Option

• Indicates a default value to be used when inserting a record.... OrderDate DATE DEFAULT SysDate ...

• Use only literal values (“...”), numbers, expressions, SQL functions such as USER, or SYSDATE.


Rules for Naming a Table

• TableName must begin with a letter.

• Up to 30-chars long.

• Alphabetic, digits, #, $, _ symbols are ok.

• Can not duplicate the name of another object already created by the user.


Oracle DatatypesDatatype Description

Varchar2(size) Variable length string

Char(size) Fixed length string

Number Real & Integer numbers.

Date Calendar & Clock

RAW Variable length binary data.

LONG Variable Length strings up to 2GB


Deciding on a Proper Width

• A character column that is not wide enough for the data you want to put in it will cause an insert to fail and result in this message

Error...ORA-1401:inserted value too large for column

• Maximum CHAR allocation is 2,000 chars.

• VARCHAR2 can store up to 4,000 chars.


NUMBER Precision

• Make a NUMBER field wide enough for your actual data, otherwise– Oracle will report an error “value too large”, or– Precision will be lost on some of the least

significant digits.

• Decimals are rounded to the next valid representation.


Table Constraints

• Enforce maintenance-table behavior. Rules can be checked when a record is– inserted– deleted– updated

• Prevent the removal of referenced tables.


Table Constraints

Constraint Description

NOT NULL This column must contain data.

UNIQUE Duplicate values are not acceptable.

PRIMARY KEY Column uniquely identifies the row.

FOREIGN KEY Refers to a key-column in anothertable.

CHECK Specifies a validation condition.


The Candidate Key

• A candidate key is a combination of one or more columns, the values of which make a unique identifier for the row.

drop table BADWEATHER;

create table BADWEATHER ( CITY VARCHAR2(13) NOT NULL , SAMPLEDATE DATE NOT NULL , NOON NUMBER(4,1), MIDNIGHT NUMBER(4,1), PRECIPITATION NUMBER,

UNIQUE (City, SampleDate) )/


The Primary Key

• A primary key is one of the candidate keys.

• Only one primary-key per table is allowed.

drop table BADWEATHER;

create table BADWEATHER ( CITY VARCHAR2(13) NOT NULL , SAMPLEDATE DATE NOT NULL , NOON NUMBER(4,1), MIDNIGHT NUMBER(4,1), PRECIPITATION NUMBER,

PRIMARY KEY (City, SampleDate) )/


The Primary Key

• An in-line primary key is an immediate definition applying on only one column.

drop table WORKER2;

create table WORKER2 ( Name VARCHAR2(25) PRIMARY KEY ,

Age NUMBER, Address VARCHAR(15)

)/


The Foreign Key

• A foreign-key is one or more columns which refer to the key of another table.

drop table WORKER2;



REFERENCES LODGING(Lodging))

/


The Check Constraint

• Checks state that values of a column must be within certain valid ranges.

drop table WORKER2;


Age NUMBER CHECK (Age between 18 and 65), Address VARCHAR(15)

)/

The condition must be TRUE for new records to be inserted


Naming Constraints

• All constraints are stored in the data dictionary.

• Name constraints are easy to manipulate– create / drop– enable / disable– alter.


Named Constraints

• • Use a table_guide name combination, where guide is– table_PK Primary Key– table_FK Foreign Key– table_UK Unique Key– table_Field_CK Check rule on Field.


Named Constraints

drop table WORKER2

/create table WORKER2 ( Name VARCHAR2(25) constraint WORKER2_PK

PRIMARY KEY,Age NUMBER constraint WORKER2_AGE_CK

CHECK (Age Between 18 and 65), Address VARCHAR(15) constraint WORKER2_Address_FK REFERENCES LODGING(Lodging)

)/


Altering Tables

• Tables can be altered in two ways– changing a column’s definition (modify), or

– adding a new column to an existing table (add)

alter table LODGING add ( constraint Lodging_PK PRIMARY KEY (Lodging) )

NOTE: Use the DESC operator to see the current table structure.


Rules for Add/Modify Table

• When adding a column do not include the NOT NULL clause.

• You may add a NOT NULL in 3 steps– 1. Add the column– 2. Fill every possible row with data.– 3. Modify the column to be NOT NULL.

• Increase CHAR/VARCHAR2 width at any time.• Add more digits to a NUMBER at any time.


Rules for Views

• Views based on a single table can insert, delete, and update the underlying table.

• You can not insert if the base table has any not null columns not included in the view.

• You can not insert, update if columns in the view are generated with functions or calculations.

• You can not maintain the base table if the view includes: group by, distinct, rowNum.


Read-Only Views

create or replace view RAIN asselect City, Precipitationfrom BADWEATHERwith read only;

Comment: Read-only views are also called SNAP-SHOTS


Create a Table from a Table

• Use the create table ... as select-statm.

drop table RAIN;

create table RAINasselect * from BADWEATHER;

You can use the name of an existing object.


Create a Table from a Table

• To copy only the structure -and no data- do as in the example below

create table RAINasselect * from BADWEATHERwhere true = false;

The condition is false and results in no row selected


Index-Only Tables

• An index-only table is stored as if the entire table were an index.

• They can be used in order to• increase performance

• enforce uniqueness of data

• Only one key can be used to index the table.

• Good idea only when data is very stable.


Index-Only Tables

drop table WORKER3;

create table WORKER3 ( Name VARCHAR2(25),


constraint WORKER3_PK PRIMARY KEY (Name))ORGANIZATION INDEX;

/


Partitioned Tables

• Large tables could be split into more than one physical

table-location. Partitioning may improve performance.create table WORKER5 ( Name VARCHAR2(25),

Age NUMBER, Address VARCHAR(15),

constraint WORKER5_PK PRIMARY KEY (Name))PARTITION by range (Name) (partition PART1 values less than ('F') tablespace Ohio1_TS, partition PART2 values less than (MAXVALUE)

tablespace Ohio2_TS);


Summary

• SQL tables are created using the commandcreate table [schema.]tablname (

column datatype [col_constraint], ... )

[table_Constraint];

• Constructing a table requires• Table name,

• Column names, datatypes, lengths,

• Integrity constraints.


Summary

• Constraint types– NOT NULL

– UNIQUE

– PRIMARY KEY

– FOREIGN KEY

– CHECK

• UNIQUE indexes are created automatically with– PRIMARY KEY and

– UNIQUE constraints

Documents

Oracle8 - The Complete Reference. Koch & Loney1 Chapter 17 Creating, Dropping, and Altering Tables and Views Presented by Victor Matos