错误：子查询只能返回一列

Question

subquery must return only one column当我尝试运行以下查询时出现错误：

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

是什么导致此错误？

Answer 1

将一个返回多列的子查询放在FROM列表中，然后从中选择。

首先，相关子查询将是一个坏主意。但是，您的查询甚至没有关联，但是 没有关联
（没有链接到外部查询），并且似乎返回了多行。这导致产生笛卡尔积的（可能非常昂贵且荒谬的）交叉连接，可能不是您的（秘密）意图。

看起来您确实想要：

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也都是毫无意义的。WHERE条件将一个sale强制为a INNER JOIN。垫子上的那个似乎毫无意义，因为您GROUP BY mat.mat-除非您有兴趣mat IS NULL？（我对此表示怀疑。）

这种情况可能可以进一步简化为：

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;

WHERE根据您的秘密数据类型和索引，可以进一步简化该条件。在dba.SE的相关答案中，提供了大量有关该情况的信息。