如何在 select 语句中使用 if else

Question

我有一个下面给出的存储过程，我想在选择查询中使用 if else

ALTER PROCEDURE updatepriority
    @sort smallint
AS 
BEGIN
    SELECT    
        Request.SubmitTime, Request.CommitTime, 
        (CASE WHEN Priority.Priority = 'Live Demo' THEN 1 WHEN Priority.Priority = 'Priority Re-do' THEN 2 ELSE 3 END) AS Sort1,
END

当@sort = 1我想使用另一个case语句并且我不想将整个语句包含在if else中

像下面的东西

IF (@sort = 1)
BEGIN
    (CASE WHEN Priority.Priority = 'Live Demo' THEN 1 WHEN Priority.Priority = 'Priority Re-do' THEN 2 ELSE 3 END) AS Sort1,
END
ELSE
BEGIN
    (CASE WHEN Priority.Priority = 'Live Demo' THEN 1 WHEN Priority.Priority = 'Conditional Live' THEN 2 WHEN Priority.Priority = 'Priority Re-do' THEN 3 ELSE 4 END) AS Sort1,
END

但是上面的查询会抛出一个错误，因为我们不能在 select 语句中使用 if else

Answer 1

是的，在sql server中没有else ，但是您可以使用它。

if(@sort=1)

BEGIN
    SELECT    Request.SubmitTime, Request.CommitTime, 
          (CASE WHEN Priority.Priority = 'Live Demo' THEN 1 WHEN Priority.Priority = 'Priority Re-do' THEN 2 ELSE 3 END) AS Sort1 ....
END

if(@sort!=1)
BEGIN
    SELECT    Request.SubmitTime, Request.CommitTime, 
          (CASE WHEN Priority.Priority = 'Live Demo' THEN 1 
            WHEN Priority.Priority = 'Conditional Live' THEN 2 
            WHEN Priority.Priority = 'Priority Re-do' THEN 3 ELSE 4 END) AS Sort1,
 ....
END

Answer 2

如果你真的不想在两个 if/else 分支中都使用 select 语句，那么唯一的选择是对 CASE 程序集使用动态 SQL，如下所示：

create table #test (
    Priority varchar(30)
);

insert #test (Priority)
values 
    ('Live Demo'), 
    ('Priority Re-do'), 
    ('Conditional Live'), 
    ('ABC');

declare @sort smallint = 1;
declare @sqlStmt varchar(max);

if(@sort=1)
BEGIN
    set @sqlStmt = '
    (CASE 
        WHEN Priority.Priority = ''Live Demo'' THEN 1 
        WHEN Priority.Priority = ''Priority Re-do'' THEN 2 
        ELSE 3 END) AS Sort1';

END
ELSE
BEGIN
    set @sqlStmt = '
    (CASE 
        WHEN Priority.Priority = ''Live Demo'' THEN 1 
        WHEN Priority.Priority = ''Conditional Live'' THEN 2 
        WHEN Priority.Priority = ''Priority Re-do'' THEN 3 
        ELSE 4 END) AS Sort1';
END

set @sqlStmt = 'select Priority.Priority, ''SubmitTime'' as SubmitTime, ' + @sqlStmt + ' from #test as Priority order by Sort1';

exec (@sqlStmt); 

当然，你可以只取你需要的（sql 动态部分..）。

Answer 3

你只需要嵌套CASE来做一些布尔逻辑

SELECT    
  Request.SubmitTime,
  Request.CommitTime,
  CASE WHEN @sort = 1 THEN
      CASE WHEN Priority.Priority = 'Live Demo' THEN 1
           WHEN Priority.Priority = 'Priority Re-do' THEN 2
           ELSE 3
      END
  ELSE
      CASE WHEN Priority.Priority = 'Live Demo' THEN 1
           WHEN Priority.Priority = 'Conditional Live' THEN 2
           WHEN Priority.Priority = 'Priority Re-do' THEN 3
           ELSE 4
      END
  END AS Sort1,
....

你甚至可以将两个分支结合起来，让它更简洁，但上面的方法是最直接的