SQL将变量赋值给SELECT语句中的函数结果

Question

我有一个SELECT语句，它多次使用函数的结果。有没有一种方法可以为其分配一个变量，以便使语句更有效，而且我不必继续使用该功能？

过度简化的假设示例代码：

DECLARE @Total Decimal(18,2)

SELECT t.ClientName AS Customer, t.ClientID,
--This function returns total sales for the Client
@Total = dbo.functionSalesTotal(t.ClientID), 
@Total AS Sales, (@Total * 0.13) AS Tax, 
(@Total + (@Total *0.13)) AS TotalIncludingTax
FROM table t

Answer 1

子查询或apply是最好的方法：

SELECT t.ClientName AS Customer, t.ClientID, 
       v.total AS Sales, (v.Total * 0.13) AS Tax, 
       (v.Total + (v.Total *0.13)) AS TotalIncludingTax
FROM table t CROSS APPLY
     (VALUES (dbo.functionSalesTotal(t.ClientID)) as v(total);