lecture 6 - urząd miasta Łodzimarmaj.math.uni.lodz.pl/itd/lecture 6.pdf · a stored procedure is...

Introduction to T-SQL Stored procedures Functions Triggers

Lecture 6

Stored procedures, functions and triggers

January 5, 2015


Conditional statements I

SQL Server provides two conditional statements: if..else and

case.

IF..ELSE statement:

IF boolean_expression

{statement1}

[

ELSE

{statement2}

]

It is also possible to nest one if..else statement inside of another.


Conditional statements II

Example (Sample if..else statements.)

DECLARE @quarter int;

SELECT @quarter = DATEPART(quarter,GETDATE())

IF(@quarter=1)

PRINT 'The first quarter'

ELSE

IF(@quarter=2)

PRINT 'The second quarter'

ELSE

IF(@quarter=3)

PRINT 'The third quarter'

ELSE

PRINT 'The fourth quarter';


Conditional statements III

The case statements has two forms. The �rst forms is as follows:

CASE input_expression

WHEN expr1 THEN result1

...

WHEN exprN THEN resultN

[

ELSE else_result

]

END

Where:

input_expression is any valid Microsoft SQL Server expression

expr1,...,exprN are the value in which the input_expression

is compared,


Conditional statements IV

result1,...resultN are the value that will be return for the CASE

statement if the expr1,...,exprN evaluates to true, respectively,

else_result is the value that will be returned if none of

result1,...resultN evaluates to true

Example (Sample case statement � the �rst form.)

select first_name, last_name, case dept_id

WHEN 1 THEN 'She/he works at IT department'

WHEN 2 THEN 'She/he works at HR department'

WHEN 3 THEN 'She/he works at Marketing department'

ELSE 'No information'

END

as 'Working place'

from Employment join Employee

on Employee.emp_id=Employment.emp_id


Conditional statements V

The second form of the case statement is as follows:

CASE

WHEN boolean_expr1 THEN result1

...

WHEN boolean_exprN THEN resultN

[

ELSE expr

]

END

Where:

boolean_expr1,...,boolean_exprN is any Boolean expression

that when it evaluates to true case will return

result1,...,resultN, respectively.


Conditional statements VI

Example (Sample case statement � the second form)

DECLARE @quarter int;

SELECT @quarter = DATEPART(quarter,GETDATE())

SELECT CASE

WHEN @quarter=1 THEN 'The first quarter'

WHEN @quarter=2 THEN 'The second quarter'

WHEN @quarter=3 THEN 'The third quarter'

ELSE 'The fourth quarter'

END


Loops I

A loop enables to execute a block of statements a given number of

times.

WHILE boolean_expression

BEGIN

statements

END

Example (Sample while statement)

declare @counter int=10

while @counter>0

begin

print @counter

set @counter=@counter-1

end


Cursors I

Operations in a relational database act on a complete set of rows.

Some applications cannot always work e�ectively with the entire

result set as a unit. These applications need a mechanism to work

with one row or a small block of rows at a time. Cursors are an

extension to result sets that provide that mechanism.

Cursors extend result processing by:

Allowing positioning at speci�c rows of the result set.

Retrieving one row or block of rows from the current position

in the result set.

Supporting data modi�cations to the rows at the current

position in the result set.


Cursors II

Supporting di�erent levels of visibility to changes made by

other users to the database data that is presented in the result

set.

Providing Transact-SQL statements in scripts, stored

procedures, and triggers access to the data in a result set.

The basic syntax to declare a cursor is as follows:

DECLARE cursor_name CURSOR

FOR select_statement


Cursors III

Cursors are examined by means of the following statements:

OPEN cursor_name � it opens a cursor

FETCH [ NEXT | PRIOR | FIRST | LAST ] FROM

cursor_name INTO @variable_name � it takes a speci�c

row from a cursor. NEXT returns the row which is after the

current row (it retrieves the �rst row for the �rst time). NEXT

is the default. PRIOR retrieves the row which is prior to the

current row. FIRST and LAST return the �rst and last row,

correspondingly.

@@FETCH_STATUS - it returns 0 if when FETCH is successful,

otherwise returns -1. This variable allows to control a cursor

inside the WHILE statement.

CLOSE cursor_name � it closes a cursor.

DEALOCATE cursor_name � it deletes a cursor reference.


Cursors IV

Example

DECLARE EmpDept_Cursor CURSOR

FOR SELECT e.emp_id,em.salary,d.name

FROM Employee e JOIN Employment em ON

e.emp_id=em.emp_id JOIN Department d ON

em.dept_id=d.dept_id

WHERE em.end_date IS NULL;

OPEN EmpDept_Cursor;

FETCH NEXT FROM EmpDept_Cursor;

WHILE @@FETCH_STATUS = 0

BEGIN

FETCH NEXT FROM EmpDept_Cursor;

END;

CLOSE EmpDept_Cursor;

DEALLOCATE EmpDept_Cursor;


A stored procedure is a sequence of T-SQL statements kept on the

server. SQL Server compiles the procedure code when it is invoked

for the �rst time. After compiling, an optimal execution plan for

the procedure is stored in the cache.

There are several bene�ts of using procedures:

Increasing security (users can execute procedures which

operate on database objects to which they have no access).

Reusing the same code by many users or applications.

Increasing performance (subsequent procedure calls are

performed on the basis of the execution plan which resides in

the cache).


The basic syntax of the CREATE PROCEDURE statement is as follows:

CREATE {PROC|PROCEDURE} procedure_name

[@parameter1 data_type, @parameter2 data_type,... ]

AS sql_statements

The ALTER PROCEDURE and DROP PROCEDURE statements allow to

modify and remove procedures, correspondingly.

ALTER {PROC|PROCEDURE} procedure_name

[@parameter1 data_type, @parameter2 data_type,...]

AS sql_statements

DROP {PROC|PROCEDURE} procedure_name


Example (Creating a procedure without parameters)

CREATE PROCEDURE sp_current_employees

AS SELECT e.emp_id, e.first_name, e.last_name


e.emp_id=em.emp_id

WHERE em.end_date IS NULL

Example (Invoking the procedure)

EXEC sp_current_employees


System stored procedures I

SQL Server provides many system stored procedures. They are

mainly applied for administrative purposes.

Example (Invoking the system procedures)

EXEC sp_who;

returns information about current users, sessions, and processes in

a server instance

EXEC sys.sp_helpdb;

provides information about all databases


Procedures with parameters I

Procedures can take two kind of parameters:

input parameters - they allows to supply some values to the

procedure.

output parameters - they allow to return some values to the

procedure's caller.


Procedures with parameters II

Example (Creating and invoking a procedure with an input

parameter.)

CREATE PROCEDURE GetEmpsFromDept

@dept varchar(40)

AS

BEGIN

SELECT e.emp_id, e.first_name, e.last_name,

em.salary


e.emp_id=em.emp_id

JOIN Department d ON em.dept_id=d.dept_id

WHERE em.end_date IS NULL AND d.name= @dept;

END;

EXEC GetEmpsFromDept 'IT'


Example (Creating and invoking a procedure with an input and

output parameter.)

CREATE PROCEDURE GetWorkdaysByID

@id int, @workdays int OUTPUT

AS

BEGIN

SELECT @workdays =

DATEDIFF(day,start_date,GETDATE())

FROM Employment

WHERE emp_id = @id AND end_date IS NULL

END;

DECLARE @id int = 1;

DECLARE @workdays int;

EXEC GetWorkdaysByID @id, @workdays OUTPUT;

SELECT @workdays;


A function is a sequence of T-SQL statements kept on the server.

It can take parameters and must return some value. SQL Server

provides two types of user-de�ned functions:

scalar functions they return values of scalar data types (e.g.

integers, strings),

table-valued functions they return tables (e.g. the result of

some select statement).


Scalar functions I

The basic syntax is as follows:

CREATE FUNCTION function_name

([@parameter1 data_type, @parameter2 data_type,...])

RETURNS data_type

[AS]

BEGIN

T-SQL_statements

RETURN scalar_expression

END

The ALTER FUNCTION and DROP FUNCTION statements allow to

modify and remove functions.


Scalar functions II

Example (Sample scalar function which takes one parameter.)

CREATE FUNCTION GetNumberofEmps(@deptName varchar(30))

RETURNS INT

BEGIN

DECLARE @number int;

SELECT @number = COUNT(*)

FROM Employment em JOIN Department d

ON em.dept_id=d.dept_id

WHERE d.name=@deptName;

RETURN @number;

END

�Invoking the function

SELECT dbo.GetNumberofEmps('HR')


Table-valued functions I

There are two types of these functions:

inline table-valued functions (They involve only the RETURN

clause followed by a single select statement).

multi-statement table-valued functions (They specify the

structure of table variables which are returned by them. Their

body can involve a sequence of T-SQL statements.)


Table-valued functions II

Inline table-valued functions




RETURNS TABLE

[AS]

RETURN

select_statement


Table-valued functions III

Example (Sample inline table-valued function with one parameter.)

CREATE FUNCTION GetDeptsByAvgSal(@avg_salary money)

RETURNS TABLE

AS

RETURN

SELECT name

FROM Department d JOIN Employment em ON

ON d.dept_id=em.dept_id

WHERE em.end_date IS NULL

GROUP BY name

HAVING AVG(em.salary)>@avg_salary


SELECT * FROM GetDeptsByAvgSal(2000)


Table-valued functions IV

Multi-statement table-valued functions




RETURNS @table_variable TABLE table_defintion

[AS]

BEGIN

T-SQL statements

RETURN

END


Table-valued functions V

Example (Sample multi-statement table-valued function with one

parameter.)

CREATE FUNCTION GetEmpsSalByDept(@deptName varchar(30))

RETURNS @emps TABLE

(emp_id int,

first_name varchar(30),

last_name varchar(40),

salary money)

AS

BEGIN

INSERT INTO @emps

SELECT e.emp_id,first_name,last_name,salary

FROM Employee e JOIN Employment em ON e.emp_id=em.emp_id

JOIN Department d ON em.dept_id=d.dept_id

WHERE d.name=@deptName;

RETURN

END


SELECT * FROM GetEmpsSalByDept('IT')


A trigger is a kind of stored procedure which SQL Server runs

automatically when the proper even occurs. SQL provides three

types of triggers:

DML triggers - they are run when users perform DML

statements (INSERT, UPDATE, DELETE) on tables or views.

DDL triggers - they are run when users perform various DDL

statements (e.g. CREATE, ALTER, DROP)

Logon triggers - they are run when users logon to the SQL

Server instance to establish a session.


DML triggers I

There are types of DML triggers:

AFTER triggers - they are de�ned only on tables. SQL Server

runs such a trigger when the proper DML statement has

performed successfully.

INSTED OF triggers - they can be de�ned both tables and on

views. SQL Server runs such a trigger instead of the proper

DML statement.

The syntax is as follows:

CREATE TRIGGER trigger_name

ON {table_name|view_name}

{AFTER|INSTEAD OF}

{[INSERT] [ , ] [UPDATE] [ , ] [DELETE]}

AS T-SQL_statements


DML triggers II

There are two special tables which can be used inside DML

triggers: INSERTED and DELETED. They are automatically managed

by SQL Server. If a trigger runs in respond to an INSERT statement

the INSERTED table stores copies of rows which were added to the

trigger table. In the case of an UPDATE statements the INSERTED

table stores copies of rows introduced by this statement. The

DELETED table stores the old rows. If a trigger runs in respond to a

DELETE statement the DELETED table stores the rows which were

removed from the trigger table.


DML triggers III

Example (Using a DML trigger to ensure that only adults people

can work in the company.)

CREATE TRIGGER TR_Adults_Emps

ON Employee

AFTER INSERT AS BEGIN

IF EXISTS(SELECT 1

FROM inserted AS i

WHERE DATEDIFF(YEAR,i.birth_date,GETDATE())<18)

BEGIN

PRINT 'An employee must be adult person';

ROLLBACK;

END;

END;


DML triggers IV

Example (Test the trigger)

insert into Employee

values('Martin','Cain','08.08.2000','M','MCAIN',null,null)


Logon triggers I


CREATE TRIGGER nazwa_wyzwalacza

ON ALL SERVER

AFTER LOGON AS

T-SQL_statements


Logon triggers II

Example (Using a logon trigger to prevent logon as Martin.)

CREATE TRIGGER trgControlLogin

ON ALL SERVER

FOR LOGON AS

BEGIN

IF

(ORIGINAL_LOGIN()= 'Martin'

AND APP_NAME() = 'Microsoft SQL Server Management

Studio')

ROLLBACK;

END;


DDL triggers I


CREATE TRIGGER trigger_name

ON {ALL SERVER | DATABASE}

{AFTER} {event_type} [ ,...n ]

AS

T-SQL_statements

ALL SERVER - the trigger scope involves the current instance.

DATABASE - the trigger scope involves the current database.

event_type - it de�nes DDL events (e.g. CREATE TABLE, DROP

TABLE, ALTER TABLE, CREATE PROCEDURE,ALTER PROCEDURE,

CREATE INDEX, ALTER INDEX, etc.


DDL triggers II

Example (Using a DDL trigger to prevent modi�cation or deletion

of tables.)

CREATE TRIGGER trgPreventDDL_AD

ON DATABASE

FOR ALTER_TABLE, DROP_TABLE

AS

BEGIN

PRINT 'Modification or deletion is not allowed';

ROLLBACK;

END;


Thank you for your attention!

lecture 6 - urząd miasta Łodzimarmaj.math.uni.lodz.pl/itd/lecture 6.pdf · a stored procedure is...

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