Jul 12, 2016

What is transaction in SQL Server

Transaction is the collection of T-SQL statements which executes in single unit. Transaction begins with a specific T-SQL statement and ends when all T-SQL statements execute successfully. If any statements fail then transaction fails. Transaction has only two types of results success or failed.

If transactions statements execute successfully then transaction is committed and changes saved successfully in database. If any statement failed then complete transaction become failed and rollback all statements execution.

When we use Transaction in SQL Server

If more than one table/view are related to each other and if we make any changes on any table/view and this change should be on related table also, here we have to use transaction.

Use transaction carefully and use it when needed because when transaction begins it locks all tables which are used in T-SQL Statement for data isolation. During transaction execution period no one can perform any operation on used tables in transaction due to region of maintaining data integrity.
Transaction use the following statements
  •          Begin transaction
  •          Commit Transaction
  •          Rollback Transaction

Example:

Suppose we have two table Users and UserProfile as bellow. Execute the following script


CREATE TABLE Users
(
ID BIGINT PRIMARY KEY,
NAME VARCHAR(100),
EMAIL VARCHAR(100),
PWD VARCHAR(100),
CREATED_DATE DATETIME
)

CREATE TABLE UserProfile
(
            ID BIGINT PRIMARY KEY,
            USERID BIGINT foreign Key references Users(ID),
            STREET VARCHAR(100),
            CITY VARCHAR(100),
            STATE VARCHAR(100),
            COUNTRY VARCHAR(100),
            PHONE VARCHAR(100),
            CAREATED_DATE DATETIME,
            MODIFIED_DATE DATETIME
)

INSERT INTO [dbo].[Users](ID,NAME,EMAIL,PWD,CREATED_DATE)
VALUES(1,'Codefari','info@codefari.com','codefari@123',GETDATE())                     

INSERT INTO [dbo].[UserProfile](ID,USERID,STREET,CITY,STATE,COUNTRY,PHONE,CAREATED_DATE,MODIFIED_DATE)
VALUES(1,1,'G-234','EAST DELHI','DELHI','INDIA','9089987767',GETDATE(),GETDATE())

SELECT * FROM Users
SELECT * FROM UserProfile



Result: 









Now we will try to insert data in both tables using transaction.
Users table have a column ID which is primary key and it is reference key in UserProfle table. UserProfile table ID is also primary key. In following script I will put existing UserProfile .ID value and it will generate an error and transaction should be failed and return same as above result. It means no any changes in Users table also.


BEGIN
BEGIN TRANSACTION trn
            BEGIN TRY
            --Insert data in Users Table
                        INSERT INTO [dbo].[Users](ID,NAME,EMAIL,PWD,CREATED_DATE)
                        VALUES(2,'Dilip','dilip@gmail.com','dilip@123',GETDATE())      
                       
            --Insert data in UserProfile table where ID is already exist                  
                        INSERT INTO [dbo].[UserProfile](ID,USERID,STREET,CITY,
                        STATE,COUNTRY,PHONE,CAREATED_DATE,MODIFIED_DATE)
                        VALUES(1,2,'G-234','EAST DELHI','DELHI','INDIA',
                        '9089987767',GETDATE(),GETDATE())
                        COMMIT TRANSACTION
            END TRY
            BEGIN CATCH
                                    ROLLBACK TRANSACTION
            END CATCH      
END

SELECT * FROM Users
SELECT * FROM UserProfile


Result:








In above script I put the UserDetails.Id value 1 so it’s generate an error and rollback transaction now in following script I changed the unique value with 2 and execute again.


BEGIN
BEGIN TRANSACTION trn
            BEGIN TRY
            --Insert data in Users Table
                        INSERT INTO [dbo].[Users](ID,NAME,EMAIL,PWD,CREATED_DATE)
                        VALUES(2,'Dilip','dilip@gmail.com','dilip@123',GETDATE())      
                       
            --Insert data in UserProfile table where ID is already exist                  
                        INSERT INTO [dbo].[UserProfile](ID,USERID,STREET,CITY,
                        STATE,COUNTRY,PHONE,CAREATED_DATE,MODIFIED_DATE)
                        VALUES(2,2,'G-234','EAST DELHI','DELHI','INDIA',
                        '9089987767',GETDATE(),GETDATE())
                        COMMIT TRANSACTION
            END TRY
            BEGIN CATCH
                                    ROLLBACK TRANSACTION
            END CATCH      
END

SELECT * FROM Users
SELECT * FROM UserProfile


Result:










Everything fine transaction commited.

Related Link

Nested Transaction in SQL Server