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For many, the question is why be concerned with implementing error handling at all Let us review this question through the following example:
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Create Procedure prInsertLeasedAsset_1 -- Insert leased asset and update total in LeaseSchedule. -- (demonstration of imperfect solution) ( @intEquipmentId int, @intLocationId int, @intStatusId int, @intLeaseId int, @intLeaseScheduleId int, @intOwnerId int, @mnyLease money, @intAcquisitionTypeID int ) As set nocount on begin transaction -- insert asset insert Inventory(EquipmentId, StatusId, LeaseScheduleId, Lease, values ( @intEquipmentId, @intStatusId, @intLeaseScheduleId, @mnyLease,
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LocationId, LeaseId, OwnerId, AcquisitionTypeID) @intLocationId, @intLeaseId, @intOwnerId, @intAcquisitionTypeID)
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7: Debugging and Error Handling
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-- update total update LeaseSchedule Set PeriodicTotalAmount = PeriodicTotalAmount + @mnyLease where LeaseId = @intLeaseId commit transaction return
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This may seem like a trivial example, and it is true that in all probability nothing would go wrong, but imagine that an error occurs on the Update statement. The error could be for any reason overflow, violation of a constraint, or inadequate security privileges, for example. As explained earlier, transactions do not automatically roll back when an error occurs. Instead, SQL Server simply commits everything that was changed when it encounters the Commit Transaction statement as if nothing unusual had happened. Unfortunately, from that moment on, the total of the lease schedule will have the wrong value.
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Tactics of Error Handling
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Most developers recognize the importance of this issue and place error handling in critical positions in their code. The result would be something like the following:
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Create Procedure prInsertLeasedAsset_2 -- Insert leased asset and update total in LeaseSchedule. -- (demonstration of not exactly perfect solution) ( @intEquipmentId int, @intLocationId int, @intStatusId int, @intLeaseId int, @intLeaseScheduleId int, @intOwnerId int, @mnyLease money, @intAcquisitionTypeID int ) As set nocount on begin transaction -- insert asset insert Inventory(EquipmentId,
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LocationId,
SQL Server 2000 Stored Procedure & XML Programming
values (
StatusId, LeaseScheduleId, Lease, @intEquipmentId, @intStatusId, @intLeaseScheduleId, @mnyLease,
LeaseId, OwnerId, AcquisitionTypeID) @intLocationId, @intLeaseId, @intOwnerId, @intAcquisitionTypeID)
If @@error <> 0 Begin Print 'Unexpected error occurred!' Rollback transaction Return 1 End -- update total update LeaseSchedule Set PeriodicTotalAmount = PeriodicTotalAmount + @mnyLease where LeaseId = @intLeaseId If @@error <> 0 Begin Print 'Unexpected error occurred!' Rollback transaction Return 1 End commit transaction return 0
This kind of solution contains substantial repetition especially if your business logic requires more than two Transact-SQL statements to be implemented. A more elegant solution is to group codes into a generic error handling procedure:
Create Procedure prInsertLeasedAsset_3 -- Insert leased asset and update total in LeaseSchedule. -- (demonstration of not exactly perfect solution) ( @intEquipmentId int, @intLocationId int, @intStatusId int, @intLeaseId int, @intLeaseScheduleId int, @intOwnerId int, @mnyLease money,
7: Debugging and Error Handling
@intAcquisitionTypeID int ) As set nocount on begin transaction -- insert asset insert Inventory(EquipmentId, StatusId, LeaseScheduleId, Lease, values ( @intEquipmentId, @intStatusId, @intLeaseScheduleId, @mnyLease, If @@error <> 0 GOTO ERR_HANDLER
LocationId, LeaseId, OwnerId, AcquisitionTypeID) @intLocationId, @intLeaseId, @intOwnerId, @intAcquisitionTypeID)
-- update total update LeaseSchedule Set PeriodicTotalAmount = PeriodicTotalAmount + @mnyLease where LeaseId = @intLeaseId If @@error <> 0 GOTO ERR_HANDLER commit transaction return 0 ERR_HANDLER: Print 'Unexpected error occurred!' Rollback transaction Return 1
This is better, but it does not deal with all the issues that need to be handled. A typical error that beginners in T-SQL make is to check the value of a global variable and then try to return or process it. Such an attempt is usually the result of a good intention, such as wanting to notify the user of an error that has occurred.
Create Procedure prInsertLeasedAsset_4 -- Insert leased asset and update total in LeaseSchedule. -- (demonstration of not exactly perfect solution) ( @intEquipmentId int, @intLocationId int, @intStatusId int,
SQL Server 2000 Stored Procedure & XML Programming
@intLeaseId int, @intLeaseScheduleId int, @intOwnerId int, @mnyLease money, @intAcquisitionTypeID int ) As set nocount on begin transaction -- insert asset insert Inventory(EquipmentId, StatusId, LeaseScheduleId, Lease, values ( @intEquipmentId, @intStatusId, @intLeaseScheduleId, @mnyLease, If @@error <> 0 GOTO ERR_HANDLER
LocationId, LeaseId, OwnerId, AcquisitionTypeID) @intLocationId, @intLeaseId, @intOwnerId, @intAcquisitionTypeID)
-- update total update LeaseSchedule Set PeriodicTotalAmount = PeriodicTotalAmount + @mnyLease where LeaseId = @intLeaseId If @@error <> 0 GOTO ERR_HANDLER commit transaction return 0 ERR_HANDLER: Print 'Unexpected error occurred: ' + Convert(varchar, @@error) - this will -- not work, -- as expected Rollback transaction Return @@error
Although something like this could work in Visual Basic, for example, in this case the stored procedure will return 0 as an error number. SQL Server sets the value of the @@error variable after each statement. It treats each statement separately, so the value
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