案例陈述中的RAISERROR

Question

你能否在T-SQL的case语句中引发错误？我总是遇到SQL case语句的问题:/

    begin try
    declare @i int 
    --set @i = (select COUNT(1) from table_name)

    select Item_Num =
        CASE (select COUNT(1) from table_name)
            when 1 then (select Item_Num from table_name)
            when 0 then (raiserror('No records in database', 0, 0))
            ELSE (raiserror('Multiple records in database', 0, 0))
        END
    from table_name

    end try
    begin catch
        declare @errormsg nvarchar(1024),
                @severity int,
                @errorstate int;

        select @errormsg = error_message(),
                @severity = error_severity(),
                @errorstate = error_state();

        raiserror(@errormsg, @severity, @errorstate);
    end catch

Answer 1

您可以通过将错误字符串转换为 int 从 case 表达式引发错误。

select case (select count(*) from mytable)
  when 1 then 100 
  when 2 then 200
  else convert(int, 'ERROR')
end

这会给出一条错误消息，例如

Conversion failed when converting the varchar value 'ERROR' to data type int.

这大约是你能得到的最好的结果。

并非所有失败的转换都会在错误消息中提供输入字符串。例如，转换为日期时间则不然。因此，如果您的 case 表达式返回日期时间，您仍然必须通过字符串到整数的转换来触发错误：

select case (select count(*) from mytable)
  when 1 then getdate()
  else convert(datetime, convert(int, 'ERROR'))
end

情况变得更糟：如果你返回一个日期，你不能显式地从 int 转换它，所以你必须求助于

convert(date, convert(char(1), convert(int, 'ERROR')))

这非常可怕，但在我看来，唯一比干净的代码更重要的是信息丰富的错误消息，所以我接受它。

Answer 2

正如我在评论中所说，我认为简单地创建一个在语句范围之外检查的标志会更容易CASE。大致如下：

--- code before the TRY...
BEGIN TRY
    DECLARE @i int 

    -- declare a variable to act as a flag
    DECLARE @my_flag as int

    -- than change your statement to simply fill the value of the flag 
    CASE (SELECT COUNT(1) FROM table_name)
         WHEN 1 THEN SET @my_flag = 1
         WHEN 0 THEN SET @my_flag = 0
         ELSE SET @my_flag = -1
     END

    IF (NOT @my_flag in (-1, 0))
    BEGIN
        SET @Item_Num = (SELECT Item_Num FROM table_name) -- consider a filter here
      END 
     ELSE
    BEGIN
        IF (-1 = @my_flag) RAISERROR('too many records', 0, 0)
        IF (0 = @my_flag) RAISERROR('no records', 0, 0) 
      END
END TRY
BEGIN CATCH 
    --- rest of the code goes here....