datamatrix c# library Creating a DML FOR Trigger in C#

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Creating a DML FOR Trigger
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The first trigger we will be looking at is a DML trigger. The following example will demonstrate how to create a trigger on a data insertion, but also what happens to that INSERT when there is a problem in the trigger itself. As we are near the end of the book, our T-SQL within the trigger will be more advanced than some of the code so far.
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Try It Out: Creating a Trigger in Query Editor
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The purpose of our example is to change a customer s account balance when a financial transaction occurs as defined by an INSERT in the TransactionDetails.Transactions table. We want to change the balance AFTER the row has been inserted into the TransactionDetails.Transactions table. This is so we do not change the customer s account balance if later in the INSERT of the row a problem occurs and the INSERT does not complete. 1. Ensure that Query Editor is running and that you are logged in with an ID that can insert objects into the database. First of all, it is necessary to give the trigger a meaningful name. Then you define the table that the trigger will be attached to, which in this case is the TransactionDetails.Transactions table. The final part of the start of the trigger will then define the type of trigger and on what actions the trigger will execute on. This will be a FOR AFTER trigger on an INSERT on the TransactionDetails.Transactions table. The first part of the code looks as follows: CREATE TRIGGER trgInsTransactions ON TransactionDetails.Transactions AFTER INSERT AS
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CHAPTER 13 TRIGGERS
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2. It is now time to enter the remainder of the code for the trigger. We need to retrieve the Amount and TransactionType from the INSERTED table to be able to use these in the update of the CustomerDetails.Customers table. We can JOIN from the INSERTED table to the TransactionDetails.TransactionTypes table to find out whether we are dealing with a credit or a debit transaction. If it is a debit, then through the use of a subquery and a CASE statement we can alter the Amount by multiplying it by -1 so that we are reducing a customer s balance. Notice the subquery includes a WHERE statement so that if we are entering a transaction type that does not affect the cash balance, such as recording a share movement, then the ClearedBalance will not be altered. The final action is to update the customer s balance, which we will do via an UPDATE statement. There is a great deal to take in, so take time over the code. Also, the two examples of running this trigger should clear up any queries you will have.
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Note This trigger does have a deliberate bug, which is included so that you can see a little later in this
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section what happens when a trigger has a bug.
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UPDATE CustomerDetails.Customers SET ClearedBalance = ClearedBalance + (SELECT CASE WHEN CreditType = 0 THEN i.Amount * -1 ELSE i.Amount END FROM INSERTED i JOIN TransactionDetails.TransactionTypes tt ON tt.TransactionTypeId = i.TransactionType WHERE AffectCashBalance = 1) FROM CustomerDetails.Customers c JOIN INSERTED i ON i.CustomerId = c.CustomerId 3. Execute the code to create the trigger in the database. We can test the trigger now by inserting a cash withdrawal or deposit relating to the two transaction types we currently have. We will list the customer balance before executing the INSERT into the TransactionDetails.Transactions table, and then we will add the row and look at the balance again to show that it has changed. Enter the following code, which inserts a withdrawal of $200 from Vic McGlynn s account: SELECT ClearedBalance FROM CustomerDetails.Customers WHERE customerId=1 INSERT INTO TransactionDetails.Transactions (CustomerId,TransactionType, Amount,RelatedProductId, DateEntered) VALUES (1,2,200,1,GETDATE()) SELECT ClearedBalance FROM CustomerDetails.Customers WHERE customerId=1
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