Sql server 执行选择案例时出错

运行此查询时，出现以下错误：

程序sp_获取随机问题，第40行 当子查询未引入EXISTS时，只能在选择列表中指定一个表达式。 Msg 116，16级，状态1，程序sp_获取随机问题，第52行 当子查询未引入EXISTS时，只能在选择列表中指定一个表达式

这是一个存储过程，旨在给出随机问题，@easy、@medium、@hard是将被随机分配的问题数。 如果用户选择只给出一个@num，我将把这个数字除以3（3个困难），然后得到这些随机问题

SELECT  case
WHEN @num=0 THEN (
    SELECT * FROM (
    SELECT top (@easy) * from TBL_PERGUNTAS as p where p.category=1 order by NEWID()) as a
    UNION ALL SELECT * FROM
    (SELECT top (@medium) * from TBL_PERGUNTAS as p where p.category=2 order by NEWID()) as b
    UNION ALL SELECT * FROM
    (SELECT top (@hard) * from TBL_PERGUNTAS as p where p.category=3 order by NEWID()) as c)
    else (
    SELECT * FROM
    (SELECT top (@num/3) * from TBL_PERGUNTAS as p where p.category=1 order by NEWID()) as a
    UNION ALL SELECT * FROM
    (SELECT top (@num/3) * from TBL_PERGUNTAS as p where p.category=2 order by NEWID()) as b
    UNION ALL SELECT * FROM
    (SELECT top (@num/3) * from TBL_PERGUNTAS as p where p.category=3 order by NEWID()) as c )
END

---

正如注释所述，您需要使用

IF/ELSE

来控制流量，请尝试以下方法：

IF @num=0 THEN 
BEGIN
    SELECT top (@easy) * 
    FROM TBL_PERGUNTAS as p 
    WHERE p.category=1 
    ORDER BY NEWID()

    UNION ALL 

    SELECT top (@medium) * 
    FROM TBL_PERGUNTAS as p 
    WHERE p.category=2 
    ORDER BY NEWID()

    UNION ALL 

    SELECT top (@hard) * 
    FROM TBL_PERGUNTAS as p 
    WHERE p.category=3 
    ORDER BY NEWID())
END
ELSE
BEGIN
    SELECT top (@num/3) * 
    FROM TBL_PERGUNTAS as p 
    WHERE p.category=1 
    ORDER BY NEWID()

    UNION ALL 

    SELECT top (@num/3) * 
    FROM TBL_PERGUNTAS as p
    WHERE p.category=2 
    ORDER BY NEWID()

    UNION ALL  

    SELECT top (@num/3) * 
    FROM TBL_PERGUNTAS as p 
    WHERE p.category=3 
    ORDER BY NEWID())
END

---

案例不用于控制流量（必须返回单个值）；如果您想要一组或另一组结果，请使用if/elsey您的代码至少可以说是一团糟。这里的问题是，您试图使用一个case表达式来控制不起作用的流。这里需要使用IF/ELSE逻辑。此外，还应避免使用sp_uu前缀。