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SQL Server 2000 Stored Procedure Programming
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+ '=' + @chvValue + ' ' + if @debug <> 0
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@chvUnit + '; '
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select @chvProperties chvProperties -- let's go another round and get another property set @intCounter = @intCounter + 1 end drop table #Properties if @debug <> 0 select '**** '+ @chvProcedure + 'END ****' return 0
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Execution in a Test Environment To debug or test a stored procedure, I execute the stored procedure from Query Analyzer with the @debug parameter set to 1.
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declare @chvResult varchar(8000) exec prGetInventoryProperties @intInventoryId = 5, @chvProperties = @chvResult OUTPUT, @debug = 1 select @chvResult Result
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Naturally, you can pass parameters either by name or by position. The result of the execution will be an elaborate printout like the one shown in Figure 7-6. Execution in the Production Environment In production, the stored procedure is called without a reference to the @debug parameter. Here, SQL Server assigns a default value to the parameter (0), and the stored procedure is executed without debug statements.
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exec prGetInventoryProperties @intInventoryId = 5, @chvProperties = @chvResult OUTPUT
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Figure 7-6.
Poor Man s Debugger
Nested Stored Procedures Two tricks can help you debug a set of nested stored procedures (that is, when a stored procedure calls another stored procedure). It is a useful practice to display the name of the stored procedure at the beginning and end of the stored procedure.
declare @chvProcedure sysname set @chvProcedure = 'prGetInventoryProperties_2' if @debug <> 0 select '**** '+ @chvProcedure + 'START ****' ... if @debug <> 0 select '**** '+ @chvProcedure + 'END ****' return 0
SQL Server 2000 Stored Procedure Programming
When you call a nested stored procedure, you need to pass the value of the @debug parameter to it as well. In this way, you will be able to see its debugging information.
exec prGetInventoryProperties @intInventoryId, @chvProperties OUTPUT, @debug
Typical Errors
You should keep the following issues in mind when you are writing your code and testing Transact-SQL programs:
w s s s s s s s s s v
NULLS Assignment of variable from resultset No records affected Wrong size or datatype Default length Rollback of triggers Warnings and lower-priority errors Nested comments Deferred name resolution Cursors Overconfidence
NULLs
Many errors are a result of the inadequate treatment of NULL values in Transact-SQL code. Developers often forget that local variables or table columns might contain NULLs. If such a value becomes part of any expression, the result will also be NULL. The proper way to test the value of an expression for NULLs is to use the IS NULL or IS NOT NULL clauses. Microsoft SQL Server treats
7:
Debugging and Error Handling
the use of = NULL as another way to type IS NULL, but <> NULL is not the equivalent of IS NOT NULL. The result of such an expression is always simply NULL. It will never be true, and stored procedures will always skip statements after the If statement when you use the <> NULL clause.
If @intInventoryId IS NULL ... If @intInventoryId = NULL ... If @intInventoryId IS NOT NULL ... If @intInventoryId <> NULL ... -- WRONG!!!
Assignment of Variable from Resultset
Earlier, we discussed assigning the value(s) for a variable(s) using the resultset of the Select statement. This technique is fine when the resultset returns precisely one record. However, if the resultset returns more than one record, the variable(s) are assigned using the value(s) from the last record in recordset. Not perfect, but in some cases, you can live with it. It is sometimes difficult to predict which record will be returned as last in the recordset. It depends on the query and the index that SQL Server has used. A more serious problem occurs when the recordset is empty. The values of the variables are changed in this case, and the code is vulnerable to several mistakes. If you do not expect the resultset to be empty, your stored procedure will fail. If you expect the values of the variables to be NULL, your stored procedure will function correctly only immediately after it is started (that is, in the first iteration of the process). In such a case, the local variables are not yet initialized and will contain NULLs. Later, when variables are initialized, their values will remain unchanged. If you are testing the contents of the variables for NULLs to find out if the record was selected, you will just process the previous record again.
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