SQL Talk

Read below points:

THROW

RAISE ERROR

TRY_ CATCH

NESTED TRY CATCH

@@TRANCOUNT
XACT_STATE
XACT_ABORT

What is Exception handling in SQL Server?

Some errors are generated by the application at runtime. These errors can be generated because of various reasons and the application is not able to handle these errors, causing the program to break and throw an exception.

Usually, these are generated because of unexpected human input such as passing an unexpected data type input e.g. passing an integer at the place of a string value, dividing a number by zero, etc.

But in real-life scenarios, the program is not supposed to be broken down, but execute flawlessly. Therefore, there is a need for a mechanism that can handle the situations that affect the program’s normal execution or cause it to break. This mechanism is called exception handling or error handling.

Error handling in SQL Server gives us control over the Transact-SQL code. For example, when things go wrong, we get a chance to do something about it and possibly make it right again. SQL Server error handling can be as simple as just logging that something happened, or it could be us trying to fix an error. It can even be translating the error in SQL language because we all know how technical SQL Server error messages could get making no sense and hard to understand. Luckily, we have a chance to translate those messages into something more meaningful to pass on to the users, developers, etc.

SQL Server also provides the exception handling mechanism like any other programming language. There are different ways to handle the exceptions. We will discuss these in the upcoming sections with the help of examples.

Consider the following program which divides the two number:

USE [master]
GO
DECLARE
	@Number1 int,
	@Number2 int,
	@Result real

SET @Number1=10
SET @Number2=0

SET @Result=@Number1/@Number2
PRINT(@Result)

Types of exceptions in SQL Server

1. System Defined Exceptions
2. User-Defined Exceptions

System Defined Exceptions: These are the exceptions that are generated by the system during the program execution. The error messages are predefined in the system and the final user will see the predefined message on the console.

User-Defined Exceptions: These are the exceptions that are thrown explicitly by the programmer. The programmer can raise exceptions at any part of the SQL code according to the logic he has implemented in the program.

The programmer can also create custom messages that he finds will be interactive and meaningful to the user. He can also store these errors into log tables to maintain a log of generated exceptions.

Raising Errors with RAISERROR

1RAISERROR (N'This is message.', -- Message text. 
210, -- Severity, 
31); -- Second argument. 
4GO 

THROW 
251000, -- error number
3'This is not a valid value for unit price.', -- message
41; --state
5GO 

1Msg 51000, Level 16, State 1, Line 1 
2This is not a valid value for unit price.

Select @@ERROR

1RAISERROR(N'Message', 16, 1);
2IF @@ERROR <> 0
3PRINT 'Error=' + CAST(@@ERROR AS VARCHAR(8));
4GO

1Msg 50000, Level 16, State 1, Line 1
2Message
3Error=0

1DECLARE @Value int;
2RAISERROR(N'Message', 16, 1);
3SET @Value = @@ERROR;
4IF @Value <> 0
5PRINT 'Error=' + CAST(@Value AS VARCHAR(8));

1Msg 50000, Level 16, State 1, Line 2
2Message
3Error=50000

Try Catch in SQL Server

• We put the SQL statements that may cause an exception in the BEGIN TRY block.
• If any exception arises, the control is immediately transferred to the BEGIN CATCH block.
• In the BEGIN CATCH block, we write the statements that should be executed if an exception is occurred in the BEGIN TRY block. That means the statements inside the BEGIN CATCH block are used for handling the flow of the program in case the exception is triggered.
• If no exception arises i.e. all the SQL statements inside the BEGIN TRY block are executed, the control is not transferred to the BEGIN CATCH block.

Now let us see the general syntax for BEGIN TRY and BEGIN CATCH block.

• BEGIN TRY  
     -- statements that may cause exceptions
  END TRY  
  Code language: SQL (Structured Query Language) (sql)

  Then you use a BEGIN CATCH...END CATCH block immediately after the TRY block:

  BEGIN CATCH  
     -- statements that handle exception
  END CATCH  
  Code language: SQL (Structured Query Language) (sql)

  The following illustrates a complete TRY CATCH construct:

  BEGIN TRY  
     -- statements that may cause exceptions
  END TRY 
  BEGIN CATCH  
     -- statements that handle exception
  END CATCH  
  Code language: SQL (Structured Query Language) (sql)

  If the statements between the TRY block complete without an error, the statements between the CATCH block will not execute. However, if any statement inside the TRY block causes an exception, the control transfers to the statements in the CATCH block.

  The CATCH block functions

  Inside the CATCH block, you can use the following functions to get the detailed information on the error that occurred:

  • ERROR_LINE() returns the line number on which the exception occurred.
  • ERROR_MESSAGE() returns the complete text of the generated error message.
  • ERROR_PROCEDURE() returns the name of the stored procedure or trigger where the error occurred.
  • ERROR_NUMBER() returns the number of the error that occurred.
  • ERROR_SEVERITY() returns the severity level of the error that occurred.
  • ERROR_STATE() returns the state number of the error that occurred.

  Note that you only use these functions in the CATCH block. If you use them outside of the CATCH block, all of these functions will return NULL.

  Nested TRY CATCH constructs

  You can nest TRY CATCH construct inside another TRY CATCH construct. However, either a TRY block or a CATCH block can contain a nested TRY CATCH, for example:

  BEGIN TRY
      --- statements that may cause exceptions
  END TRY
  BEGIN CATCH
      -- statements to handle exception
      BEGIN TRY
          --- nested TRY block
      END TRY
      BEGIN CATCH
          --- nested CATCH block
      END CATCH
  END CATCH
  Code language: SQL (Structured Query Language) (sql)

  SQL Server TRY CATCH examples

  First, create a stored procedure named usp_divide that divides two numbers:

  CREATE PROC usp_divide(
      @a decimal,
      @b decimal,
      @c decimal output
  ) AS
  BEGIN
      BEGIN TRY
          SET @c = @a / @b;
      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;  
      END CATCH
  END;
  GO
  Code language: SQL (Structured Query Language) (sql)

  In this stored procedure, we placed the formula inside the TRY block and called the CATCH block functions ERROR_* inside the CATCH block.

  Second, call the usp_divide stored procedure to divide 10 by 2:

  DECLARE @r decimal;
  EXEC usp_divide 10, 2, @r output;
  PRINT @r;
  Code language: SQL (Structured Query Language) (sql)

  Here is the output

  5
  Code language: SQL (Structured Query Language) (sql)

  Because no exception occurred in the TRY block, the stored procedure completed at the TRY block.

  Third, attempt to divide 20 by zero by calling the usp_divide stored procedure:

  DECLARE @r2 decimal;
  EXEC usp_divide 10, 0, @r2 output;
  PRINT @r2;
  Code language: SQL (Structured Query Language) (sql)

  The following picture shows the output:

  

  Because of division by zero error which was caused by the formula, the control was passed to the statement inside the CATCH block which returned the error’s detailed information.
  
  

  SQL Serer TRY CATCH with transactions

  Inside a CATCH block, you can test the state of transactions by using the XACT_STATE() function.

  • If the XACT_STATE() function returns -1, it means that an uncommittable transaction is pending, you should issue a ROLLBACK TRANSACTION statement.
  • In case the XACT_STATE() function returns 1, it means that a committable transaction is pending. You can issue a COMMIT TRANSACTION statement in this case.
  • If the XACT_STATE() function return 0, it means no transaction is pending, therefore, you don’t need to take any action.

  It is a good practice to test your transaction state before issuing a COMMIT TRANSACTION or ROLLBACK TRANSACTION statement in a CATCH block to ensure consistency.

  Using TRY CATCH with transactions example

  First, set up two new tables sales.persons and sales.deals for demonstration:

  CREATE TABLE sales.persons
  (
      person_id  INT
      PRIMARY KEY IDENTITY, 
      first_name NVARCHAR(100) NOT NULL, 
      last_name  NVARCHAR(100) NOT NULL
  );
  
  CREATE TABLE sales.deals
  (
      deal_id   INT
      PRIMARY KEY IDENTITY, 
      person_id INT NOT NULL, 
      deal_note NVARCHAR(100), 
      FOREIGN KEY(person_id) REFERENCES sales.persons(
      person_id)
  );
  
  insert into 
      sales.persons(first_name, last_name)
  values
      ('John','Doe'),
      ('Jane','Doe');
  
  insert into 
      sales.deals(person_id, deal_note)
  values
      (1,'Deal for John Doe');
  Code language: SQL (Structured Query Language) (sql)

  Next, create a new stored procedure named usp_report_error that will be used in a CATCH block to report the detailed information of an error:

  CREATE PROC usp_report_error
  AS
      SELECT   
          ERROR_NUMBER() AS ErrorNumber  
          ,ERROR_SEVERITY() AS ErrorSeverity  
          ,ERROR_STATE() AS ErrorState  
          ,ERROR_LINE () AS ErrorLine  
          ,ERROR_PROCEDURE() AS ErrorProcedure  
          ,ERROR_MESSAGE() AS ErrorMessage;  
  GO
  Code language: SQL (Structured Query Language) (sql)

  Then, develop a new stored procedure that deletes a row from the sales.persons table:

  CREATE PROC usp_delete_person(
      @person_id INT
  ) AS
  BEGIN
      BEGIN TRY
          BEGIN TRANSACTION;
          -- delete the person
          DELETE FROM sales.persons 
          WHERE person_id = @person_id;
          -- if DELETE succeeds, commit the transaction
          COMMIT TRANSACTION;  
      END TRY
      BEGIN CATCH
          -- report exception
          EXEC usp_report_error;
          
          -- Test if the transaction is uncommittable.  
          IF (XACT_STATE()) = -1  
          BEGIN  
              PRINT  N'The transaction is in an uncommittable state.' +  
                      'Rolling back transaction.'  
              ROLLBACK TRANSACTION;  
          END;  
          
          -- Test if the transaction is committable.  
          IF (XACT_STATE()) = 1  
          BEGIN  
              PRINT N'The transaction is committable.' +  
                  'Committing transaction.'  
              COMMIT TRANSACTION;     
          END;  
      END CATCH
  END;
  GO
  Code language: SQL (Structured Query Language) (sql)

  In this stored procedure, we used the XACT_STATE() function to check the state of the transaction before performing COMMIT TRANSACTION or ROLLBACK TRANSACTION inside the CATCH block.

  After that, call the usp_delete_person stored procedure to delete the person id 2:

  EXEC usp_delete_person 2;
  Code language: SQL (Structured Query Language) (sql)

  There was no exception occurred.

  Finally, call the stored procedure usp_delete_person to delete person id 1:

  EXEC usp_delete_person 1;
  Code language: SQL (Structured Query Language) (sql)

  The following error occurred:

  

  In this tutorial, you have learned how to use the SQL Server TRY CATCH construct to handle exceptions in stored procedures.

Throw custom exception in SQL Server stored procedure

In SQL Server, you can also throw custom exceptions. Throwing a custom exception means you can cause an exception to encounter an error at any part of your code according to your logic.

We use the THROW statement or the RAISERROR function for throwing a custom exception. Both the functions do almost the same work. However, there are some differences between the two.

• The RAISERROR was introduced in SQL Server 7.0 and the THROW statement was introduced in the SQL Server 2012. Microsoft suggests using the THROW statement as it is easier to use and has more functionalities than the RAISERROR function.

• The RAISERROR function keeps the record of only the last exception thrown. However, it is not the case with the THROW statement. The THROW statement keeps the record of all the recent exceptions.

• You can also use the THROW function without using any parameters specified. But, you cannot do this with the RAISERROR function.
• The THROW statement threw an error. It takes three parameters:
  • Error number, which should be always greater than or equal to 50000 and less than the INT range.
  • Error message, which you want to show to the user. It should be enough informative to the user.
  • Error state, the current state of the error. It can have values between 0 to 255 as it supports the TINYINT data type.
• You can also use the THROW() function without any parameter to throw an exception and transfer the control to the CATCH block, but there must be a CATCH block to handle the exception.
  

Note

1. A TRY..CATCH block combination catches all the errors that have a severity between 11 and 19.

2. The CATCH block is executed only if there is an error occurs in T-SQL statements within TRY block otherwise the CATCH block is ignored.

3. Each TRY block is associated with only one CATCH block and vice versa

4. TRY and CATCH blocks can’t be separated with the GO statement. We need to put both TRY and CATCH blocks within the same batch.

5. TRY..CATCH blocks can be used with transactions. We check the number of open transactions by using @@TRANCOUNT function in Sql Server.

6. XACT_STATE function within the TRY..CATCH block can be used to check whether an open transaction is committed or not. It will return -1 if the transaction is not committed else returns 1.