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Using stored procedures

to querySQL Server databases

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CREATE Procedure

PROCEDURE_NAME

@PARAMETER_NAME DATA_TYPE

=

DefaultValue

AS

SQL

_commands

Procedure definition:

Procedure call/execution:

Option 1: Providing the parameter value

Exec

PROCEDURE_NAME

Parameter_Value

Option

2:

Parameter left out

-

> the implicit value is used

Exec

PROCEDURE_NAME

Exec

PROCEDURE_NAME

default

If no value is explicitly specified for the input parameter when

the procedure is executed then the default value will be used.

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Optional parameters in proceduresExamples:

Procedure definition:

Procedure execution:

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General rules:

Parameters must be separated by commas ( , );

Optional parameters (with specified default values)may be mixed with the mandatory ones (with nodefault values);

At execution time, parameter order is relevant only incases when values alone are specified while parameternames are left out;

When optional parameters (with default values)precede the mandatory ones, in order to use theirdefault values all other parameters (required) must bespecified according to the syntax:

@ParameterName = Value

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Multiple parameters in proceduresExamples:

Procedure definition:

Procedure execution:

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The alternative structure: IF ELSEIF

CONDITION_Statement

BEGIN

SQL

_Commands

END

[

ELSE

BEGIN

SQL

_Commands

END

]

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The repetitive structure: WHILE

WHILECONDITION_StatementBEGIN

SQL_Commands

BREAK

SQL_Commands

END

BREAKcauses an immediate exit from the loop; execution continueswith the first line of code which follows after END

DECLARE @v int=1

WHILE@v

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A transaction consists of a single instructionor an assembly of instructions acting as asingle processing unit: either all operationsare executed or none is, thus ensuring data

coherence.Purpose: transactions should preventactions leading to data errors.

Operations belonging to the sametransaction cannot be partly executed:if one of them fails than all operationsare canceled.

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Ending transactions:by Committing when all constituentoperations were successfully performed -> theirconsequences finally become apparent in thedatabase.

by Rolling Back when all constituentoperations are canceled due to an exception or a

specified constraint and the database reverts tothe same state that preceded transactionexecution.

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Transactions are mainly used for update actionsperformed by stored procedures;

To check whether previously executed

statements resulted in errors, @@Errorsystemfunction can be used in logical tests controllingalternative structures;

To get the value generated for autonumber(identity) fields, Scope_Identity() systemfunction should be used.

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@@ROWCOUNT - system function -> returns the number of

records affected by the previously executed query

@@TRANCOUNT - system function -> returns the numberof uncommitted transactions for the current connection

@@ERROR - system function -> Returns the error numberfor the previously executed SQL statement

ROLLBACK TRANSACTION | COMMIT TRANSACTION> endtransactions by canceling /committing

RAISERROR(message to display, severity level, identification code)

Parameters:

1: User-defined error message

2: Number 0 - 25 (19 for fatal errors)3: Number 0 - 255 used when debu in SQL code

T i i d E l

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Stored

procedure to

remove a

certain

examination

and transfer

associated

marks (if

any) to

anotherexisting

examination:

Transactions in proceduresExample:

T E

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Stored procedure

to remove a

certain exam and

transferassociated marks

(if any) to a new

exam that must

be generated by

the system:

Prerequisites:1. The primary key

in Examtable

(ExamId) is an

identity field(autonumber)

2. Cascade delete

enabled for

relationships

involving Exam

as parent table

Transact ons n proce uresExamp e:

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Useful functions :

ERROR_NUMBER()returns the error number

ERROR_LINE()returns the number of the code line

where the error has occurredERROR_SEVERITYreturns a number smaller than 25corresponding to error severity

ERROR_MESSAGE()returns the message describing

the error

BEGIN TRY

SQL-commands

END TRY

BEGIN CATCH

Code executed when errors occur (messages to display etc.)

END CATCH

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BEGIN TRYBEGIN TRANSACTION -- a transaction is started

-- INSERT, UPDATE, DELETE statements follow:

-- If preceding commands were successful:COMMIT -- the transaction is completed

-- results are saved to the databaseEND TRY

BEGIN CATCH-- Error messages may be displayed here

-- the uncommitted transaction is canceled:

IF @@TRANCOUNT > 0 ROLLBACKEND CATCH