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DDL Triggers in sql server

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In this video we will discuss DDL Triggers in sql server.



In SQL Server there are 4 types of triggers
1. DML Triggers - Data Manipulation Language. Discussed in Parts 43 to 47 of SQL Server Tutorial.
2. DDL Triggers - Data Definition Language
3. CLR triggers - Common Language Runtime
4. Logon triggers

What are DDL triggers
DDL triggers fire in response to DDL events - CREATE, ALTER, and DROP (Table, Function, Index, Stored Procedure etc...). For the list of all DDL events please visit https://msdn.microsoft.com/en-us/library/bb522542.aspx

Certain system stored procedures that perform DDL-like operations can also fire DDL triggers. Example - sp_rename system stored procedure

What is the use of DDL triggers
  • If you want to execute some code in response to a specific DDL event
  • To prevent certain changes to your database schema
  • Audit the changes that the users are making to the database structure
Syntax for creating DDL trigger
CREATE TRIGGER [Trigger_Name]
ON [Scope (Server|Database)]
FOR [EventType1, EventType2, EventType3, ...],
AS
BEGIN
   -- Trigger Body
END

DDL triggers scope : DDL triggers can be created in a specific database or at the server level. 

The following trigger will fire in response to CREATE_TABLE DDL event. 
CREATE TRIGGER trMyFirstTrigger
ON Database
FOR CREATE_TABLE
AS
BEGIN
   Print 'New table created'
END

To check if the trigger has been created
  1. In the Object Explorer window, expand the SampleDB database by clicking on the plus symbol.
  2. Expand Programmability folder
  3. Expand Database Triggers folder
find ddl triggers sql server

Please note : If you can't find the trigger that you just created, make sure to refresh the Database Triggers folder.

When you execute the following code to create the table, the trigger will automatically fire and will print the message - New table created
Create Table Test (Id int)

The above trigger will be fired only for one DDL event CREATE_TABLE. If you want this trigger to be fired for multiple events, for example when you alter or drop a table, then separate the events using a comma as shown below.

ALTER TRIGGER trMyFirstTrigger
ON Database
FOR CREATE_TABLE, ALTER_TABLE, DROP_TABLE
AS
BEGIN
   Print 'A table has just been created, modified or deleted'
END

Now if you create, alter or drop a table, the trigger will fire automatically and you will get the message - A table has just been created, modified or deleted. 

The 2 DDL triggers above execute some code in response to DDL events

Now let us look at an example of how to prevent users from creating, altering or dropping tables. To do this modify the trigger as shown below.

ALTER TRIGGER trMyFirstTrigger
ON Database
FOR CREATE_TABLE, ALTER_TABLE, DROP_TABLE
AS
BEGIN
   Rollback 
   Print 'You cannot create, alter or drop a table'
END

To be able to create, alter or drop a table, you either have to disable or delete the trigger.

To disable trigger
1. Right click on the trigger in object explorer and select "Disable" from the context menu 
2. You can also disable the trigger using the following T-SQL command
DISABLE TRIGGER trMyFirstTrigger ON DATABASE

To enable trigger
1. Right click on the trigger in object explorer and select "Enable" from the context menu 
2. You can also enable the trigger using the following T-SQL command
ENABLE TRIGGER trMyFirstTrigger ON DATABASE

To drop trigger
1. Right click on the trigger in object explorer and select "Delete" from the context menu 
2. You can also drop the trigger using the following T-SQL command
DROP TRIGGER trMyFirstTrigger ON DATABASE

Certain system stored procedures that perform DDL-like operations can also fire DDL triggers. The following trigger will be fired when ever you rename a database object using sp_rename system stored procedure.

CREATE TRIGGER trRenameTable
ON DATABASE
FOR RENAME
AS
BEGIN
    Print 'You just renamed something'
END

The following code changes the name of the TestTable to NewTestTable. When this code is executed, it will fire the trigger trRenameTable
sp_rename 'TestTable', 'NewTestTable'

The following code changes the name of the Id column in NewTestTable to NewId. When this code is executed, it will fire the trigger trRenameTable
sp_rename 'NewTestTable.Id' , 'NewId', 'column'

2 comments:

  1. I have a table category and a table product. When delete one key in cateogry, i want to delete all item in product have parentid like this key. How can i build tringger for this function?

    Thanks

    ReplyDelete
  2. i have inserted a items in cart and i didn't go for the checkout, i have to delete that item from cart automatically if i quit the shopping and didn't checkout. How cani solve this??

    ReplyDelete

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