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Query Languages: How to build or interrogate a relational database
Structured Query Language (SQL)
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SQL
SQL is a query language for relational databases. Contains:
Data Definition Language to define databases Data Manipulation Language to manipulate
databases. SQL is widely accepted and is used by most
relational DBMSs. Is being standardized.
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The importance of SQL
Since SQL is used in almost all relational databases, once you know SQL you can probably construct and manipulate databases in all RDBMs.
Knowing SQL makes you a (beginning) ORACLE, Informix, SyBase, AdaBas, and so on programmer!
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Functionalities of SQL
SQL provides On-line and embedded use. Precompilation of embedded queries. Dynamic database definition and alteration. Maintenance of indexes View mechanism Authorization mechanism Automatic concurrency control Logging and database recovery Report formatting
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Tables in SQL
SQL recognizes Base Tables
real tables that physically exist in the database. There are physically stored records and possibly physically stored indexes directly corresponding to the table
Views virtual tables that do not physically exist but look to the
user as if they do
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Database Spaces
DBSpace is a section of physical disk. It consists of
Base tables Indices Views
All can be dynamically dropped from DBSpaces. DBSpaces allow the DB administrator to distribute
data accesses over different disks.
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Indexes
As we know, indexes can improve search performance.
Cost: more space needed and slower insertion. Indexes can be defined over any combination of
attributes in a base table. Automatically maintained in SQL. Users never directly use an index.
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Views
Correspond to external schemas. Derived from one or more base tables or views. Computed dynamically.
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Operations in SQL
For tables: CREATE, ALTER, DROP
For indexes CREATE, DROP
For views: CREATE, DROP
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Creating tables
CREATE <table name>
(<coldecl> [,<coldecl>*], [, <pkdef> [, <fkdef>*]);
<coldecl> :=
<col><type>[NOT NULL]
<type> :=
integer|smallint|float(p)|
decimal(p,q)|char(n)|
varchar(n)|long varchar|
date|time
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Creating tables continued
<pkdef> :=
PRIMARY KEY (<colname>
[,<colname>*]
<fkdef> :=
FOREIGN KEY (<colname>[,<colname>*])
REFERENCES <table>
[ ON DELETE <effect>]
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More on creating tables
<effect> :=
RESTRICT | CASCADE | SET NULL
What happens when the tuple in the referenced table with that value is deleted RESTRICT: Do not delete as long as there tuples
in other table with that foreign key value CASCADE: Delete all tuples with that foreign key
value SET NULL: Set value of foreign key to NULL.
(Note violates referential integrity).
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Example 1
CREATE TABLE Student
(sid CHAR(5) NOT NULL,
sname VARCHAR(20),
address VARCHAR(70),
PRIMARY KEY (sid));
OR
CREATE TABLE Student
(sid CHAR(5) PRIMARY KEY,
sname VARCHAR(20),
address VARCHAR(70));
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Example 2
CREATE TABLE Enrol
(sid CHAR(5) NOT NULL,
cid CHAR(5) NOT NULL,
grade INT,
PRIMARY KEY(sid, cid),
FOREIGN KEY (sid)
REFERENCES Student
ON DELETE CASCADE
FOREIGN KEY (cid)
REFERENCES Course
ON DELETE RESTRICT);
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Altering tables I
ALTER TABLE <table name>
ADD {<coldecl>|
<pkdef>|
<fkdef>};
ALTER TABLE Enrol
ADD edate DATE;
adds a new column to the table grade. For existing tuples, the value is set to NULL.
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Altering tables II
ALTER TABLE <table name>
DROP {PRIMARY KEY|
<fkname>};
Note that care must be taken when dropping columns.
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Dropping tables
Tables can be dropped at any time. Dropping a table deletes both the definition and
data. Also, all views, indexes and foreign key
definitions referring to this table are dropped.
DROP TABLE <table name>;
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Creating indexes
CREATE [UNIQUE] INDEX
<index> ON <table>
(<colname> [<order>]
[,<colname> [<order>]*]);
<order>:= ASC | DESC
Creates an index on named columns. With UNIQUE, no two tuples can have the same values for the indexes columns.
Example:CREATE INDEX Course
ON Enrol (cid);
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Data manipulation
Having created the tables, indexes and views, we now need to populate the database and retrieve information from it.
In other words, we want to manipulate the data.
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Retrieval
SELECT [DISTINCT] <items>
FROM <table> [, <table>*]
[WHERE <pred>]
[GROUP BY <attrs>
[HAVING <pred>]]
[ORDER BY <attrs> ];
Corresponds to a JOIN-SELECT-PROJECT expression in relational algebra.
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Predicates
The predicate <pred> is a condition formed by parentheses and boolean operators AND, OR and NOT.
A condition has the form <attr><op>{<value>|<attr>}
and an operator is one of < | =< | > | >= | = | !=
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WHERE clauses
In general, WHERE clauses are constructed as in relational algebra, but with some additions
LIKE string May contain wildcard characters %, which matches
any string, and _, which matches a single character.
IN (set of values) Tests for set membership
BETWEEN c1 AND c2
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Example
Find Student IDs and grades for those students who read CS51T
SELECT sid, grade
FROM Enrol
WHERE cid = ‘CS51T’;
Compare
sid, grade ( cid = ‘CS51T’(Enrol))
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Example continued
We can embellish the way in which the result appears by including format strings in the SELECT
Example
SELECT Student as sid, grade
FROM Enrol
WHERE cid = ‘CS51T’;
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DISTINCT
DISTINCT is used to make sure that we do not get any duplicate values.
ExampleSELECT DISTINCT cid
FROM Enrol
WHERE grade > 70;
First, find the various course numbers that qualify and then remove duplicates.
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More examples
The use of * in the SELECT returns all attributesSELECT *
FROM Enrol
WHERE cid = ‘CS51T’;
Find all students who obtained 60 or more for CS51TSELECT sid
FROM Enrol
WHERE cid = ‘CS51T’
AND grade >= 60;
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Yet more examples
Find all results for either or CS51T or CS51SSELECT *
FROM Enrol
WHERE cid IN
(‘CS51S’, ‘CS51T’);
Find results for CS coursesSELECT *
FROM Enrol
WHERE cid LIKE ‘CS%’;
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Ordering results
Get all results for CS51S and CS51T but order them by result
SELECT sid, cid, grade
FROM Enrol
WHERE cid IN
(‘CS51S’, ‘CS51T’)
ORDER BY grade DESC;
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Subqueries
Notice that the result of a SELECT clause is a table which can be used in another WHERE clause.
Find course titles of the courses for which 123 was registeredSELECT title
FROM Course
WHERE cid IN
(SELECT cid FROM Enrol
WHERE sid = ‘123’);
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Table labels
Sometimes we need to interrogate the same table twice.
We use table labels Example: Get IDs from those students who did both
CS51S and CS51T
SELECT DISTINCT sid
FROM Enrol as E1, Enrol as E2
WHERE E1.Sid = E2.Sid
AND E1.Cid = ‘CS51S’
AND E2.Cid = ‘CS51T’;
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Table labels can usually be avoided
We could formulate the same query as
SELECT sid
FROM Enrol
WHERE cid = ‘CS51S’
AND sid IN
(SELECT sid
FROM Enrol WHERE cid = ‘CS51T’);
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Use of ALL in WHERE clauses
Queries that look at all tuples satisfying a particular predicate.
Get the IDs of the students all of whose results are over 70.SELECT sidFROM Enrol as E1WHERE 70 < ALL (SELECT grade FROM Enrol as E2 WHERE E1.sid = E2.sid);
Forms of ALL:< ALL, <= ALL, = ALL, >= ALL, > ALL
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Union
Union allows one to union tuples from different tables.
Get Student IDs for all students whose name starts with a ‘J’ or who obtained an A for CS51T.
SELECT sid FROM Student
WHERE sname LIKE ‘J%’
UNION
SELECT sid FROM Enrol
WHERE cid = ‘CS51T’
AND grade > 70;
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Intersect
Allows one to intersect Get all IDs for students whose name begins with
a ‘J’ and who obtained an A for CS51S
SELECT sid FROM Student
WHERE sname LIKE ‘J%’
UNION
SELECT sid FROM Enrol
WHERE cid = ‘CS51S’
AND grade > 70;
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EXISTS and NOT EXISTS
Counterpart of ALL Find name of students who have not obtained an
A for any course
SELECT sname FROM Student
WHERE NOT EXISTS
(SELECT * FROM Enrol
WHERE sid = Student.sid
AND grade > 70);
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Analysis of data
In order to help do some primitive analysis of data, SQL has some built-in functions COUNT(*) COUNT(DISTINCT <attr>) SUM([DISTINCT]<item>)
where <item> may be an abstraction and does not need to be a single attribute.
AVG([DISTINCT]<item>) MAX(<item>) MIN(<item>)
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Some simple examples of data analysis in SQL How many students are registered for at least one
courseSELECT COUNT(DISTINCT sid)
FROM Enrol;
Find the average grade for CS51SSELECT AVG(grade)
FROM Enrol
WHERE cid = ‘CS51S’;
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Another example
How many students were above the average for CS51T?
SELECT COUNT(*)
FROM Enrol
WHERE grade >
(SELECT AVG(grade)
FROM Enrol
WHERE cid = ‘CS51T’);
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Yet another example
What is the name of the student who got the best mark for CS51T?
SELECT sname
FROM Student
WHERE sid IN
(SELECT sid
FROM Enrol
WHERE grade =
(SELECT MAX(grade)
FROM Enrol
WHERE cid = ‘CS51T’));
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GROUP BY
A relation can be partitioned into groups according to some value. Analysis can then be done on these groups.
What are the averages for the various courses?
SELECT cid, AVG(grade)
FROM Enrol
GROUP BY cid;
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HAVING
After partitioning, we can disqualify groups. What is average results for courses with
enrollment of more than 10? SELECT cid, AVG(grade) FROM Enrol
GROUP BY cid
HAVING COUNT(*) > 10;
COUNT is applied to each group separately.
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Insertion
INSERT INTO {<table>|<view>}
[(<attr>] [,<attr>*])]
{VALUES (<items>|
<select statement>)};
Example
INSERT INTO Enrol
(cid, sid, grade)
VALUES (‘CS51T’, ‘123’, 67);
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Insertion through a SELECT statement For each course, get the average and insert into a
RES table
INSERT INTO RES (cid, average)
SELECT cid, AVG(grade)
FROM Enrol
GROUP BY cid;
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Deletion
DELETE FROM <table>
[<WHERE clause>];
Example DELETE FROM Enrol
WHERE cid = ‘CS51T’;
Difference between DELETE and DROP DELETE FROM Enrol;
DELETE empties the table but leaves the table and indexes.
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Updating tables
UPDATE <table>
SET <attr> = <expr>
[, <attr> = <expr>*]
[<WHERE CLAUSE>];
Example: Give everybody 10 extra marks for CS35A
UPDATE Enrol
SET grade = grade + 10
WHERE cid = ‘CS51T’;
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Views
Views are derived tables whose definition is stored and whose content is computed.
Can be used as base table for retrieval and view definition.
Exact condition for updating an open problem. Currently only update iff
derived form single base table and, has rows and attributes corresponding to a
unique and distinct row in base table.
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Advantages of views
Views are SQL’s external schemas. They are useful Users are immune to database growth Users are immune to database restructuring
(logical data independence) Simplified user perception Different views of same data for different users Automatic security for hidden data.
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Creation and deletion of views
CREATE VIEW <view>
[(<colname>[,<colname>*])]
AS select-statement;
ExampleCREATE VIEW CourseAvg
(Cid,Average)
AS SELECT cid, Avg(grade)
FROM Enrol
GROUP BY cid;
DeletionDELETE VIEW <view>;
DELETE VIEW CourseAvg;
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The view update problem
The view CourseAvg as defined above cannot be updated, as any updates cannot be translated into the base table.
The DB administrator should decide whether a view is updatable.
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Authorization Mechanism
The authorization mechanism allows one to give other users permission to access and update data in a view or table.
The owner must explicitly grant necessary privileges to others, as by default the owner has all privileges and others have none.
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GRANT and REVOKE
GRANT <privilege>
ON <table> | <view>
TO <user> [,<user>*] |
PUBLIC
[WITH GRANT OPTION];
REVOKE <privilege>
ON <table> | <view>
FROM <user> [,<user>*] |
PUBLIC;
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Grantable privileges
These privileges are allowed: SELECT INSERT UPDATE DELETE ALTER INDEX
permission to create or drop indexes on a table. ALL