SQL Server - Creating Simple Trigger

A trigger is a database object that is attached to a table. In many aspects it is similar to a stored procedure. As a matter of fact, triggers are often referred to as a "special kind of stored procedure." The main difference between a trigger and a stored procedure is that the former is attached to a table and is only fired when an INSERT, UPDATE or DELETE occurs. You specify the modification action(s) that fire the trigger when it is created.

Here are some trigger design tips to keep in mind:

Avoid using nested triggers
By default, if a trigger is changing other tables, the triggers declared for these tables are not fired. The "allow nested triggers" server option sets databases to have the opposite behavior. Triggers are nested when a trigger performs an action that initiates another trigger, which can initiate another trigger and so on. Triggers can be nested up to 32 levels. It is very difficult to follow the logic of nested triggers and they can affect performance.

Avoid using recursive triggers

There are two types of recursion:

Direct recursion occurs when a trigger fires and performs an action that causes the same trigger to fire again. You can prevent that from happening by setting the "recursive trigger" database option to OFF.

Indirect recursion occurs when a trigger fires and performs an action that causes a trigger on another table to fire. This second trigger causes an update to occur on the original table, which causes the original trigger to fire again. This can be prevented with the "nested triggers" server option.

The following shows how to create a trigger that displays the current system time when a row is inserted into the table to which it is attached.

SET NOCOUNT ON

CREATE TABLE Source (Sou_ID int IDENTITY, Sou_Desc varchar(10))
go
CREATE TRIGGER tr_Source_INSERT
ON Source
FOR INSERT
AS
PRINT GETDATE()
go
INSERT Source (Sou_Desc) VALUES ('Test 1')

-- Results --

Apr 28 2001  9:56AM

 
FOR MORE FOLLOW THESE LINKS-http://msdn.microsoft.com/en-us/library/ms189799.aspxhttp://www.blurtit.com/q654983.html

What is a Trigger

A trigger is a special kind of a store procedure that executes in response to certain action on the table like insertion, deletion or updation of data. It is a database object which is bound to a table and is executed automatically. You can’t explicitly invoke triggers. The only way to do this is by performing the required action no the table that they are assigned to.

Types Of Triggers

There are three action query types that you use in SQL which are INSERT, UPDATE and DELETE. So, there are three types of triggers and hybrids that come from mixing and matching the events and timings that fire them.

Basically, triggers are classified into two main types:-

(i) After Triggers (For Triggers)
(ii) Instead Of Triggers

(i) After Triggers

These triggers run after an insert, update or delete on a table. They are not supported for views.
AFTER TRIGGERS can be classified further into three types as:

(a) AFTER INSERT Trigger.
(b) AFTER UPDATE Trigger.
(c) AFTER DELETE Trigger.

(ii) Instead Of Triggers

These can be used as an interceptor for anything that anyonr tried to do on our table or view. If you define an Instead Of trigger on a table for the Delete operation, they try to delete rows, and they will not actually get deleted (unless you issue another delete instruction from within the trigger)
INSTEAD OF TRIGGERS can be classified further into three types as:-

(a) INSTEAD OF INSERT Trigger.
(b) INSTEAD OF UPDATE Trigger.
(c) INSTEAD OF DELETE Trigger.

The following shows how to create a trigger that displays the current system time when a row is inserted into the table to which it is attached.

SET NOCOUNT ON

CREATE TABLE Source (Sou_ID int IDENTITY, Sou_Desc varchar(10))
go
CREATE TRIGGER tr_Source_INSERT
ON Source
FOR INSERT
AS
PRINT GETDATE()
go
INSERT Source (Sou_Desc) VALUES ('Test 1')

-- Results --

Apr 28 2001  9:56AM


FOR MORE FOLLOW THESE LINKS-http://msdn.microsoft.com/en-us/library/ms189799.aspxhttp://www.blurtit.com/q654983.html

Hello,

The following shows how to create a trigger that displays the current system time when a row is inserted into the table to which it is attached.

SET NOCOUNT ON

CREATE TABLE Source (Sou_ID int IDENTITY, Sou_Desc varchar(10))

go

CREATE TRIGGER tr_Source_INSERT

ON Source

FOR INSERT

AS

PRINT GETDATE()

go

INSERT Source (Sou_Desc) VALUES ('Test 1')

-- Results --

--Apr 28 2001  9:56AM

Hope this is helpful !

Thanks

Creates a trigger, which is a special kind of stored procedure that executes automatically when a user attempts the specified data-modification statement on the specified table. Microsoft® SQL Server™ allows the creation of multiple triggers for any given INSERT, UPDATE, or DELETE statement.

Syntax

CREATE TRIGGER trigger_name
ON { table | view }
[ WITH ENCRYPTION ]
{
  { { FOR | AFTER | INSTEAD OF } { [ INSERT ] [ , ] [ UPDATE ] [ , ] [ DELETE ] }
    [ WITH APPEND ]
    [ NOT FOR REPLICATION ]
    AS
    [ { IF UPDATE ( column )
      [ { AND | OR } UPDATE ( column ) ]
        [ ...n ]
    | IF ( COLUMNS_UPDATED ( ) { bitwise_operator } updated_bitmask )
        { comparison_operator } column_bitmask [ ...n ]
    } ]
    sql_statement [ ...n ]
  }
}

Arguments

trigger_name

Is the name of the trigger. A trigger name must conform to the rules for identifiers and must be unique within the database. Specifying the trigger owner name is optional.

Table | view

Is the table or view on which the trigger is executed and is sometimes called the trigger table or trigger view. Specifying the owner name of the table or view is optional. A view can be referenced only by an INSTEAD OF trigger.

WITH ENCRYPTION

Indicates that SQL Server will convert the original text of the CREATE TRIGGER statement to an obfuscated format. Note that obfuscated triggers can be reverse engineered because SQL Server must de-obfuscate triggers for execution. In SQL Server 2000, the obfuscated text is visible in the syscomments system table and may be susceptible to de-obfuscation attempts.

Using WITH ENCRYPTION prevents the trigger from being published as part of SQL Server replication.

AFTER

Specifies that the trigger is fired only when all operations specified in the triggering SQL statement have executed successfully. All referential cascade actions and constraint checks also must succeed before this trigger executes.

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 of the triggering SQL statement, thus overriding the actions of the triggering statements.

At most, one INSTEAD OF trigger per INSERT, UPDATE, or DELETE statement can be defined on a table or view. However, it is possible to define views on views where each view has its own INSTEAD OF trigger.

INSTEAD OF triggers are not allowed on updateable views WITH CHECK OPTION. SQL Server will raise an error if an INSTEAD OF trigger is added to an updateable view WITH CHECK OPTION specified. The user must remove that option using ALTER VIEW before defining the INSTEAD OF trigger.

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

Are keywords that specify which data modification statements, when attempted against this table or view, activate the trigger. At least one option must be specified. Any combination of these in any order is allowed in the trigger definition. If more than one option is specified, separate the options with commas.

For INSTEAD OF triggers, the DELETE option is not allowed on tables that have a referential relationship specifying a cascade action ON DELETE. Similarly, the UPDATE option is not allowed on tables that have a referential relationship specifying a cascade action ON UPDATE.

WITH APPEND

Specifies that an additional trigger of an existing type should be added. Use of this optional clause is needed only when the compatibility level is 65 or lower. If the compatibility level is 70 or higher, the WITH APPEND clause is not needed to add an additional trigger of an existing type (this is the default behavior of CREATE TRIGGER with the compatibility level setting of 70 or higher.) For more information, see sp_dbcmptlevel.

WITH APPEND cannot be used with INSTEAD OF triggers or if AFTER trigger is explicitly stated. WITH APPEND can be used only when FOR is specified (without INSTEAD OF or AFTER) for backward compatibility reasons. WITH APPEND and FOR (which is interpreted as AFTER) will not be supported in future releases.

NOT FOR REPLICATION

Indicates that the trigger should not be executed when a replication process modifies the table involved in the trigger.

AS

Are the actions the trigger is to perform.

sql_statement

Is the trigger condition(s) and action(s). Trigger conditions specify additional criteria that determine whether the attempted DELETE, INSERT, or UPDATE statements cause the trigger action(s) to be carried out.

The trigger actions specified in the Transact-SQL statements go into effect when the DELETE, INSERT, or UPDATE operation is attempted.

Triggers can include any number and kind of Transact-SQL statements. A trigger is designed to check or change data based on a data modification statement; it should not return data to the user. The Transact-SQL statements in a trigger often include control-of-flow language. A few special tables are used in CREATE TRIGGER statements:

• deleted and inserted are logical (conceptual) tables. They are structurally similar to the table on which the trigger is defined, that is, the table on which the user action is attempted, and hold the old values or new values of the rows that may be changed by the user action. For example, to retrieve all values in the deleted table, use:

  SELECT * FROM deleted 

• In a DELETE, INSERT, or UPDATE trigger, SQL Server does not allow text, ntext, or image column references in the inserted and deleted tables if the compatibility level is equal to 70. The text, ntext, and image values in the inserted and deleted tables cannot be accessed. To retrieve the new value in either an INSERT or UPDATE trigger, join the inserted table with the original update table. When the compatibility level is 65 or lower, null values are returned for inserted or deleted text, ntext, or image columns that allow null values; zero-length strings are returned if the columns are not nullable.

  If the compatibility level is 80 or higher, SQL Server allows the update of text, ntext, or image columns through the INSTEAD OF trigger on tables or views.

n

Is a placeholder indicating that multiple Transact-SQL statements can be included in the trigger. For the IF UPDATE (column) statement, multiple columns can be included by repeating the UPDATE (column) clause.

IF UPDATE (column)

Tests for an INSERT or UPDATE action to a specified column and is not used with DELETE operations. More than one column can be specified. Because the table name is specified in the ON clause, do not include the table name before the column name in an IF UPDATE clause. To test for an INSERT or UPDATE action for more than one column, specify a separate UPDATE(column) clause following the first one. IF UPDATE will return the TRUE value in INSERT actions because the columns have either explicit values or implicit (NULL) values inserted.

Note  The IF UPDATE (column) clause functions identically to an IF, IF...ELSE or WHILE statement and can use the BEGIN...END block. For more information, see http://msdn.microsoft.com/en-us/library/aa226017%28v=sql.80%29.aspx.

UPDATE(column) can be used anywhere inside the body of the trigger.

column

Is the name of the column to test for either an INSERT or UPDATE action. This column can be of any data type supported by SQL Server. However, computed columns cannot be used in this context. For more information, see http://msdn.microsoft.com/en-us/library/aa258271%28v=sql.80%29.aspx.

IF (COLUMNS_UPDATED())

Tests, in an INSERT or UPDATE trigger only, whether the mentioned column or columns were inserted or updated. COLUMNS_UPDATED returns a varbinary bit pattern that indicates which columns in the table were inserted or updated.

The COLUMNS_UPDATED function returns the bits in order from left to right, with the least significant bit being the leftmost. The leftmost bit represents the first column in the table; the next bit to the right represents the second column, and so on. COLUMNS_UPDATED returns multiple bytes if the table on which the trigger is created contains more than 8 columns, with the least significant byte being the leftmost. COLUMNS_UPDATED will return the TRUE value for all columns in INSERT actions because the columns have either explicit values or implicit (NULL) values inserted.

COLUMNS_UPDATED can be used anywhere inside the body of the trigger.

bitwise_operator

Is the bitwise operator to use in the comparison.

updated_bitmask

Is the integer bitmask of those columns actually updated or inserted. For example, table t1 contains columns C1, C2, C3, C4, and C5. To check whether columns C2, C3, and C4 are all updated (with table t1 having an UPDATE trigger), specify a value of 14. To check whether only column C2 is updated, specify a value of 2.

comparison_operator

Is the comparison operator. Use the equal sign (=) to check whether all columns specified in updated_bitmask are actually updated. Use the greater than symbol (>) to check whether any or some of the columns specified in updated_bitmask are updated.

column_bitmask

Is the integer bitmask of those columns to check whether they are updated or inserted.