Counter of Festivals

Ashok Blog for SQL Learners and Beginners and Experts

Tuesday, 3 May 2016

TRY…CATCH Block Explanation in SQL Server

TRY CATCH  Block (Exception Handling in SQL Server)

The TRY or CATCH block can contain a single T-SQL statement or a series of statements. The CATCH block must follow immediately after the TRY block. The TRY/CATCH block cannot span more than a single batch. In addition, TRY/CATCH block cannot span an IF/ELSE statement.

Example of TRY…CATCH:

BEGIN TRY
DECLARE @X INT
---- Divide by zero to generate Error
SET @X = 1/0
PRINT 'Command after error in TRY block'
END TRY
BEGIN CATCH
PRINT 'Error Detected'
END CATCH
PRINT 'Command after TRY/CATCH blocks'

Above code will return following result:
Error Detected
Command after TRY/CATCH blocks


If all the statements within the TRY block are executed successfully, then processing does not enter the CATCH block, but instead skips over the CATCH block and executes the first statement following the END CATCH statement. Removing SET statement in above code PRINT ‘Error Detected’ statement is not executed, but the PRINT statement within the TRY block is executed, as well as the PRINT statement after the TRY/CATCH block. TRY/CATCH blocks can be nested.
Limitation of TRY…CATCH:
  • Compiled errors are not caught.
  • Deferred name resolution errors created by statement level recompilations. (If process is terminated by Kill commands or broken client connections TRY…CATCH will be not effective)
  • Errors with a severity greater than 10 that do not terminate their database connection are caught in the TRY/CATCH block.
For errors that are not trapped, SQL Server 2005 passes control back to the application immediately, without executing any CATCH block code.

Similar to C++, Java and other languages SQL Server also has a mechanism to handle exceptions by using TRY-CATCH construct. The TRY block contains the SQL statements that may raise an error and CATCH block contains the handling mechanism to process the error. When any error is raised in the TRY block the control is immediately transferred to the CATCH block, where the Error is handled.
–> Following rules should be taken care off while using TRY-CATCH constructs:
– A TRY block must be followed immediately by the CATCH block.
– Both TRY & CATCH blocks must be inside a Batch, Stored Procedure or a Trigger.
– Only Errors with severity between 10 & 20 that do not close the database connection are caught & handled by TRY-CATCH constructs.
– As per MS BOL, Errors that have a severity of 20 or higher that cause the Database Engine to close the connection will not be handled by the TRY…CATCH block. And Errors that have a severity of 10 or lower are considered warnings or informational messages, and are not handled by TRY…CATCH blocks.
–> let’s check how to use TRY-CATCH block:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
USE [tempdb]
GO
 
--// Create a test Stored Procedure
CREATE PROC testPrc (@val VARCHAR(10))
AS
BEGIN
 SELECT 1/@val AS operation
END
GO
 
--// Test for Divide by 0 (Divide by zero error encountered.)
BEGIN TRY
 EXEC testPrc '0'
END TRY
BEGIN CATCH
 SELECT
 ERROR_NUMBER() AS ERROR_ID,
 ERROR_MESSAGE() AS ERROR_MSG,
 ERROR_SEVERITY() AS ERROR_SEVERITY,
 ERROR_STATE() AS ERROR_STATE,
 ERROR_PROCEDURE() AS ERROR_PROCEDURE,
 ERROR_LINE() AS ERROR_LINE
END CATCH
GO
 
--// Test for Datatype conversion (Conversion failed when converting the varchar value 'a' to data type int.)
BEGIN TRY
 EXEC testPrc 'a'
END TRY
BEGIN CATCH
 SELECT
 ERROR_NUMBER() AS ERROR_ID,
 ERROR_MESSAGE() AS ERROR_MSG,
 ERROR_SEVERITY() AS ERROR_SEVERITY,
 ERROR_STATE() AS ERROR_STATE,
 ERROR_PROCEDURE() AS ERROR_PROCEDURE,
 ERROR_LINE() AS ERROR_LINE
END CATCH
GO
 
--// Test nested TRY-CATCH for "Divide by 0" & "Datatype conversion" errors both.
BEGIN TRY
 EXEC testPrc 'a'
END TRY
BEGIN CATCH
 SELECT 'outer block',
 ERROR_NUMBER() AS ERROR_ID,
 ERROR_MESSAGE() AS ERROR_MSG,
 ERROR_SEVERITY() AS ERROR_SEVERITY,
 ERROR_STATE() AS ERROR_STATE,
 ERROR_PROCEDURE() AS ERROR_PROCEDURE,
 ERROR_LINE() AS ERROR_LINE
 
 BEGIN TRY
 SELECT 1/0 AS operation
 END TRY
 BEGIN CATCH
 SELECT 'inner block',
 ERROR_NUMBER() AS ERROR_ID,
 ERROR_MESSAGE() AS ERROR_MSG,
 ERROR_SEVERITY() AS ERROR_SEVERITY,
 ERROR_STATE() AS ERROR_STATE,
 ERROR_PROCEDURE() AS ERROR_PROCEDURE,
 ERROR_LINE() AS ERROR_LINE
 END CATCH
 
END CATCH
GO
 
--// Test for violation of PK Constraint (Violation of PRIMARY KEY constraint 'PK__testTable__2C3393D0'. Cannot insert duplicate key in object 'dbo.testTable'.)
BEGIN TRY
 CREATE TABLE testTable (a INT PRIMARY KEY)
 
 INSERT INTO testTable VALUES(1)
 INSERT INTO testTable VALUES(1)
END TRY
BEGIN CATCH
 SELECT
 ERROR_NUMBER() AS ERROR_ID,
 ERROR_MESSAGE() AS ERROR_MSG,
 ERROR_SEVERITY() AS ERROR_SEVERITY,
 ERROR_STATE() AS ERROR_STATE,
 ERROR_PROCEDURE() AS ERROR_PROCEDURE,
 ERROR_LINE() AS ERROR_LINE
END CATCH
GO
 
SELECT * FROM testTable -- Contains single record with value 1
 
--// Test for recreating a table that already exists (There is already an object named 'testTable' in the databASe.)
BEGIN TRY
 CREATE TABLE testTable (a INT PRIMARY KEY)
END TRY
BEGIN CATCH
 SELECT
 ERROR_NUMBER() AS ERROR_ID,
 ERROR_MESSAGE() AS ERROR_MSG,
 ERROR_SEVERITY() AS ERROR_SEVERITY,
 ERROR_STATE() AS ERROR_STATE,
 ERROR_PROCEDURE() AS ERROR_PROCEDURE,
 ERROR_LINE() AS ERROR_LINE
END CATCH
GO
 
--// Test for inserting NULL value on Primary Key column (Cannot insert the value NULL into column 'a', table 'tempdb.dbo.testTable'; column does not allow nulls. INSERT fails.)
BEGIN TRY
 INSERT INTO testTable VALUES(NULL)
END TRY
BEGIN CATCH
 SELECT
 ERROR_NUMBER() AS ERROR_ID,
 ERROR_MESSAGE() AS ERROR_MSG,
 ERROR_SEVERITY() AS ERROR_SEVERITY,
 ERROR_STATE() AS ERROR_STATE,
 ERROR_PROCEDURE() AS ERROR_PROCEDURE,
 ERROR_LINE() AS ERROR_LINE
END CATCH
GO
 
--// Final Cleanup
DROP TABLE     testTable
DROP PROC testPrc
GO


TRY…CATCH  Block in Stored Procedures

Since the release of SQL Server 2005, you’ve been able to handle errors in your T-SQL code by including a TRY…CATCH block that controls the flow of your script should an error occur, similar to how procedural languages have traditionally handled errors. The TRY…CATCH block makes it easy to return or audit error-related data, as well as take other actions. And within the block—specifically, the CATCH portion—you’ve been able to include a RAISERROR statement in order to re-throw error-related data to the calling application. However, with the release of SQL Server 2012, you now have a replacement for RAISERROR, the THROW statement, which makes it easier than ever to capture the error-related data.
In this article, we’ll look at the TRY…CATCH block used with both the RAISERROR and THROW statements. The examples are based on a table I created in the AdventureWorks2012 sample database, on a local instance of SQL Server 2012. Listing 1 shows the T-SQL script I used to create the LastYearSales table.

USE AdventureWorks2012;
GO

IF OBJECT_ID('LastYearSales', 'U') IS NOT NULL
DROP TABLE LastYearSales;
GO

SELECT
  BusinessEntityID AS SalesPersonID,
  FirstName + ' ' + LastName AS FullName,
  SalesLastYear
INTO
  LastYearSales
FROM
  Sales.vSalesPerson
WHERE
  SalesLastYear > 0;
GO
Listing 1: Creating the LastYearSales table

The script should be fairly straightforward. I use a SELECT…INTO statement to retrieve data from the Sales.vSalesPerson view and insert it into the newly created table. However, to demonstrate how to handle errors, we need to add one more element to our table: a check constraint that ensures the SalesLastYear value is never less than zero. Listing 2 shows the ALTERTABLE statement I used to add the constraint.

ALTER TABLE LastYearSales
ADD CONSTRAINT ckSalesTotal CHECK (SalesLastYear >= 0);
GO
Listing 2: Adding a check constraint to the LastYearSales table

The constraint makes it easy to generate an error when updating the table. All I have to do is try to add a negative amount to the SalesLastYear column, an amount large enough to cause SQL Server to throw an error. Once we’ve created our table and added the check constraint, we have the environment we need for the examples in this article. You can just as easily come up with your own table and use in the examples. Just be sure you have a way of violating a constraint or you come up with another mechanism to generate an error. The goal is to create a script that handles any errors.

Working with the TRY…CATCH Block

Once we’ve set up our table, the next step is to create a stored procedure that demonstrates how to handle errors. The procedure, UpdateSales, modifies the value in the SalesLastYear column in the LastYearSales table for a specified salesperson. It works by adding or subtracting an amount from the current value in that column. Listing 3 shows the script I used to create the procedure. Notice that I include two input parameters—@SalesPersonID and @SalesAmt—which coincide with the table’s SalesPersonID and SalesLastYear columns.

USE AdventureWorks2012;
GO

IF OBJECT_ID('UpdateSales', 'P') IS NOT NULL
DROP PROCEDURE UpdateSales;
GO

CREATE PROCEDURE UpdateSales
  @SalesPersonID INT,
  @SalesAmt MONEY = 0
AS
BEGIN
  BEGIN TRY
    BEGIN TRANSACTION;
      UPDATE LastYearSales
      SET SalesLastYear = SalesLastYear + @SalesAmt
      WHERE SalesPersonID = @SalesPersonID;
    COMMIT TRANSACTION;
  END TRY
  BEGIN CATCH
    IF @@TRANCOUNT > 0
    ROLLBACK TRANSACTION;

    DECLARE @ErrorNumber INT = ERROR_NUMBER();
    DECLARE @ErrorLine INT = ERROR_LINE();
    DECLARE @ErrorMessage NVARCHAR(4000) = ERROR_MESSAGE();
    DECLARE @ErrorSeverity INT = ERROR_SEVERITY();
    DECLARE @ErrorState INT = ERROR_STATE();

    PRINT 'Actual error number: ' + CAST(@ErrorNumber AS VARCHAR(10));
    PRINT 'Actual line number: ' + CAST(@ErrorLine AS VARCHAR(10));

    RAISERROR(@ErrorMessage, @ErrorSeverity, @ErrorState);
  END CATCH
END;
GO
Listing 3: Creating a stored procedure that contains a Try…Catch block

The main body of the procedure definition, enclosed in the BEGIN…END block, contains the TRY…CATCH block, which itself is divided into the TRY block and the CATCH block. The TRY block starts with BEGINTRY and ends with ENDTRY and encloses the T-SQL necessary to carry out the procedure’s actions. In this case, I include an UPDATE statement that adds the @SalesAmount value to the SalesLastYear column. The statement is enclosed in BEGINTRANSACTION and COMMITTRANSACTION statements to explicitly start and commit the transaction. Examples vary in terms of where they include the transaction-related statements. (Some don’t include the statements at all.) Just keep in mind that you want to commit or rollback your transactions at the appropriate times, depending on whether an error has been generated.
If the UPDATE statement runs successfully, the SalesLastYear value is updated and the operation is completed, in which case, the code in the CATCH block is never executed. However, if the UPDATE statement fails and SQL Server generates an error, the transaction is terminated and the database engine jumps to the CATCH block. The CATCH block starts with BEGINCATCH and ends with ENDCATCH and encloses the statements necessary to handle the error.
For the stored procedure in Listing 3, the first step I take in the CATCH block is to roll back the transaction if it is still running. I start by using the @@TRANCOUNT function to determine whether any transactions are still open. @@TRANCOUNT is a built-in SQL Server function that returns the number of running transactions in the current session. In this case, there should be only one (if an error occurs), so I roll back that transaction.
Next, I declare a set of variables based on system functions that SQL Server makes available within the scope of the CATCH block. The functions return error-related information that you can reference in your T-SQL statements. Currently, SQL Server supports the following functions for this purpose:
  • ERROR_NUMBER(): The number assigned to the error.
  • ERROR_LINE(): The line number inside the routine that caused the error.
  • ERROR_MESSAGE(): The error message text, which includes the values supplied for any substitutable parameters, such as times or object names.
  • ERROR_SEVERITY(): The error’s severity.
  • ERROR_STATE(): The error’s state number.
  • ERROR_PROCEDURE(): The name of the stored procedure or trigger that generated the error.
For this example, I use all but the last function, though in a production environment, you might want to use that one as well.
After I declare the variables, I include two PRINT statements that display the values of the @ErrorNumber and @ErrorLine variables (along with some explanatory text). The reason I do this is to demonstrate the difference between what the actual values are and what the RAISERROR statement returns, as you’ll see shortly.
The RAISERROR statement comes after the PRINT statements. The statement returns error information to the calling application. Generally, when using RAISERROR, you should include an error message, error severity level, and error state. The rules that govern the RAISERROR arguments and the values they return are a bit complex and beyond the scope of this article, but for the purposes of this example, I simply pass in the @ErrorMessage, @ErrorSeverity, and @ErrorState variables as arguments.
NOTE: For more information about the RAISERROR statement, see the topic “RAISERROR (Transact-SQL)” in SQL Server Books Online.
That’s basically all you need to do to create a stored procedure that contains a TRY…CATCH block. In a moment, we’ll try out our work. But first, let’s retrieve a row from the LastYearSales table to see what the current value is for salesperson 288. Listing 4 shows the SELECT statement I used to retrieve the data.

SELECT FullName, SalesLastYear
FROM LastYearSales
WHERE SalesPersonID = 288

Listing 4: Retrieving date from the LastYearSales table

Not surprisingly, the statement returns the name and total sales for this salesperson, as shown in Listing 5.
As you can see, Rachel Valdez shows over $1.3 million dollars in sales for last year.

FullName  SalesLastYear
Rachel Valdez  1307949.7917

Listing 5: Data retrieved from the LastYearSales table

Now let’s try out the UpdateSales stored procedure. Just for fun, let’s add a couple million dollars to Rachel Valdez’s totals. Listing 6 shows how I use the EXEC statement to call the procedure and pass in the salesperson ID and the $2 million.

EXEC UpdateSales 288, 2000000;

Listing 6: Running the UpdateSales stored procedure

The stored procedure should run with no problem because we’re not violating the check constraint. If we were to execute the SELECT statement again (the one in Listing 4), our results would look similar to those shown in Listing 7.

Notice all the extra cash.

FullName  SalesLastYear
Rachel Valdez  3307949.7917

Listing 7: Viewing the updated sales amount in the LastYearSales table

Now let’s look what happens if we subtract enough from her account to bring her totals to below zero. In listing 8, I run the procedure once again, but this time specify -4000000 for the amount.

EXEC UpdateSales 288, -4000000;

Listing 8: Causing the UpdateSales stored procedure to throw an error

As you’ll recall, after I created the LastYearSales table, I added a check constraint to ensure that the amount could not fall below zero. As a result, the stored procedure now generates an error, which is shown in Listing 9.

 (0 row(s) affected)
Actual error number: 547
Actual line number: 9
Msg 50000, Level 16, State 0, Procedure UpdateSales, Line 27

The UPDATE statement conflicted with the CHECK constraint "ckSalesTotal". The conflict occurred in database "AdventureWorks2012", table "dbo.LastYearSales", column 'SalesLastYear'.

Listing 9: The error message returned by the UpdateSales stored procedure

As expected, the information we included in the CATCH block has been returned. But notice that the actual error number (547) is different from the RAISERROR message number (50000) and that the actual line number (9) is different from the RAISERROR line number (27). In theory, these values should coincide. But as I mentioned earlier, the rules that govern RAISERROR are a bit quirky.

Working with the THROW Statement

To simplify returning errors in a CATCH block, SQL Server 2012 introduced the THROW statement. With the THROW statement, you don’t have to specify any parameters and the results are more accurate. You simply include the statement as is in the CATCH block.
NOTE: You can use the THROW statement outside of the CATCH block, but you must include parameter values to do so. For more information about the THROW statement, see the topic “THROW (Transact-SQL)” in SQL Server Books Online.

To demonstrate the THROW statement, I defined an ALTER PROCEDURE statement that modifies the UpdateSales procedure, specifically the CATCH block, as shown in Listing 10.

ALTER PROCEDURE UpdateSales
  @SalesPersonID INT,
  @SalesAmt MONEY = 0
AS
BEGIN
  BEGIN TRY
    BEGIN TRANSACTION;
      UPDATE LastYearSales
      SET SalesLastYear = SalesLastYear + @SalesAmt
      WHERE SalesPersonID = @SalesPersonID;
    COMMIT TRANSACTION;
  END TRY
  BEGIN CATCH
    IF @@TRANCOUNT > 0
    ROLLBACK TRANSACTION;

    DECLARE @ErrorNumber INT = ERROR_NUMBER();
    DECLARE @ErrorLine INT = ERROR_LINE();

    PRINT 'Actual error number: ' + CAST(@ErrorNumber AS VARCHAR(10));
    PRINT 'Actual line number: ' + CAST(@ErrorLine AS VARCHAR(10));

    THROW;
  END CATCH
END;
GO
Listing 10: Altering the UpdateSales stored procedure

Notice that I retain the @ErrorNumber and @ErrorLine variable declarations and their related PRINT statements. I do so only to demonstrate the THROW statement’s accuracy. In actually, I need only to roll back the transaction and specify the THROW statement, without any parameters.
Now let’s execute the stored procedure again, once more trying to deduct $4 million from the sales amount, as shown in Listing 11.

EXEC UpdateSales 288, -4000000;

Listing 11: Causing the UpdateSales stored procedure to throw an error

Once again, SQL Server returns an error. Only this time, the information is more accurate. As you can see in Listing 12, the message numbers and line numbers now match. No longer do we need to declare variables or call system functions to return error-related information to the calling application.

 (0 row(s) affected)
Actual error number: 547
Actual line number: 8
Msg 547, Level 16, State 0, Procedure UpdateSales, Line 8
The UPDATE statement conflicted with the CHECK constraint "ckSalesTotal". The conflict occurred in database "AdventureWorks2012", table "dbo.LastYearSales", column 'SalesLastYear'.

Listing 12: The error message returned by the UpdateSales stored procedure


As you can see, SQL Server 2012 makes handling errors easier than ever. Even if you’ve been using the TRY…CATCH block for a while, the THROW statement should prove a big benefit over RAISERROR. And if you’re new to error handling in SQL Server, you’ll find that the TRY…CATCH block and the THROW statement together make the process a fairly painless one, one well worth the time and effort it takes to learn and implement them.