error : subquery must return only one column

Question

I am getting the error subquery must return only one column when I try to run the following query:

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

What is causing this error?

Answer 1

Put a subquery that returns multiple columns in the FROM list and select from it.

A correlated subquery would be a bad idea to begin with. However, your query is not even correlated, but unrelated (no link to outer query) and seems to return multiple rows. This leads to (possibly very expensive and nonsensical) cross join producing a cartesian product, probably not your (secret) intention.

Looks like you really want:

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);

Both LEFT JOIN are also pointless. The one on sale is forced to a INNER JOIN by the WHERE condition. The one on mat seems pointless, since you GROUP BY mat.mat - except if you are interested in mat IS NULL? (I doubt it.)

The case can probably be further simplified to:

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;

The WHERE condition can probably be simplified further, depending on your secret data types and indices. A boatload of information on precisely that case in this related answer on dba.SE.