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Composite Transact-SQL Constructs Batches, Scripts, and Transactions
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Nested Transactions
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SQL Server allows you to nest transactions. Basically, this feature means that a new transaction can start even though the previous one is not complete:
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Begin transaction ... Begin transaction ... Commit transaction ... Commit transaction
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Usually this situation occurs when one stored procedure containing a transaction calls another stored procedure that also contains a transaction. In the following example, prCompleteOrder completes an order by setting its completion date and changing the status of the order, and then looping through associated order items and calling prCompleteOrderItem to complete each of them; prCompleteOrderItem sets the completion date of an order item to the last ChargeLog date associated with that OrderItem. Both of these procedures contain a transaction:
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Alter Procedure prCompleteOrder_1 -- complete all orderItems and then complete order @intOrderId int, @dtsCompletionDate smalldatetime As set nocount on Declare @intErrorCode int, @i int, @intCountOrderItems int, @intOrderItemId int Select @intErrorCode = @@Error
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SQL Server 2000 Stored Procedure Programming
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If @intErrorCode = 0 Begin Transaction -- complete order If @intErrorCode = 0 Begin Update [Order] Set CompletionDate = @dtsCompletionDate, OrderStatusId = 4 -- completed Where OrderId = @intOrderId Select @intErrorCode = @@Error End
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-- loop through OrderItems and complete them If @intErrorCode = 0 Begin Create Table #OrderItems( id int identity(1,1), OrderItemId int) Select @intErrorCode = @@Error End
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-- collect orderItemIds If @intErrorCode = 0 Begin Insert Into #OrderItems(OrderItemId) Select ItemId From OrderItem Where OrderId = @intOrderId Select @intErrorCode = @@Error End
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Composite Transact-SQL Constructs Batches, Scripts, and Transactions
If @intErrorCode = 0 Begin Select @intCountOrderItems = Max(Id), @i = 1 From #OrderItems Select @intErrorCode = @@Error End while @intErrorCode = 0 and @i <= @intCountOrderItems Begin If @intErrorCode = 0 Begin Select @intOrderItemId = OrderItemId From #OrderItems Where id = @I Select @intErrorCode = @@Error End If @intErrorCode = 0 Exec @intErrorCode = prCompleteOrderItem_1 @intOrderItemId If @intErrorCode = 0 Set @i = @i + 1 End If @intErrorCode = 0 and @@trancount > 0 Commit Transaction Else Rollback Transaction return @intErrorCode Go
Alter Procedure prCompleteOrderItem_1 -- Set CompletionDate of OrderItem to date
SQL Server 2000 Stored Procedure Programming
-- of last ChargeLog record associated with OrderItem. @intOrderItemId int As set nocount on Declare @intErrorCode int Select @intErrorCode = @@Error If @intErrorCode = 0 Begin Transaction -- Set CompletionDate of OrderItem to date -- of last ChargeLog record associated with OrderItem. If @intErrorCode = 0 Begin update OrderItem Set CompletionDate = (Select Max(ChargeDate) from ChargeLog where ItemId = @intOrderItemId) Where ItemId = @intOrderItemId Select @intErrorCode = @@Error End If @intErrorCode = 0 and @@trancount > 0 Commit Transaction Else Rollback Transaction Return @intErrorCode
In the case of nested transactions, no Commit statements except the outer one will save changes to the database. Only after the last transaction is committed will all changes to the database become permanent. Up to that point, it is still possible to roll back all changes. The interesting question is how SQL Server knows which transaction is the last one. It keeps the number of opened transactions
6:
Composite Transact-SQL Constructs Batches, Scripts, and Transactions
in the @@trancount global variable for each user connection. When SQL Server encounters a Begin Transaction statement, it increments the value of the @@trancount, and when SQL Server encounters a Commit Transaction statement, it decrements the value of the @@trancount. Therefore, the only effect of a nested (internal) Commit Transaction statement is a change to the @@trancount value. Only the outer Commit Transaction statement (when @@trancount = 1) stores changes in data tables rather than in the transaction log. Figure 6-3 shows an academic example that does not contain any real processing, but that demonstrates the effect of nested stored procedures on the @@trancount global variable. An interesting inconsistency to observe is in the behavior of the Rollback Transaction statement. No matter how many transaction levels deep execution extends, the Rollback Transaction statement will cancel all changes caused by all transactions (and bring the @@trancount value down to zero).
Figure 6-3.
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