Lecture 5hjjkujku

7/28/2019 Lecture 5hjjkujku

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Database ManagementSystems II

Lecture 5


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Last Lecture Relational Algebra

SQL (DDL, DML, SELECT)

Any questions?


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This lecture Views

SQL extensions (specifically T-SQL)

Transaction Basics

Triggers

Stored Procedures


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ViewsAviewis a virtual table

A view is based on base tables ordefining tables

In SQL, CREATE VIEW statement isused to define views


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Views (contd.)Example

Create a view of student names & their majoring

department

CREATE VIEW STUD_MAJAS

SELECT S.Name, D.NameFROM STUDENT S INNER JOINDEPARTMENT D ON(S.Major = D.DeptID)


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Views (contd.)Another example

View of departments and their average salary

CREATE VIEW DEPT_SAL (Dept, AvgSalary)ASSELECT D.Name, AVG(E.Salary)

FROM EMPLOYEE E INNER JOINDEPARTMENT D ON(E.Dept = D.DeptID)

GROUP BY D.Name


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Views (contd.) Now, we can query the view similar to a

table

Example..

List the departments with average salary >

30,000SELECT Dept

FROM DEPT_SAL

WHERE AvgSalary > 30,000


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Views (contd.) Dropping a view...

DROP VIEW DEPT_SAL


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Views (contd.) Updating a view can be ambiguous

Views containing aggregate functions are not

updateable

Example

UPDATE DEPT_SAL

SET AvgSalary = 40000WHERE Dept = CSE

**This is not possible


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Views (contd.) Views containing a join can be ambiguous

UPDATE V1

SET A = a

WHERE C = e

A B

b 1

B C

1 d

1 e

A B C

b 1 d

b 1 e

A B V1 = A B


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Views (contd.) Thus, in many DBMSs, views are

updateable only if they are defined on a

single base table.

Views are utilized for

Security mechanism

Convenience


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Transaction Basics Concepts

Atomicity Consistency Isolation Durability

T-SQL Commands BEGIN TRANSACTION Begin transaction COMMIT TRANSACTION End transaction ROLLBACK TRANSACTION Cancel transaction


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Transaction Basics (contd.) Transferring Rs. 1000 from Account 1234 to Account 2345

Begin Transaction

update Accountset balance = balance - 1000where account_no = 1234

update Account

set balance = balance + 1000where account_no = 2345

Commit transaction


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Programming in T-SQL Similar to a programming language,

certain extensions have been made in

SQL to program simple server-sidelogic.

Some of the statements include:Variables

Selection conditions IF () ELSE

Looping

WHILE ()


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T-SQL: Variables A Transact-SQL local variable is an object that can

hold a single data value of a specific type.

Variables in batches and scripts are typically used:

As a counter either to count the number of timesa loop is performed or to control how many timesthe loop is performed

To hold a data value to be tested by a control-of-flow statement

To save a data value to be returned by a storedprocedure return code.


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T-SQL: Variables (contd.)

The DECLARE statement initializes aTransact-SQL variable by:

Assigning a name. The name must have a single@ as the first character.

Assigning a system-supplied or user-defined datatype and a length. For numeric variables, a

precision and scale are also assigned. Setting the value to NULL.

For example

DECLARE @DeptName VARCHAR(20)


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Now we can use the variable in ourscript

For example...

SELECT building

FROM Dept

WHERE deptname = @DeptName


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A variable can also have a valueassigned by being referenced in a select

list.

DECLARE @EmpIDVariable INT

SELECT @EmpIDVariable =

MAX(EmployeeID)

FROM Employees


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T-SQL: IF statement

Imposes conditions on the execution of aTransact-SQL statement.

IF (SELECT AVG(price) FROM titles) < 15BEGIN

PRINT Inside the IF statement

PRINT Multiple statements have to be encoded in BEGIN END

END

ELSE

PRINT 'Average title price is more than 15.'


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T-SQL: WHILE statement

Sets a condition for the repeated execution ofan SQL statement or statement block.

The statements are executed repeatedly aslong as the specified condition is true.

The execution of statements in the WHILEloop can be controlled from inside the loopwith the BREAK and CONTINUE keywords.


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T-SQL: WHILE statement(contd.)

BREAK

Causes an exit from the innermost

WHILE loop. Any statements appearingafter the END keyword, marking theend of the loop, are executed.

CONTINUE

Causes the WHILE loop to restart,ignoring any statements after theCONTINUE keyword.


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T-SQL: WHILE statement(contd.)

Example

WHILE (SELECT AVG(price) FROM titles) < $30

BEGIN

UPDATE titles

SET price = price * 2 SELECT MAX(price) FROM titles

IF (SELECT MAX(price) FROM titles) > $50BREAK

ELSE

CONTINUE

END

PRINT 'Too much for the market to bear'


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Stored Procedures/Functions

Business logic is maintained in database tierfor data intensive operations

E.g. Calculating all interest earned in bankaccounts

In SQL Server 2005, Stored Procedure/

Functions can be written in T-SQL (we will study only this)

Any .NET Language


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Stored Procedures/Functions(contd.)

Example

create table emp(

id varchar(200) primary key,

hrs_worked int,

rate float);


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Function Example

CREATE FUNCTION func_calc_salary (@id varchar) RETURNS floatas

begindeclare @hrs_worked_ intdeclare @rate_ floatdeclare @amt_ float

select @rate_ = e.rate, @hrs_worked_ = e.hrs_worked

from emp ewhere e.id = id

set @amt_ = @rate_* @hrs_worked_return @amt_

end;


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Calling the function created previously

declare @temp float

exec @temp = calc_salary'e1'

print @temp


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Procedure Example (with output parameter)

CREATE PROCEDURE proc_calc_salary (@id varchar, @salary float output)as

begindeclare @hrs_worked_ intdeclare @rate_ realdeclare @amt_ float

select @rate_ = e.rate, @hrs_worked_ = e.hrs_worked

from emp ewhere e.id = id

set @amt_ = @rate_* @hrs_worked_set @salary = @amt_

end;


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Calling a procedure

declare @temp float

exec proc_calc_salary'e1', @temp OUTPUT

print @temp


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Triggers

Triggers are useful in enforcingbusiness rules and data integrity.

More powerful than general constraints.

For example,

The employees salary is always less thanhis/her managers salary


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T-SQL: Triggers (contd.)

We will learn DML triggers

CREATE TRIGGERtrigger_nameON { table| view}{

{ { FOR | AFTER | INSTEAD OF }

{ [ INSERT ] [, ] [ UPDATE ] [, ]

[DELETE ] }

ASsql_statement[...n]

}

}


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AFTER Specifies that the trigger is fired only when all operations

specified in the triggering SQL statement have executedsuccessfully.

AFTER is the default, if FOR is the only keyword specified. AFTER triggers cannot be defined on views.INSTEAD OF Specifies that the trigger is executed instead ofthe triggering

SQL statement, thus overriding the actions of the triggeringstatements.

At most, one INSTEAD OF trigger per INSERT, UPDATE, orDELETE statement can be defined on a table or view.

[DELETE] [,] [INSERT] [,] [UPDATE] } Are keywords that specify which data modification statements,

when attempted against this tableor view, activate the trigger.At least one option must be specified.


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deleted and inserted are logical(conceptual) tables. They are

structurally similar to the table on whichthe trigger is defined, that is, the tableon which the user action is attempted,

and hold the old values or new valuesof the rows that may be changed by theuser action.


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Example

CREATE TRIGGER check_salON EmployeeFOR INSERT, UPDATEASBEGIN

DECLARE @emp_sal FLOAT,@mgr_sal FLOAT

SELECT @emp_sal = salary

FROM Inserted

SELECT @mgr_sal = E.salaryFROM Inserted I, Dept D, Employee EWHERE I.dno = D.dno AND D.mgr = E.nic

IF @emp_sal > @mgr_sal

ROLLBACK TRANSACTIONEND


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Summary

Views

Transaction Basics

T-SQL extensions

Stored Procedures

Triggers

Documents

Lecture 5hjjkujku