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Mastering Exception Handling in SQL Server with the THROW Statement

SQL Server THROW Statement: Mastering Exception Handling in SQL Server

As a database developer, handling exceptions is an essential part of your daily routine. In SQL Server, the THROW statement is an important tool that allows you to throw an error and take appropriate action, such as rolling back the transaction or rerunning the code block.

In this article, well explore the basics of the THROW statement, learn its syntax, and compare it with the RAISERROR statement.

SQL Server THROW Statement Overview

Syntax and Parameters

The THROW statement is used to raise an exception and is often used in conjunction with the CATCH block and the TRY CATCH construct. The syntax of the THROW statement is as follows:

“`

THROW [ { error_number | @local_variable },

{ message | ‘@local_variable’ },

{ state | @local_variable } ]

“`

The parameters are as follows:

– error_number: The error number is a user-defined integer that uniquely identifies the error.

It must be greater than or equal to 50000. – message: The message is an error message that provides more information about the error.

You can use a string literal or a local variable. – state: The state is an integer that indicates the state of the exception.

It must be between 0 and 255. THROW vs.

RAISERROR

The RAISERROR statement is similar to the THROW statement in that it raises an exception, but it has some key differences. The RAISERROR statement has the following syntax:

“`

RAISERROR ( { message_id | message_string | @local_variable }

{ , severity , state }

[ , argument [ ,…n ] ] )

[ WITH option [ ,…n ] ]

“`

The parameters for RAISERROR are as follows:

– message_id: The id of the message in sys.messages or the message string itself.

– severity: The severity level of the error. Must be between 1 and 25.

– state: The user-defined state of the error. – argument: An optional argument list to use with the message string.

– option: An optional argument to specify additional error handling behavior. The RAISERROR statement allows you to use printf formatting styles with the message string as well as the FORMATMESSAGE() function to build dynamic error messages.

However, THROW allows for cleaner, more concise code when raising simple errors.

SQL Server THROW Statement Examples

Example 1: Raising an Exception

Suppose you want to raise an error when a condition is not met in a stored procedure. Heres an example:

“`

CREATE PROCEDURE dbo.usp_InsertEmployee

@Name varchar(50),

@Salary decimal(9,2)

AS

BEGIN

IF @Name IS NULL OR @Salary IS NULL OR @Salary < 0

BEGIN

THROW 50001, ‘Invalid parameter values’, 1;

END

INSERT INTO dbo.Employee (Name, Salary)

VALUES (@Name, @Salary);

END

“`

In this example, we throw an exception with error number 50001 if any of the input parameters are invalid. We also provide a custom error message and the state of the error.

Example 2: Rethrowing an Exception

When handling exceptions in a CATCH block, you may need to rethrow the exception to a higher level to take appropriate action. Heres an example:

“`

BEGIN TRY

— Code block that throws an exception

INSERT INTO dbo.Employee (Name, Salary)

VALUES (‘John Doe’, 1000000);

END TRY

BEGIN CATCH

— Check for primary key violation

IF ERROR_NUMBER() = 2627

BEGIN

— Rethrow the exception with custom message

THROW 50002, ‘Employee already exists’, 1;

END

END CATCH

“`

In this example, we insert a record into the Employee table with a primary key constraint. If the insert fails due to a primary key violation, we catch the exception in the CATCH block, check the error number with the ERROR_NUMBER() function, and rethrow the exception with a custom message.

Conclusion

In this article, we have learned the basics of the SQL Server THROW statement, its syntax and parameters, and compared it with the RAISERROR statement. We also demonstrated how to use the THROW statement to raise and handle exceptions in a stored procedure.

With this knowledge, you can now handle exceptions more effectively in your SQL Server applications.

Using THROW Statement to Rethrow an Exception and FORMATMESSAGE() Function to Substitute Parameters in the Message Text

In the previous section, we explored the basics of the THROW statement, its syntax and parameters, and how it compares to the RAISERROR statement. We also looked at some examples of using the THROW statement to raise and rethrow exceptions in SQL Server.

In this section, we will dive deeper into rethrowing exceptions and explore the FORMATMESSAGE() function to build dynamic error messages.

Rethrowing Exceptions with THROW

In SQL Server, you can use the CATCH block to handle exceptions that occur in a TRY block. In some cases, you may need to rethrow the exception to a higher level or to the client application to take appropriate action.

When rethrowing an exception with the THROW statement, you can preserve the original error number, message, and state or customize them with new values. Here’s an example of rethrowing an exception with a customized error message:

“`

BEGIN TRY

— Some code block that throws an exception

SELECT 1/0;

END TRY

BEGIN CATCH

— Rethrow the exception with a custom message

DECLARE @ErrorMessage NVARCHAR(MAX) = ‘An error occurred: ‘ + ERROR_MESSAGE()

THROW 50001, @ErrorMessage, 1

END CATCH

“`

In this example, we use the ERROR_MESSAGE() function to retrieve the original error message and append it to a custom message. We then pass the custom message as the second parameter to the THROW statement and a custom error number and state as the first and third parameters, respectively.

Formatting Error Messages with FORMATMESSAGE()

In some cases, you may need to build a dynamic error message that includes information from multiple columns or variables. The FORMATMESSAGE() function provides a flexible way to build such messages.

The FORMATMESSAGE() function retrieves a message from the sys.messages catalog view and replaces placeholders with the values passed in as arguments. Here’s the syntax of the FORMATMESSAGE() function:

“`

FORMATMESSAGE ( message_string, parameter1, parameter2,…,parameter n )

“`

The parameters are as follows:

– message_string: The format string that contains placeholders for the parameter values.

– parameter1, parameter2,…,parameter n: The parameter values to substitute for the placeholders in the message_string. Here’s an example of using the FORMATMESSAGE() function to build a dynamic error message:

“`

BEGIN TRY

— Some code block that throws an exception

DECLARE @TableName NVARCHAR(50) = ‘Employees’

DECLARE @ErrorID INT = 102

— Rethrow the exception with a dynamic error message

DECLARE @ErrorMessage NVARCHAR(MAX) = FORMATMESSAGE(‘A validation error occurred while processing table %s (error ID: %d).’, @TableName, @ErrorID)

THROW 50001, @ErrorMessage, 1

END TRY

BEGIN CATCH

— Handle the exception

END CATCH

“`

In this example, we declare variables @TableName and @ErrorID and use them as parameters to the FORMATMESSAGE() function. We then pass the formatted message as the second parameter to the THROW statement, along with a custom error number and state.

Conclusion

In this section, we have explored more advanced techniques for using the THROW statement in SQL Server to rethrow exceptions and build dynamic error messages using the FORMATMESSAGE() function. By mastering these techniques, you can write more efficient and effective error handling code in your SQL Server applications.

In summary, this article has covered the basics and advanced techniques of using the THROW statement in SQL Server to raise and rethrow exceptions, and the FORMATMESSAGE() function to build dynamic error messages. We compared the THROW statement to the RAISERROR statement and demonstrated how to handle exceptions and customize error messages with different levels of complexity.

The importance of error handling in SQL Server cannot be overstated, and the use of the THROW statement and FORMATMESSAGE() function can improve the efficiency and effectiveness of your applications. Always remember to apply these techniques in your code to ensure better error handling in SQL Server.

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