Commonly used statements of SQL triggers (summary)

This article summarizes some commonly used SQL trigger statements for everyone. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to everyone.

1. Create a simple trigger

CREATE TRIGGER 触发器名称
ON 表名
FOR INSERT、UPDATE 或 DELETE
AS
    T-SQL 语句

Note: The trigger name is not quoted.

2. Delete triggers

drop trigger 触发器名称

Delete multiple triggers:

drop trigger 触发器名称,触发器名称

3. Rename triggers

Rename with Query Analyzer

exec sp_rename 原名称, 新名称

sp_rename is a stored procedure that comes with SQL Server. It is used to change the names of user-created objects in the current database, such as table names, lists, index names, etc. .

4. INSERT, UPDATE or DELETE

INSTEAD OF

Execute the trigger statement, but do not execute the SQL statement that triggers the trigger, such as trying to When a record is deleted, the statement specified by the trigger will be executed, and the delete statement will no longer be executed at this time. Example:

create trigger f
on tbl
instead of delete
as
    insert into Logs...

IF UPDATE(column name)

Check whether a column has been updated. It is used for insert or update, but cannot be used for delete. Example:

create trigger f
on tbl
for update
as
    if update(status) or update(title)
        sql_statement --更新了 status 或 title 列

inserted, deleted

These are two virtual tables. inserted saves the table formed by the records affected after insert or update, and deleted saves the table formed before delete or update. The table formed by the affected records. Example:

create trigger tbl_delete
on tbl
for delete
as
    declare @title varchar(200)
    select @title=title from deleted
    insert into Logs(logContent) values('删除了 title 为：' + title + '的记录')

Explanation: If the field value of the field type text or image is retrieved from the inserted or deleted virtual table, the obtained value will be null.

5. View all triggers in the database

use 数据库名
go
select * from sysobjects where xtype='TR'
sysobjects 保存着数据库的对象，其中 xtype 为 TR 的记录即为触发器对象。在 name 一列，我们可以看到触发器名称。

6. sp_helptext View trigger content

use 数据库名
go
exec sp_helptext '触发器名称'

7. sp_helptrigger is used to view the attributes of the trigger.

sp_helptrigger has two parameters: the first parameter is the table name; the second parameter is the trigger type, which is char(6) Type, which can be INSERT, UPDATE, or DELETE. If omitted, the properties of all types of triggers in the specified table will be displayed.

Example:

use 数据库名
go
exec sp_helptrigger tbl

8. Recursion, nested triggers

There are two types of recursion, indirect recursion and direct recursion. We explain it with an example as follows. Suppose there are tables 1 and 2 named T1 and T2 respectively, and there are flip-flops G1 and G2 on T1 and T2 respectively.

Indirect recursion: operate on T1 to trigger G1, G1 to operate on T2 to trigger G2, G2 to operate on T1 to trigger G1 again...

Direct recursion: operate on T1 to trigger G1 , G1 operates on T1 to trigger G1 again...

Nested trigger

is similar to indirect recursion. Indirect recursion must form a loop, while nested triggers do not necessarily have to form a loop. A ring can be triggered continuously from T1->T2->T3... in this way, allowing up to 32 levels of nesting.

Set direct recursion

By default, direct recursion is prohibited. There are two methods to set it to allow:

T-SQL: exec sp_dboption 'dbName', ' recursive triggers', true

EM: Right click on the database->Properties->Options.

Set indirect recursion and nesting

By default, indirect recursion and nesting are allowed. There are two ways to disable it:

T-SQL: exec sp_configure 'nested triggers', 0 --The second parameter is 1, which means it is allowed

EM: Right click on the registration->Properties->Server Settings.

9. Trigger rollback

We have seen that many registration systems cannot change the user name after registration, but this is mostly determined by the application. If By directly opening the database table to make changes, you can also change its user name. By using rollback in the trigger, you can cleverly realize that the user name cannot be changed.

use 数据库名
go
create trigger tr
on 表名
for update
as
    if update(userName)
        rollback tran

The key lies in the last two sentences, which are explained as follows: If the userName column is updated, roll back the transaction.

10. Disable and enable triggers

Disable: alter table table name disable trigger trigger name

Enable: alter table table name enable trigger Trigger name

If there are multiple triggers, separate the trigger names with commas.

If you replace "Trigger Name" with "ALL", it means disabling or enabling all triggers of the table.

11. Determine insertion, modification and deletion

create TRIGGER [updatetest] ON [dbo].[TEST] 
FOR INSERT, UPDATE, DELETE 
AS
begin
    DECLARE
    @IsInsert bit,
    @IsUpdate bit,
    @IsDelete bit
    IF EXISTS(SELECT 1 FROM inserted) AND NOT EXISTS(SELECT 1 FROM deleted)
        SET @IsInsert = 1
    ELSE
        SET @IsInsert = 0
     
    IF EXISTS(SELECT 1 FROM inserted) AND EXISTS(SELECT 1 FROM deleted)
        SET @IsUpdate = 1
    ELSE
        SET @IsUpdate = 0
     
    IF NOT EXISTS(SELECT 1 FROM inserted) AND EXISTS(SELECT 1 FROM deleted)
        SET @IsDelete = 1
    ELSE
        SET @IsDelete = 0 
     
    if (@IsUpdate=1)
       PRINT 'updated '
     if (@IsInsert=1)
       PRINT 'insert '
     if (@IsDelete=1)
       PRINT 'delete 'end

Recommended learning: MySQL tutorial

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