datamatrix c# library T-SQL ESSENTIALS in Visual C#

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CHAPTER 11 T-SQL ESSENTIALS
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@@ERROR
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This is the most basic of error handling. It has served SQL Server developers well over the years, but it can be cumbersome. When an error occurs, such as you have seen as we have gone through the book creating and manipulating objects, a global variable, @@ERROR, would have been populated with the SQL Server error message number. Similarly, if you try to do something with a set of data that is invalid, such as dividing a number by zero or exceeding the number of digits allowed in a numeric data type, then SQL Server will populate this variable for you to inspect. The downside is that the @@ERROR variable setting only lasts for the next statement following the line of code that has been executed; therefore, when you think there might be problems, you need to either pass the data to a local variable or inspect it straight away. The first example demonstrates this.
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Try It Out: Using @@ERROR
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1. The first example will try and divide 100 by zero, which is an error. We will then list out the error number, and then again list out the error number. Enter the following code and execute it: SELECT 100/0 SELECT @@ERROR SELECT @@ERROR 2. It is necessary in this instance to check both the Results and Messages tab. The first tab is the Messages tab, which will show you the error that encountered. As expected, we see the Divide by zero error. Msg 8134, Level 16, State 1, Line 1 Divide by zero error encountered. (1 row(s) affected) (1 row(s) affected) 3. Moving to the Results tab, you should see three result sets, as shown in Figure 11-45. The first, showing no information, would be where SQL Server would have put the division results, had it succeeded. The second result set is the number from the first SELECT @@ERROR. Notice the number corresponds to the msg number found in the messages tab. The third result set shows a value of 0. This is because the first SELECT @@ERROR worked successfully and therefore set the system variable to 0. This demonstrates the lifetime of the value within @@ERROR.
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CHAPTER 11 T-SQL ESSENTIALS
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Figure 11-45. Showing @@ERROR in multiple statements 4. When we use the RAISERROR function, it will also set the @@ERROR variable, as we can see in the following code. However, the value will be set to 0 using our preceding example. This is because the severity level was below 11. RAISERROR (50001,1,1,243) SELECT @@ERROR 5. When the code is executed, you can see that @@ERROR is set to 0, as shown in Figure 11-46.
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Figure 11-46. When severity is too low to set @@ERROR 6. By changing the severity to 11 or above, the @@ERROR setting will now be set to the message number within the RAISERROR. RAISERROR (50001,11,1,243) SELECT @@ERROR 7. The preceding code produces the same message as seen within our RAISERROR example, but as you can see in Figure 11-47, the error number setting now reflects that value placed in the msgnum parameter.
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Figure 11-47. With a higher severity, the message number is set. Although a useful tool, it would be better to use the next error-handling routine to be demonstrated, TRY...CATCH.
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It can be said that no matter what, any piece of code has the ability to fail and generate some sort of error. For the vast majority of this code, you will want to trap any error that occurs, check
CHAPTER 11 T-SQL ESSENTIALS
what the error is, and deal with it as best you can. As you saw previously, this could be done one statement at a time using @@ERROR to test for any error code. A new and improved functionality exists whereby a set of statements can try and execute, and if any statement has an error, it will be caught. This is known as a TRY...CATCH block. Surrounding code with the ability to try and execute a slice of code and to catch any errors and try to deal with them has been around for quite a number of years in languages such as C++. Gladly, we now see this within SQL Server. The syntax is pretty straightforward. There are two blocks of code. The first block, BEGIN TRY, is where there is one or more T-SQL statements that you wish to try and run. If any of statements have an error, then no further processing within that block will execute, and processing will switch to the second block, BEGIN CATCH. BEGIN TRY { sql_statement | statement_block } END TRY BEGIN CATCH { sql_statement | statement_block } END CATCH When you generate your own error via a RAISERROR, then a bit of care has to be taken with the severity setting, as this will determine how your code works within a TRY...CATCH scenario. If you raise an error with a severity level of 0 to 10, then although an error is generated and will be received by the calling program, whether that is Query Editor or a program such as C#, then processing will continue without moving to the CATCH block. This can be seen as a warning level. Changing the severity level to 11 or above will transfer the control to the CATCH block of code. Once within the CATCH block you can raise a new error, or raise the same error by using values stored within SQL Server system functions. The system functions that can be used to find useful debugging information are detailed here: ERROR_LINE(): The line number that caused the error or performed the RAISERROR command. This is will be physical rather than relative, i.e., you don t have to remove blank lines within the T-SQL to get the correct line number, unlike some software that does require this. ERROR_MESSAGE(): The text message. ERROR_NUMBER(): The number associated with the message. ERROR_PROCEDURE(): If you are retrieving this within a stored procedure or trigger, the name of it will be contained here. If you are running ad hoc T-SQL code, then the value will be NULL. ERROR_SEVERITY(): The numeric severity value for the error. ERROR_STATE(): The numeric state value for the error. TRY...CATCH blocks can be nested, and when an error occurs, the error will be passed to the relevant CATCH section. This would be done when you wanted an overall CATCH block for general statements, and then you could perform specific testing and have specific error handling where you really think an error might be generated.
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