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SQL Transaction

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SQL Transaction is helpful to execute one more statements as a set. If a transaction is successful, all the changes made in that transaction will be applied to the table. If any single statement inside the transaction encounters error then changes made in that transaction will be erased, or rolled back. Following are the list of things that you should remember while working the SQL server Transactions.

  • Every transaction should start with BEGIN TRANSACTION, BEGIN TRAN, or BEGIN TRANSACTION Transaction_Name
  • Every Transaction must end with either COMMIT, or ROLLBACK statements.
  • COMMIT TRANSACTION: This statement tells the SQL to save the changes made between the BEGIN and COMMIT. There are multiple ways to write this statement. Either you can write COMMIT, COMMIT TRAN, COMMIT TRANSACTION, or COMMIT TRANSACTION Transaction_Name
  • ROLLBACK TRANSACTION: This statement tells the SQL to erase all the changes made between the BEGIN and ROLLBACK. There are multiple ways to write this statement. Either you can write ROLLBACK, ROLLBACK TRAN, ROLLBACK TRANSACTION, or ROLLBACK TRANSACTION Transaction_Name

In this article we will show you list of various example that can explain the SQL Transaction. They are: BEGIN TRANSACTION, COMMIT TRANSACTION, ROLLBACK TRANSACTION, named transactions, Transactions inside the IF ELSE, and Transactions inside the TRY CATCH block.

SQL Transaction Example

In this example we will place a INSERT INTO SELECT statement inside the BEGIN, and COMMIT transaction. As you can see, it will select top four records from Employee table, and store them in Employee Records table.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO

BEGIN TRAN

INSERT INTO [dbo].[EmployeeRecords] (
	         [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
SELECT TOP 4 [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales]
FROM [dbo].[Employee]

COMMIT TRANSACTION

OUTPUT

SQL Transaction 2

Let us see the inserted records in Employee Records table.

SQL Transaction 3

Transaction in SQL server is not limited to single statement. We can place multiple statement inside one transaction. In this example we will place one Insert Statement, and Update Statement

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO

BEGIN TRANSACTION

INSERT INTO [dbo].[EmployeeRecords] (
	    [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
     VALUES (5, 'SQL', 'Server', 'Education', 'Teaching', 10000, 200)

UPDATE [dbo].[EmployeeRecords]
	SET [Education] = 'Tutorials',
		[YearlyIncome] = 98000
	WHERE [EmpID] = 5
	   
COMMIT TRANSACTION

OUTPUT

SQL Transaction 4

Let me show you the records in Employee table after that transaction.

SQL Transaction 5

Auto Rollback in SQL Transaction

In SQL Server, Statements inside a transaction is executed as a set, and if one statement fails, then remaining statements with not execute. This process is also called as Auto Rollback.

This time we will deliberately fail one statement inside the Transaction. As you can from the below screenshot, Update statement will return an error because we are entering the string (VARCHAR) information into the float data type.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO

BEGIN TRANSACTION

INSERT INTO [dbo].[EmployeeRecords] (
	    [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
     VALUES (6, 'Tutorial', 'Gateway', 'Education', 'Learning', 65000, 1400)

UPDATE [dbo].[EmployeeRecords]
	SET [Education] = 'Masters',
	    [YearlyIncome] = 'Wrong Data'
	WHERE [EmpID] = 6
	   
COMMIT TRANSACTION

OUTPUT

SQL Transaction 6

As you can from the below screenshot, though there is an error at Update statement. We are unable to see the inserted record because, it is roll backed by the SQL.

SQL Transaction 7

Let me remove the BEGIN TRANSACTION, and COMMIT TRANSACTION from the above code snippet, and execute the statements.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO

INSERT INTO [dbo].[EmployeeRecords] (
	         [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
     VALUES (6, 'Tutorial', 'Gateway', 'Education', 'Learning', 65000, 1400)

UPDATE [dbo].[EmployeeRecords]
	SET [Education] = 'Masters',
		[YearlyIncome] = 'Wrong Data'
	WHERE [EmpID] = 6

OUTPUT

SQL Transaction 8

As you can see from the below screenshot, it has inserted the new record of Emp Id 6 but failed to update.

SQL Transaction 9

Rollback Transaction in SQL Server

Rollback transaction in SQL is useful to roll back any transaction to either beginning of the transaction, or to the transaction save point. You can use this SQL Rollback to remove the half completed rows, or to handle errors. For example, if your transaction is inserting new record, and it throws an error then you can use this rollback transaction to revert the table to original position.

In this example, we will insert a new record into the Employee table, and apply the rollback transaction after the insert. To demonstrate this we are using Select Statement inside a Transaction, and outside the transaction. First select statement will show you the table records before the transaction is completed.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO

BEGIN TRANSACTION

INSERT INTO [dbo].[EmployeeRecords] (
	         [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
     VALUES (7, 'SQL Server', 'Tutorial', 'Masters', 'Learn', 55000, 1250)

SELECT * FROM [dbo].[EmployeeRecords]

ROLLBACK TRANSACTION

SELECT * FROM [dbo].[EmployeeRecords]

OUTPUT

SQL Transaction 10

Though there is no error in the above statement. As you can see from the above screenshot, it has not inserted the record.

Named Transactions in SQL Server

SQL Server allows you to name your transaction. When you are working with multiple transactions in one query, it is always advisable to use the named transaction.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO
-- AddEmployee is the Transaction name
BEGIN TRANSACTION AddEmployee

INSERT INTO [dbo].[EmployeeRecords] (
	    [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
     VALUES (7, 'SQL Server', 'Tutorial', 'Masters', 'Learn', 55000, 1250)

COMMIT TRANSACTION AddEmployee

SELECT * FROM [dbo].[EmployeeRecords]

OUTPUT

SQL Transaction 11

SQL Transaction in IF ELSE statement

SQL Server Transactions are much useful, if we place them inside any conditional statements such as: IF ELSE. For instance, checking for the existing records in employee table before the insertion, and if it is there then rollback, else commit etc.

Below statement is a simple example, where we declared a variable (assuming user going to enter the value). Next, we used insert statement, next within the If we are checking whether the @sales is less than 1000 or not. If it is true then ROLLBACK the transaction otherwise, Commit the transaction.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO
DECLARE @Sales FLOAT = 250.0

BEGIN TRANSACTION AddEmployee

INSERT INTO [dbo].[EmployeeRecords] (
	    [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
     VALUES (8, 'Dave', 'Rob', 'High School', 'Professional', 85000, @Sales)

IF @Sales < 1000
	BEGIN
	    ROLLBACK TRANSACTION AddEmployee
	    PRINT 'Sorry! Sales Amount Should be More than 1000'
	END
ELSE
	BEGIN
		COMMIT TRANSACTION AddEmployee
		PRINT 'Inserted the Record Successfully'
	END		
SELECT * FROM [dbo].[EmployeeRecords]

OUTPUT

SQL Transaction 12

Let me show you the records

SQL Transaction 13

This time we changed the variable @Sales value to 1450.02

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO
DECLARE @Sales FLOAT = 1450.02

BEGIN TRANSACTION AddEmployee

INSERT INTO [dbo].[EmployeeRecords] (
	    [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
     VALUES (8, 'Dave', 'Rob', 'High School', 'Professional', 85000, @Sales)

IF @Sales < 1000
	BEGIN
	    ROLLBACK TRANSACTION AddEmployee
	    PRINT 'Sorry! Sales Amount Should be More than 1000'
	END
ELSE
	BEGIN
		COMMIT TRANSACTION AddEmployee
		PRINT 'Inserted the Record Successfully'
	END		
SELECT * FROM [dbo].[EmployeeRecords]

OUTPUT

SQL Transaction 14

let me show you the result data in employee table.

SQL Transaction 15

SQL Transaction in TRY CATCH

SQL Server Transactions are very useful, if we place them inside TRY CATCH block. For instance, if there is an error inside the transaction then you can use the catch block to rollback the transaction to original position. Below statement is a simple example, where we placed one select, and one update statement inside the transaction. If you observe closely, our update statement will throw an error because of data type conflict.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO
BEGIN TRY
	BEGIN TRANSACTION AddEmployee

	INSERT INTO [dbo].[EmployeeRecords] (
			[EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
		 VALUES (9, 'Chong', 'Lee', 'Tutorials', 'Developer', 66500, 1950)

	UPDATE [dbo].[EmployeeRecords]
		SET [Education] = 'Bachelors',
		    [YearlyIncome] = 'Hey You gotme Wrong!'
		WHERE [EmpID] = 9

	COMMIT TRANSACTION AddEmployee
	PRINT 'Inserted the Record Successfully'
END TRY
BEGIN CATCH
	ROLLBACK TRANSACTION AddEmployee
	PRINT 'Sorry! There is a Data Type Mismatch for yearly Income'
END CATCH
	
SELECT * FROM [dbo].[EmployeeRecords]

OUTPUT

SQL Transaction 16

and the data inside the table is:

SQL Transaction 17

Let me change the yearly income value to 91567

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO
BEGIN TRY
	BEGIN TRANSACTION AddEmployee

	INSERT INTO [dbo].[EmployeeRecords] (
				 [EmpID], [FirstName], [LastName], [Education], [Occupation], [YearlyIncome], [Sales])
		 VALUES (9, 'Chong', 'Lee', 'Tutorials', 'Developer', 66500, 1950)

	UPDATE [dbo].[EmployeeRecords]
		SET [Education] = 'Bachelors',
			[YearlyIncome] = 91567 -- We changed it
		WHERE [EmpID] = 9

	COMMIT TRANSACTION AddEmployee
	PRINT 'Inserted the Record Successfully'
END TRY
BEGIN CATCH
	ROLLBACK TRANSACTION AddEmployee
		PRINT 'Sorry! There is a Data Type Mismatch for yearly Income'
END CATCH
	
SELECT * FROM [dbo].[EmployeeRecords]

OUTPUT

SQL Transaction 18

and the data inside the Employee table is:

SQL Transaction 19

SQL Transaction Common Mistake

In this example we will show you, One of the common mistake that’s going to happen while using the transactions in SQL query. As you can see, we are using simple Update statement after the BEGIN TRANSACTION but forgot to mention COMMIT, or ROLLBACK

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO

BEGIN TRANSACTION 
	UPDATE [dbo].[EmployeeRecords]
		SET [YearlyIncome] = 125896
		   WHERE [EmpID] = 6

OUTPUT

SQL Transaction 20

Let me open one more query window (click New query in SSMS), and write a simple select statement to select all the records in Employee Records table. From the below screenshot you can see, the execution is taking longer than usual. This is because, by default Select statement will return the committed data, and we forgot to commit the update.

SQL Transaction 21

Let me set the Isolation level to Read Uncommitted. This will read the uncommitted data.

SQL CODE

--SQL Server Transaction Example
USE [SQL Tutorial]
GO
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
SELECT * FROM [dbo].[EmployeeRecords]

OUTPUT

SQL Transaction 22

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