A trigger is an action that is performed behind-the-scenes when an event occurs on a table or a view.
You create a trigger using SQL code. Later on, we will start analyzing the code necessary to do this. To assist you with skeleton code, open a Query window. Then, in the Template Explorer, expand the Triggers node. Drag Create T-SQL Trigger (New Menu) and drop it in the window:
Code: SQL
or
CREATE TRIGGER <Schema_Name,
sysname,
Schema_Name>.<Trigger_Name,
sysname,
Trigger_Name>
ON <Schema_Name, sysname, Schema_Name>.<Table_Name, sysname, Table_Name>
AFTER <Data_Modification_Statements, , INSERT,DELETE,UPDATE>
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
-- Insert statements for trigger here
END
GO
Unlike a stored procedure, you never have to execute a trigger. The operating system (through the object(s) event(s)) and the database engine take care of this. A trigger acts behind the scenes when the object to which it is associated fires the appropriate event. In fact, the event fires whether the object received a change or not (whether a record was created, edited, deleted, or not).
Microsoft SQL Server supports three types of triggers: DML, DDL, and logon.
A DML trigger is a procedure that acts as a result of a data manipulation language (DML) event occurring on a table. This means that the trigger must be created in connection to a table of a non-system database.
An insert trigger is a DML trigger that acts when a new record is added to its intended table. Such a trigger uses the INSERT keywork. The primary formula to create an INSERT DML trigger on a table is:
CREATE TRIGGER Schema_Name.TriggerName
ON TableName
AFTER/FOR INSERT
AS
TriggerCode
The statement starts with CREATE TRIGGER. If you want to specify a schema, type it, followed by a a period and the name of the trigger. The name follows the rules we have applied so far to database objects.
After specifying the name of the trigger, write ON followed by the name of the table on which the trigger will apply. Of course, the table must exist in the database. If the table belongs to a schema other than dbo, the trigger must use the same schema. Put it another way, if you precede the name of the a trigger withe a schema, the table must belong to the same schema and you must precede the name of the table with the same schema. Of course, you must have previously created the schema and the table must have been assigned to that schema.
In our formula, we assume that the trigger will apply when a record has been added to the table. Therefore, you use either the AFTER INSERT or the FOR INSERT expression.
To start the SQL code that constitutes the trigger, write AS, and then write your code.
After creating an INSERT trigger, at the right time (when its intended event fires), it will execute. When this happens, the database engine automatically and internally creates a temporary table named inserted. This table holds a copy of the records that were created. You can access those records if you want.
Instead of record insertion time, a DML trigger can act when a record has been updated on a table. To support this operation, you can use the following formula:
CREATE TRIGGER TriggerName
ON TableName
AFTER/FOR UPDATE
AS
TriggerCode
The new keyword in this formula is UPDATE. This indicates that the DML trigger will act when the record has been updated. Everything else is as described for the INSERT operator. Remember to use either AFTER UPDATEor FOR UPDATE.
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Practical Learning: Creating a DML Trigger
When a record has been removed from a table, you can apply a DML trigger in response. To make it possible, you can use the following formula:
CREATE TRIGGER TriggerName
ON TableName
AFTER/FOR DELETE
AS
TriggerCode
This time, the formula uses the DELETE operator as in AFTER DELETE or FOR DELETE. The other things follow the same description we saw for the INSERT operator.
When a DELETE trigger has acted on a table, the database engine creates a special temporary table named deleted. This table holds a copy of the records that were deleted. Eventually, if necessary, you can access this table to find out about those records.
A trigger is a database object. As such, it has a name. It can be modified. It can also be deleted.
If the behavior of a trigger is not appropriate, you can change it. The formula to modify a trigger is:
ALTER TRIGGER schema_name.trigger_name
ON schema_name.table_name
AFTER , UPDATE>
AS
statement
To get skeleton code generated for you, open a Query window. In the Templates Explorer, expand the Triggers node, drag the Alter option and drop it in the Query window:
--====================================== -- Alter T-SQL Trigger Template --====================================== USE <database_name, sysname, AdventureWorks> GO ALTER TRIGGER <schema_name, sysname, Sales>.<trigger_name, sysname, uStore> ON <schema_name, sysname, Sales>.<table_name, sysname, Store> AFTER <data_modification_statements, , UPDATE> AS <T-SQL_statement, , UPDATE Sales.Store SET ModifiedDate = GETDATE() FROM inserted WHERE inserted.CustomerID = Sales.Store.CustomerID> GO
If you do not need a trigger anymore, you can remove it. The formula to do this is:
DROP TRIGGER TriggerName
After the DROP TRIGGER expression, enter the name (of the trigger).
As you may know already, Microsoft SQL Server 2008 (unlike some previous versions) requires that a person logs in order to access a database or one of its objects. In the same way, as we mentioned already, when a user uses an object like a table, a view, etc, the database engine must authenticate who the user is, what permissions that user has and on what objects. Transact-SQL allows you to specify an alternate accont under which a trigger must execute. To provide this information, when creating the trigger, after the ON expression and before specifing the type of trigger, type WITH EXECUTE AS followed by the login name of the alternate account you want to use. The formula to follow:
ALTER TRIGGER schema_name.trigger_name
ON schema_name.table_name
WITH EXECUTE AS LoginName
AFTER , UPDATE>
AS
statement
Here us an example:
CREATE TRIGGER Management.WithCustomers
ON Management.Customers
WITH EXECUTE AS N'Pat Kat'
AFTER INSERT
AS
BEGIN
INSERT INTO Management.DatabaseOperations
VALUES(default, N'Customers', SUSER_SNAME(),
N'Processed a deposit', GETDATE());
END
GO
As you can imagine, this solution allows you to let a user with more restrictive permissions to run a trigger on behalf of a user with less restrictive rights.
Although we created only one trigger for a table so far, you can go farther than that:
DML triggers present many other characteristics.
You know that, to assist a user with data entry, you can specify that a column would allow or not allow null values. If a column is marked as NOT NULL, during data entry, if the user does not or cannot provide a value for that column, the record cannot be created. If you create a DML trigger that must act against that table, if the nullity rule is violated, the trigger will not run.
You know that you can create a check constraint on a table to make sure that every new record, or a record that is being edited, follows a certain rule. You know that if the record does not abide by that rule, the record will not be created or changed. If a DML trigger is supposed to act on the table and if this rule is not respected, the trigger would fail.
One of the limitations of a check constraint is that it applies only to the table that owns it. A DML trigger can be created to perform a check constraint on more than one table. This provides its advantage over the normal check constraint.
You probably know already about data relationships and referential integrity. This makes sure that, when a record is edited in a parent table, the change is also made on the child table. This also means that the integrity is applied to more than one table. When a DML trigger runs, if a referential rule is violated, the trigger, which also checks the referential integrity, fails.
Consider the following tables in a database:
CREATE DATABASE SmallBusiness;
GO
USE SmallBusiness;
GO
CREATE TABLE Customers
(
CustomerID int identity(1, 1) primary key not null,
AccountNumber nchar(10),
FullName nvarchar(50)
);
GO
CREATE TABLE Management.DatabaseOperations
(
EmployeeName nvarchar(50),
ActionPerformed nvarchar(50),
TimePerformed time
);
GO
From what we have seen so far, when a user opens a table or a view to perform data entry, when a new record has been created, the table or view fires an event. We saw that, using a DML trigger, you can make a notification. For example, you can fill out a log to keep track of the changes. By default, when a record is submitted, it gets saved. In some cases, when a user has opened a table and tried to make a change, such as adding a new record, editing an existing record, or deleting a record, instead of accepting the change, you can dismiss it. You can then use a DML trigger to make a note. This is the basis of another category of DML triggers: an "instead of" trigger.
While an AFTER/FOR trigger acts on a table after the action has occurred, you may want to do something before the event fires. For example, you may want to prevent the user from adding a new record on a table, or from changing an existing record, or from deleting a record. Of course, it is better to take care of this before the action is performed. One way you can do this is by creating an "instead of" trigger.
While an AFTER trigger can be applied to a table only, an "instead of" trigger can be associated with either a table or a view. Therefore, to create an "instead of" trigger, use the following formula:
CREATE TRIGGER TriggerName
ON TableOrView
INSTEAD OF INSERT/UPDATE/DELETE
AS
TriggerCode
You start with the CREATE TRIGGER expression followed by a name for the trigger. After the name of the trigger, type ON followed by the name of either a table or a view on which the trigger will act.
From our review of the AFTER trigger, the new expression here is INSTEAD OF. This expression is followed by the type of operation to perform:
To start the triggering code, type AS and write the desired code.
If you use the INSTEAD OF expression, the trigger starts when the table or view is opened but before a change has taken place. The difference with the AFTER trigger is that, this time, you can perform some action(s) before the change is made on the table or view. This also implies that, if the code of the trigger is to create a new record, at this time, the record doest not yet exist, which means you cannot catch that record. At this time also, you can prevent the record from being created (since it has not yet been created anyway). For example, the following code will not accept that a new record be added to the table:
USE SmallBusiness;
GO
CREATE TRIGGER CreateCustomer
ON Customers
INSTEAD OF INSERT
AS
BEGIN
INSERT INTO Management.DatabaseOperations
VALUES(SUSER_SNAME(),
N'Attempt to create new record', GETDATE())
END
GO
If you want to get a copy of the record that was affected by the event, you can access it from the inserted (for anINSERT or UPDATE trigger) or from the deleted (for a DELETE) trigger. Here is an example:
USE SmallBusiness; GO DROP TRIGGER CreateCustomer; GO CREATE TRIGGER CreateCustomer ON Customers INSTEAD OF INSERT AS BEGIN INSERT INTO Customers SELECT AccountNumber, FullName FROM inserted END GO
An AFTER/FOR and an INSTEAD OF triggers have many differences. For example:
You probably know that the creation of a database uses a Data Definition Language (DDL) command. You may also know another example of a DDL command that involves creating a table. Each one of these creation operations fires an event.
A DDL trigger is a trigger that acts when a certain type of DDL event fires. These include the creation, modification, and removal of an object, not its records. This is the primary difference with a DML trigger that fires when a record is added or acted upon.
A DDL trigger gives you the opportunity to do some administrative work in response to the event. For example, you can get a notification, or notify someone else using an automatically generated email, that an object (and what object) has been created, or you can use a DDL trigger to discard the operation.
You create a DDL trigger using code. The basic formula is:
CREATE TRIGGER TriggerName
ON DATABASE/ALL SERVER
FOR/AFTER WhatEvent
AS
TriggerCode
You start a DDL trigger with the CREATE TRIGGER expression followed by a name for the new trigger. The name follows the same rules we have applied to objects so far. After the name of the trigger, type the ON keyword:
After specifying the object (the whole server or only the current database) on which the trigger will act, type eitherFOR or AFTER. This is followed by the event against which the trigger will act. As mentioned already, the events are DDL commands. To specify the event, use the formula of the command with the words separated by an underscore. For example, if you want the trigger to act when a CREATE TABLE command is executed, specify the event asCREATE_TABLE.
After specifying the event that will fire, type AS followed by the normal code of the trigger.
Here is an example that makes a note and adds it (the note) to a table when a new table has been created:
USE SmallBusiness;
GO
CREATE TRIGGER LogNewTableCreation
ON DATABASE
FOR CREATE_TABLE
AS
BEGIN
INSERT INTO Management.DatabaseOperations
VALUES(SUSER_SNAME(),
N'A new table was created', GETDATE())
END
GO
Whenever a new table is created in the current database, the trigger runs, gets the name of the user who created the table, the date and time the table was created, and a small message. These pieces of information are then stored in a log table.
As mentioned for DML triggers, you manage DDL triggers by modifying or deleting them. These are done using the same description we saw for DML triggers.
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