error : subquery must return only one column

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 sumtotal2FROM (   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   ) m1JOIN  (   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_bFROM   sale s JOIN   stx  x ON x.saleid = s.idJOIN   mat  m ON m.matid = x.matidWHERE (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.

Instead of subquery select statement

SELECT mat.mat  as mat, sum(stx.total)  as sumtotal

Try this statement

SELECT sum(stx.total)  as sumtotal