当其中一个为空时，我不能减去两个不同的表

Question

I have 2 tables rsales and rreturn and I have this following code. It runs properly when both table have values, but the problem is that when one of them is empty or no data is stored, it doesn't display a result. Is there something wrong with my code?

$result = mysql_query("SELECT category, (SELECT SUM(s.total)-SUM(r.total) FROM rsales AS s WHERE r.pcode=s.pcode) as total, r.pcode FROM rreturn AS r GROUP BY r.pcode;");

Answer 1

MySQL doesn't support a FULL OUTER JOIN , so you need to emulate it.

One way to do it:

SELECT IFNULL(sr.category,ss.category) AS category
     , IFNULL(s.total,0)-IFNULL(r.total,0) AS total
  FROM (
         SELECT cr.pcode
           FROM rreturn cr
          GROUP BY cr.pcode
          UNION 
         SELECT cs.pcode
           FROM rsales cs
          GROUP BY cs.pcode
       ) c
  LEFT
  JOIN ( SELECT sr.pcode
              , MAX(sr.category) AS category
              , SUM(sr.total) AS total
           FROM rreturn sr
          GROUP BY sr.pcode
       ) r
    ON r.pcode = c.pcode
  LEFT
  JOIN ( SELECT ss.pcode
              , MAX(ss.category) AS category
              , SUM(sr.total) AS total
           FROM rsales ss
          GROUP BY ss.pcode
       ) s
    ON s.pcode = c.pcode

The inline view c gets a full list of all pcode that appear in either rsales or rreturn.

Then inline view r gets a total for each pcode from rreturn, and s does the same from rsales .

We can then use a LEFT JOIN operation to match the rows based on pcode.

(This isn't the only way to return the specified resultset, just one example.)