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Triggers and Transaction Management

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Objective

To be able to learn the implementation of triggers and SQLServer transactions

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Scope

Introduction to Triggers

Creating a trigger

Transaction Management

Locks

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Introduction to Trigger

SQL Server provides a mechanism called a trigger for

enforcing procedural integrity constraints.

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How trigger work

A trigger is a mechanism that is invoked when a particularaction occurs on a particular table. Each trigger has threegeneral parts:

Name

ActionExecution

The action of the trigger either be an INSERT, an UPDATE,or a DELETE statement.

The execution part of the trigger usually contains a storedprocedure or a batch.

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Creating a Trigger

SQL Server 2005 allows us to create trigger using eitherTransact SQL or CLR programming languages such as C#and Visual Basic 2005.

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Creating a Trigger (Transact-SQL)

A trigger is created using the CREATE TRIGGER statement, which hasthe follow form:

Syntax:

CREATE TRIGGER [schema_name.]trigger_nameON table_name | view_name[WITH dml_trigger_option]{FOR | AFTER|INSTEAD OF} { [INSERT][,][UPDATE][,][DELETE]}[WITH APPEND]AS sql_statement | EXTERNAL NAME method_name

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Transaction Management

A transaction is a logical unit of work.

Any single SQL statement is considered to be a single unitof work whether it affects only a single row or multiple rowswithin the table.

In SQL Server a transaction is implicit.

Explicit transactions can be defined by the user by includingthe SQL statements enclosed within BEGIN TRANSACTIONand COMMIT TRANSACTION.

In creating explicitly defined transactions, the following

statement are used.

BEGIN TRANSACTION COMMIT TRANSACTION ROLLBACK TRANSACTION SAVE TRANSACTION

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Database consistency / Concurrency

When two or more users access the database concurrently,SQL Server uses locking to ensure that their actions dontinterfere each other.

Locking prevents one user from reading the data which isbeing changed by the other users.

It also prevents users from making more than one changeto a record at any one time.

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Isolation Levels

SQL Server supports file isolation levels:

READ UNCOMMITEDREAD COMMITEDREPEATABLE READSNAPSHOTSERIALIZABLE

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Deadlock

A deadlock is a special situation in which two transactionsblock the progress of each other.

The first transaction has a lock on some database objectthat the other transaction wants to access and vice versa.

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Summary

Introduction to Triggers

Creating a trigger

Transaction Management

Locks