如果声明内部案例嵌入

Question

How to embed and if statement inside a case. This is what I have so far.

DECLARE @LDCCode as int
       DECLARE @InvoiceDate as datetime
       DECLARE @PaymentSemiMonthlyDays AS int
       SET @LDCCode = 20
       SET @InvoiceDate = '5/16/2012'
       DECLARE @InvDateDayMonth AS INT
       DECLARE @ReturnDate AS DATETIME
       SET @ReturnDate = @InvoiceDate
       DECLARE @PaymentDOM AS INT
       DECLARE @PaymentDays AS INT
       DECLARE @PaymentSemiMonthlyOffset AS INT


SET @ReturnDate = CASE WHEN NOT @PaymentDOM IS NULL THEN
   @ReturnDate + (@PaymentDOM - DATEPART(day, DateAdd(mm,1,@ReturnDate))) 
WHEN NOT @PaymentDays IS NULL THEN 
            DATEADD(Day,@PaymentDays,@ReturnDate)
WHEN NOT @PaymentSemiMonthlyOffset IS NULL THEN                     
    IF @LDCCode = 40 AND @InvDateDayMonth > 11 AND @InvDateDayMonth < 26 
       SELECT @ReturnDate + (@PaymentSemiMonthlyOffset - DATEPART(day, DateAdd(mm,1,@ReturnDate)))
    ELSE
       SELECT @ReturnDate + (@PaymentSemiMonthlyDays - DATEPART(day, DateAdd(mm,1,@ReturnDate)))    
    END 

Answer 1

You can't combine logic in that manner, but you can add a nested CASE your existing CASE statement:

SET @ReturnDate = (CASE WHEN NOT @PaymentDOM IS NULL THEN
    @ReturnDate + (@PaymentDOM - DATEPART(day, DateAdd(mm,1,@ReturnDate))) 
WHEN NOT @PaymentDays IS NULL THEN 
    DATEADD(Day,@PaymentDays,@ReturnDate)
WHEN NOT @PaymentSemiMonthlyOffset IS NULL THEN
    CASE WHEN @LDCCode = 40 AND @InvDateDayMonth > 11 AND @InvDateDayMonth < 26 THEN
        @ReturnDate + (@PaymentSemiMonthlyOffset - DATEPART(day, DateAdd(mm,1,@ReturnDate)))
    ELSE
        @ReturnDate + (@PaymentSemiMonthlyDays - DATEPART(day, DateAdd(mm,1,@ReturnDate)))
    END   
END)

Though, if this is in a stored procedure, I might just opt for simpler IF/THEN/ELSE structure instead of the CASE :

IF @PaymentDOM IS NULL
    SET @ReturnDate = @ReturnDate + (@PaymentDOM - DATEPART(day, DateAdd(mm, 1, @ReturnDate))) 
ELSE IF NOT @PaymentDays IS NULL
    SET @ReturnDate = DATEADD(Day,@PaymentDays,@ReturnDate)
ELSE IF NOT @PaymentSemiMonthlyOffset IS NULL
BEGIN
    IF @LDCCode = 40 AND @InvDateDayMonth > 11 AND @InvDateDayMonth < 26
        SET @ReturnDate = @ReturnDate + (@PaymentSemiMonthlyOffset - DATEPART(day, DateAdd(mm, 1, @ReturnDate)))
    ELSE
        SET @ReturnDate = @ReturnDate + (@PaymentSemiMonthlyDays - DATEPART(day, DateAdd(mm, 1, @ReturnDate)))
END

Answer 2

Since all of the expressions add something to @ReturnDate , and since you calculate the day of a month after @ReturnDate multiple times, I think this can be simplified as:

DECLARE @Plus1M INT;
SET @Plus1M = DATEPART(DAY, DATEADD(MONTH, 1, @ReturnDate));

SET @ReturnDate = DATEADD(DAY, 
  COALESCE(
    @PaymentDays, -- if @PaymentDays IS NULL, this will be skipped
    @PaymentDOM - @Plus1M,  -- if @PaymentDOM is NULL, this will be skipped
    CASE WHEN @PaymentSemiMonthlyOffset IS NOT NULL THEN
      CASE WHEN @LDCCode = 40 AND @InvDateDayMonth BETWEEN 12 AND 25
        THEN @PaymentSemiMonthlyOffset 
        ELSE @PaymentSemiMonthlyDays 
      END - @Plus1M
    END
  ), 
  @ReturnDate
);

But if you want to write it as verbose and redundant expressions then:

SET @ReturnDate = CASE 
  WHEN @PaymentDOM IS NOT NULL THEN
    DATEADD(DAY, @PaymentDOM 
    - DATEPART(DAY, DATEADD(MONTH, 1, @ReturnDate)), @ReturnDate) 
  WHEN @PaymentDays IS NOT NULL THEN 
    DATEADD(DAY, @PaymentDays, @ReturnDate)
  WHEN @PaymentSemiMonthlyOffset IS NOT NULL THEN
    CASE WHEN @LDCCode = 40 AND @InvDateDayMonth BETWEEN 12 AND 25 THEN
      DATEADD(DAY, @PaymentSemiMonthlyOffset 
      - DATEPART(DAY, DATEADD(MONTH, 1, @ReturnDate)), @ReturnDate)
    ELSE
      DATEADD(DAY, @PaymentSemiMonthlyDays 
      - DATEPART(DAY, DATEADD(MONTH, 1, @ReturnDate)), @ReturnDate)   
    END 
  END;

Tough to verify though, because your existing query doesn't parse and you didn't show us sample data or desired results.

I would avoid using the @Datetime + INT shorthand and would rather spell out the DATEADD operations, because the shorthand doesn't work with the newer types ( DATE , DATETIME2 , etc). I'd also avoid shorthand like mm as several of these abbreviations are non-obvious. Three more characters makes that much more obvious. Also the NOT ...something... IS NULL is not very intuitive to read. I would rather say "the light is not red" than "not the light is red."