我可以在 WHILE 循环中使用 CASE 语句吗？

Question

这就是我正在做的：

while (@counter < 3 and @newBalance >0)
begin   
CASE
when  @counter = 1 then  ( @monFee1 = @monthlyFee, @newBalance = @newBalance-@fee)
when  @counter = 2 then  ( @monFee2 = @monthlyFee, @newBalance = @newBalance-@fee)
END
@counter = @counter +1
end

我收到此错误：

关键字“CASE”附近的语法不正确。

不知道为什么。请帮忙！

Answer 1

不，SQL中的CASE结构是为了返回一个值，而不是为了程序流程。您需要将其分解为 IF 语句。

Answer 2

对于您所提议的内容，您应该使用 IF 语句

While (@counter < 3 and @newBalance >0)
Begin
    If @Counter = 1 Then
        Begin
            Set @monFee1 = @monthlyFee
            Set @newBalance = @newBalance-@fee
        End

    If @Counter = 2 Then
        Begin
            Set @monFee2 = @monthlyFee
            Set @newBalance = @newBalance-@fee
        End

    Set @counter = @counter +1 
End

Answer 3

CASE 语句不像过程代码中的同类语句那样用于分支逻辑。它将返回结果集中的结果，以便当您将值分配给变量时，您可以确定该值是什么而不是分配给哪个变量。

不幸的是，

(@monFee1 = @counter, @newBalance = @newBalance-@fee)

不返回值。

使用 If/Else 分支逻辑的替代方法是

while (@counter < 3 and @newBalance >0)
begin   
    IF @counter = 1
    THEN
        SET @monFee1 = @monthlyFee
        SET @newBalance = @newBalance-@fee
    END
    ELSE IF @counter = 2
    BEGIN
        SET @monFee2 = @monthlyFee
        SET @newBalance = @newBalance-@fee
    END
        SET @counter = @counter +1
end

Answer 4

你不应该在这里使用 while 循环。您实际上只检查了一次 @newBalance 的值。考虑：

@monFee1 = @monthlyFee
@newBalance = @newBalance-@fee

IF @newBalance > 0
BEGIN
   @monFee2 = @monthlyFee
   @newBalance = @newBalance-@fee
END 

Answer 5

由于 CASE 是一个表达式，因此您可以在 SET 赋值语句中使用它。

WHILE (@counter < 3 and @newBalance >0) 
BEGIN
  SET @monFee1 = CASE WHEN @Counter=1
    THEN @monthlyFee ELSE @monFee1 END
  SET @monFee2 = CASE WHEN @Counter=2
    THEN @monthlyFee ELSE @monFee2 END
  SET @newBalance = @newBalance - CASE WHEN @Counter in (1, 2)
    THEN @fee ELSE 0 END
  SET @counter = @counter +1 
END

它也可以在 SELECT 赋值语句中。

WHILE (@counter < 3 and @newBalance >0) 
BEGIN
  SELECT
    @monFee1 = CASE WHEN @Counter=1
      THEN @monthlyFee ELSE @monFee1 END,
    @monFee2 = CASE WHEN @Counter=2
      THEN @monthlyFee ELSE @monFee2 END,
    @newBalance = @newBalance - CASE WHEN @Counter in (1, 2)
      THEN @fee ELSE 0 END,
    @counter = @counter +1 
END

PS：祝你的衰老报告好运......