SQL Server 2000中的触发器使用2

示例 　　在本例中，创建了一个德国客户表和一个墨西哥客户表。放置在视图上的INSTEAD OF触发器将把更新操作重新定向到适当的基表上。这时发生的插入是对CustomersGer表的插入而不是对视图的插入。 　　创建两个包含客户数据的表：

SELECT * INTO CustomersGer FROM Customers WHERE Customers.Country = ' Germany ' SELECT * INTO CustomersMex FROM Customers WHERE Customers.Country = ' Mexico ' GO

　　在该数据上创建视图：

CREATE VIEW CustomersView AS SELECT * FROM CustomersGer UNION SELECT * FROM CustomersMex GO

　　创建一个在上述视图上的INSTEAD OF触发器：

CREATE TRIGGER Customers_Update2 ON CustomersView INSTEAD OF UPDATE AS DECLARE @Country nvarchar(15) SET @Country = (SELECT Country FROM Inserted) IF @Country = 'Germany' BEGIN UPDATE CustomersGer SET CustomersGer.Phone = Inserted.Phone FROM CustomersGer JOIN Inserted ON CustomersGer.CustomerID = Inserted.CustomerID END ELSE IF @Country = 'Mexico' BEGIN UPDATE CustomersMex SET CustomersMex.Phone = Inserted.Phone FROM CustomersMex JOIN Inserted ON CustomersMex.CustomerID = Inserted.CustomerID END

　　通过更新视图，测试触发器：

UPDATE CustomersView SET Phone = ' 030-007xxxx' WHERE CustomerID = 'ALFKI' SELECT CustomerID, Phone FROM CustomersView WHERE CustomerID = 'ALFKI' SELECT CustomerID, Phone FROM CustomersGer WHERE CustomerID = 'ALFKI'

　　那么具体的讲，对于多列数据，如何计算方差呢？：

CREATE TRIGGER [calT1T2T3] ON dbo.DCLB FOR INSERT,UPDATE AS update P SET /**//* 计算方差的触发器 */ P.T1=(I.P1+I.P2+I.P3+I.P4+I.P5+I.P6), P.T2=(I.Y1+I.Y2+I.Y3+I.Y4+I.Y5+I.Y6 ), P.T3=SQRT(P.T1*P.T1+P.T2*P.T2) FROM DCLB AS P INNER JOIN Inserted AS I ON P.SID = I.SID

　　触发器的使用很方便，而且也很简单，重要的是理解inserted过程。可将UPDATE语句看成两步操作：即捕获数据前像(before image)的DELETE语句，和捕获数据后像(after image)的INSERT语句。当在定义有触发器的表上执行UPDATE语句时，原始行（前像）被移入到deleted表，更新行（后像）被移入到inserted表。触发器检查deleted表和inserted表以及被更新的表，来确定是否更新了多行以及如何执行触发器动作。