TRIGGER in SQL Server (T-SQL)

A trigger is a special kind of a store procedure that fire implicitly when certain event occurs in database server. We can’t invoke triggers explicitly. It can be used to perform administrative tasks in database such as regulating database operation.

Types of TRIGGER:

1. DDL TRIGGER:

DDL triggers execute in response to a variety of Data Definition Language (DDL) events such as CREATE, ALTER, DROP, GRANT, DENY, REVOKE, UPDATE STATISTICS statements and certain system stored procedures that perform DDL like action. DDL triggers do not fire in response to events that affect local or global temporary tables and stored procedures.

• DDL triggers do not support INSTEAD OF feature.
• DDL triggers do not have feature of inserted and deleted tables like DML triggers.

Example :

CREATE TRIGGER PreventDropTable

ON DATABASE            

FOR DROP_TABLE  --DROP_TABLE is DDL Event See link DDL events 
          

AS

PRINT 'Drop table action not allowed.'

ROLLBACK;

GO

Droping table to test trigger action
DROP TABLE [EMPLOYEE]; It will not drop table due to triggering action

We can disable or enable trigger
--Disabling database trigger
DISABLE TRIGGER PreventDropTable ON DATABASE;

--Enabling database trigger

ENABLE TRIGGER PreventDropTable ON DATABASE;

2. DML TRIGGER:

DML trigger is performed on DML operation (INSERT, UPDATE, DELETE) to enforce rule and regulation on table. Trigger can prevent incorrect, unauthorized, inconsistent changes to data. 

A). AFTER TRIGGER (FOR TRIGGER)

After triggers run after INSERT, UPDATE or DELETE operation on table. They do not supported for views. 
AFTER TRIGGERS can be classified further into three types as: 

     i). AFTER INSERT Trigger

This trigger is fired after an INSERT on the table. Let’s create the trigger as:-

--Creating a sample table

CREATE TABLE empProfile

(id INT IDENTITY(1,1),

name VARCHAR(50),

gender VARCHAR(6),

create_date DATETIME,

update_date DATETIME

)

--Creating trigger to automatic update log

CREATE TRIGGER setCreateDate ON [dbo].[empProfile]

FOR  INSERT

AS

      --AUTOMATIC populate create_date

      UPDATE empProfile SET create_date = GETDATE() WHERE id IN (SELECT ID FROM inserted)

      PRINT 'AFTER INSERT trigger fired.'

GO

Inserting value in database to test trigger action

INSERT INTO empProfile (name,gender)

SELECT 'MDF', 'MALE'

UNION ALL

SELECT 'ZTE', 'FEMALE'

UNION ALL

SELECT 'John', 'MALE'

UNION ALL

SELECT 'Abc', 'MALE'

SELECT * FROM empProfile

     ii). AFTER UPDATE Trigger

This trigger is fired after an update on the table.

Example:

--Creating trigger to automatic update log

CREATE TRIGGER setUpdateDate ON [dbo].[empProfile]

FOR  UPDATE

AS

--AUTOMATIC populate create_date

UPDATE empProfile SET update_date = GETDATE() WHERE id IN (SELECT ID FROM inserted)

      PRINT 'AFTER Update trigger fired.'

GO

UPDATE empProfile SET name='XYZ' WHERE id=1

UPDATE empProfile SET name='MDF' WHERE id=3

     iii). AFTER DELETE Trigger

This trigger is fired after a delete on the table. 

Example:

CREATE TRIGGER onDeleteTest ON [dbo].[empProfile]

AFTER  DELETE

AS

      PRINT 'AFTER DELETE trigger fired.'

GO

Testing trigger action

DELETE FROM empProfile WHERE ID=2

A). Instead Of Triggers

Instead of trigger override default action of DML operation on table or view. After defining INSTEAD OF trigger if we want to delete a row then Delete operation trying to delete but actually row will not get deleted unless you issue another delete instruction with in trigger.

Advantage of INSTEAD OF trigger:

• It can enable views that would not be updatable to support updates. A view based on multiple base tables must use an INSTEAD OF trigger to support inserts, updates, and deletes that reference data in more than one table.
• It can enable you to code logic that can reject DML operation according to condition.
• It enable to ignoring parts of a batch.

Note: INSTEAD OF DELETE and INSTEAD OF UPDATE trigger cannot be defined on table if a table has foreign key defined by using DELETE or UPDATE cascading action then INSTEAD OF DELETE and INSTEAD OF UPDATE cannot be defined.

INSTEAD OF trigger can be classified further into three types: 

     i). INSTEAD OF INSERT Trigger. 
Example :

CREATE TRIGGER onInsertTest ON [dbo].[empProfile]

INSTEAD OF  INSERT

AS

IF ((SELECT gender FROM inserted)='FEMALE')

RAISERROR('Can not INSERT FEMALE employee',16,1);

ELSE

INSERT INTO empProfile (name, gender)

SELECT name, gender FROM inserted

GO

Testing trigger action

INSERT INTO empProfile (name, gender)

SELECT 'DEVA', 'FEMALE'

     ii). INSTEAD OF UPDATE Trigger. 

Example :

CREATE TRIGGER onUpdateTest ON [dbo].[empProfile]

INSTEAD OF  UPDATE

AS

IF ((SELECT gender FROM inserted)='FEMALE')

RAISERROR('Can not update FEMALE employee',16,1);

ELSE

UPDATE empProfile SET name = (SELECT name FROM inserted) WHERE ID =(SELECT id FROM inserted)

GO

Testing trigger action

UPDATE empProfile set NAME='INDRA' WHERE ID = 2

     iii). INSTEAD OF DELETE Trigger. 

Example :

CREATE TRIGGER onDeleteTest ON [dbo].[empProfile]

INSTEAD OF  DELETE

AS

IF EXISTS( SELECT 1 FROM deleted WHERE gender ='FEMALE')

RAISERROR('Can not delete FEMALE employee',16,1);

ELSE

DELETE FROM empProfile WHERE ID IN (SELECT D.ID FROM deleted D)

GO

Testing trigger action

DELETE FROM empProfile WHERE ID IN (3,4)

3. LOGON TRIGGER

 Logon triggers fire in response to the LOGON event that is raised when a user sessions is being established. 

Example :

CREATE TRIGGER logging_logon

ON ALL SERVER FOR LOGON

AS

BEGIN

INSERT INTO logHistoryTbl (sys_user, loginTime)

SELECT SYSTEM_USER,GETDATE()

END

GO

Transaction in SQL Server

Transaction:

 A transaction is a logical, atomic unit of work that contains one or more SQL statements.

Transaction groups SQL statements so that they are either all committed (which means they are applied to the database), or all rolled back (which means they are undone from the database).

Property of Transaction:

ACID: (atomicity, consistency, isolation, durability)

In computer science, ACID is a set of properties that guarantee that database transactions are processed reliably

Atomicity: Modification on the data in the database either fails or succeeds. The beginning of such a modification starts with a transaction and ends when a transaction finishes (either by a commit or a rollback).

Consistency: The transaction takes the database from one consistent state to another consistent state.

Isolation: The effect of a transaction is not visible to other transactions until the transaction is committed. 

Durability: Changes made by committed transactions are permanent.

Type of Transaction:

1.    Auto commit mode : On any DML operation, commit fires implicitly.(e.g. SQL Server)

2.    Implicit mode: commit is must before logout. (e.g. Oracle 9i & 10g)

3.    Explicit Mode: User will apply commit as per his requirement.

Example 1:

BEGIN TRANSACTION;

BEGIN TRY

    DELETE FROM TB1                   --DELETED ALL ROW

    SELECT * FROM TB1                 --NO ROW HERE

    DELETE FROM TB1 WHERE ID = 'ABC'; --HERE IS ERROR ('ABC' is not integer value)

END TRY

BEGIN CATCH

    SELECT

        ERROR_NUMBER() AS ErrorNumber

        ,ERROR_SEVERITY() AS ErrorSeverity

        ,ERROR_STATE() AS ErrorState

        ,ERROR_PROCEDURE() AS ErrorProcedure

        ,ERROR_LINE() AS ErrorLine

        ,ERROR_MESSAGE() AS ErrorMessage;

    IF @@TRANCOUNT > 0

        ROLLBACK TRANSACTION; --ROLL BACK HERE

        SELECT * FROM TB1    --IT WILL GIVE ALL ROW

END CATCH;

IF @@TRANCOUNT > 0

    COMMIT TRANSACTION;

GO

Cursor in SQL Server (with example)

What is a Cursor?

Cursor is a variable in SQL Server Database which is used for row-by row operations. It indicates the current position in the result set so that it is known as Cursor.

Script for taking backup of all databases by using cursor.

DECLARE @dbName VARCHAR(50) --DATABASE NAME

DECLARE @path VARCHAR(500)  --BACKUP PATH

DECLARE @fileName VARCHAR(100) --FILE NAME FOR BACKUP

DECLARE @fileDate VARCHAR(100) --FILE DATE FOR NAME WITH FILE NAME

SET @path = 'C:\Backup\'       --BACKUP PATH

SELECT @fileDate = CONVERT(VARCHAR, GETDATE(),105)

DECLARE dbName_cursor CURSOR FOR   --DECLARATION AND INITILIZING CURSOR

SELECT [name] FROM [master].[dbo].[sysdatabases]

WHERE [name] NOT IN ('master','model','msdb','tempdb')

OPEN dbName_cursor        --OPEN CURSOR

FETCH NEXT FROM dbName_cursor INTO @dbName

WHILE @@FETCH_STATUS = 0

BEGIN

    SET @fileName = @path+@dbName+'_'+@fileDate+'.bak'

    BACKUP DATABASE @dbName TO DISK = @fileName

   

    FETCH NEXT FROM dbName_cursor INTO @dbName

END

CLOSE dbName_cursor      --CLOSING CURSOR

DEALLOCATE dbName_cursor --DEALLOCATION MEMORY

JOIN in SQL SERVER

JOIN

Join enable to retrieve data from two or more tables based on logical relationships between the tables.

--TABLE JOIN CONCEPT--

INNER JOIN

SELECT T1.*, T2.*

FROM TB1 T1 INNER JOIN TB2 T2 ON T1.ID=T2.ID

LEFT OUTER JOIN

SELECT T1.*, T2.*

FROM TB1 T1 LEFT OUTER JOIN TB2 T2 ON T1.ID=T2.ID

RIGHT OUTER JOIN

SELECT T1.*, T2.*

FROM TB1 T1 RIGHT OUTER JOIN TB2 T2 ON T1.ID=T2.ID

FULL OUTER JOIN

SELECT T1.*, T2.*

FROM TB1 T1 FULL OUTER JOIN TB2 T2 ON T1.ID=T2.ID

CROSS JOIN

<no picture>

SELECT T1.*, T2.*

FROM TB1 T1 CROSS JOIN TB2 T2

--(both are equivalent)--

SELECT T1.*, T2.*

FROM TB1 T1 INNER JOIN TB2 T2 ON 1=1

LEFT OUTER JOIN - WHERE IS NULL

SELECT T1.*, T2.* FROM TB1 T1 LEFT OUTER JOIN TB2 T2 ON T1.ID=T2.ID

WHERE T2.ID IS NULL

RIGHT OUTER JOIN - WHERE IS NULL

SELECT T1.*, T2.* FROM TB1 T1 RIGHT OUTER JOIN TB2 T2 ON T1.ID=T2.ID

WHERE T1.ID IS NULL

FULL OUTER JOIN - WHERE IS NULL

SELECT T1.*, T2.* FROM TB1 T1 FULL OUTER JOIN TB2 T2 ON T1.ID=T2.ID

WHERE T1.ID IS NULL OR T2.ID IS NULL 

SELF JOIN - (A REAL EXAMPLE)

SELECT T1.id, T1.name, T2.name AS manager_name

FROM employee T1 left outer JOIN employee t2

ON T1.managerId=T2.id

Common Table Expression (CTE) is MS SQL Server

Common Table Expression (CTE) is a temporary result set that is defined within the execution scope of a single SELECT, INSERT, UPDATE, DELETE, or CREATE VIEW statement. A CTE is similar to a derived table in that it is not stored as an object and lasts only for the duration of the query. A CTE can be self-referencing and can be referenced multiple times in the same query unlike a derived table.

In Common Table Expression we can create a recursive query. Reference the resulting table multiple times in the same statement. CTEs can be defined in user-defined routines, such as functions, stored procedures, triggers, or views.

EXAMPLE : 1

UPDATE or SELECT single row from a table which has many realted duplicates row

WITH CTE (config_element, rowNum)

AS

(

SELECT config_element,

ROW_NUMBER() OVER(PARTITION BY config_element ORDER BY config_element) AS rowNum

FROM [myDb].[dbo].[temp_configuration]

)

--UPDATE CTE SET config_element='factoryShortKeys' WHERE rowNum=2

SELECT * from CTE WHERE rowNum = 2

GO

EXAMPLE : 2

Using multiple CTE in single select query

WITH CTE_test1 AS (SELECT 100 AS Col1),

CTE_test2 AS (SELECT COL1+1 AS Col2 FROM CTE_test1)

SELECT CTE_test1.Col1, CTE_test2.Col2 FROM CTE_test1 

CROSS JOIN CTE_test2

GO

EXAMPLE : 3

Using multiple CTE in single select query

WITH CTE_test1  AS (SELECT 100 AS Col1),

CTE_test2 AS (SELECT COL1+1 AS Col2 FROM CTE_test1)

SELECT CTE_test1.Col1, CTE_test2.Col2 FROM CTE_test1 

CROSS JOIN CTE_test2

GO

For further detail see link:

http://blog.sqlauthority.com/2011/05/10/sql-server-common-table-expression-cte-and-few-observation/