错误:子查询必须只返回一列

Question

我subquery must return only one column尝试运行以下查询时收到错误:

SELECT mat.mat as mat1, sum(stx.total ) as sumtotal1,
  (
    SELECT mat.mat  as mat, sum(stx.total)  as sumtotal
    FROM stx 
      LEFT JOIN mat ON stx.matid = mat.matid
      LEFT JOIN sale ON stx.saleid = sale.id
    WHERE stx.date BETWEEN '2013-05-01' AND '2013-08-31' 
      AND sale.userid LIKE 'A%'
    GROUP BY mat.mat
) AS MyField
FROM stx 
  LEFT JOIN mat ON stx.matid = mat.matid
  LEFT JOIN sale ON stx.saleid = sale.id
WHERE stx.date BETWEEN '2013-05-01' AND '2013-08-31'
  AND sale.userid LIKE 'B%'
GROUP BY mat.mat

导致此错误的原因是什么？

Answer 1

放置一个子查询,返回FROM列表中的多个列并从中进行选择.

一个相关的子查询开始时是一个坏主意.但是,您的查询甚至不是相关的,而是无关的(没有链接到外部查询)并且似乎返回多行.这导致(可能非常昂贵和荒谬的)交叉连接产生笛卡尔积,可能不是你的(秘密)意图.

看起来你真的想要:

SELECT m1.mat AS mat1, m1.sumtotal AS sumtotal1
      ,m2.mat AS mat2, m2.sumtotal AS sumtotal2
FROM (
   SELECT mat.mat, sum(stx.total) AS sumtotal
   FROM   stx 
   LEFT   JOIN mat ON mat.matid = stx.matid
   LEFT   JOIN sale ON stx.saleid = sale.id
   WHERE  stx.date BETWEEN '2013-05-01' AND '2013-08-31'
   AND    sale.userid LIKE 'A%'
   GROUP  BY mat.mat
   ) m1
JOIN  (
   SELECT mat.mat, sum(stx.total) AS sumtotal
   FROM   stx 
   LEFT   JOIN mat ON mat.matid = stx.matid
   LEFT   JOIN sale ON sale.id = stx.saleid
   WHERE  stx.date BETWEEN '2013-05-01' AND '2013-08-31' 
   AND    sale.userid LIKE 'b%'
   GROUP  BY mat.mat
   ) m2 USING (mat);

两者LEFT JOIN都没有意义.在一个sale被强制到一个INNER JOIN通过WHERE条件.垫子上的那个似乎毫无意义,因为你GROUP BY mat.mat- 除非你感兴趣mat IS NULL？(我对此表示怀疑.)

案件可能会进一步简化为:

SELECT m.mat
      ,sum(CASE WHEN s.userid LIKE 'A%' THEN x.total END) AS total_a
      ,sum(CASE WHEN s.userid LIKE 'B%' THEN x.total END) AS total_b
FROM   sale s 
JOIN   stx  x ON x.saleid = s.id
JOIN   mat  m ON m.matid = x.matid
WHERE (s.userid LIKE 'A%' OR s.userid LIKE 'B%')
AND    x.date BETWEEN '2013-05-01' AND '2013-08-31'
GROUP  BY 1;

该WHERE情况大概可以进一步简化,这取决于你的秘密数据类型和索引.关于dba.SE的相关答案中关于该案例的大量信息.