从多个表中选择数据

Question

I have this code, and it runs very well. I have problem on how to select from another table. This code shows how to select and sum() from 2 different tables. What do I do if I am going to select from another table called tblproductlist . I am selecting 3 tables in one query. I don't know how to do it. I am searching codes, but it's hard for me to apply.

$result = mysql_query("SELECT (
 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;");

I tried this code but it returns an error.

$result = mysql_query("SELECT productlist.*,
(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;");

for example I have this ff value.

tblproductlist

pcode | pname |
111   | wire  |

tablersales

|  total    | pcode |
|  200      |  111  |
|  200      |  111  |

tablerreturn

|  total    | pcode |
|  200      |  111  |

so the output after subtracting its total must be something like this.

pcode | pname | total
111   | wire  | 200   |

Answer 1

Very loosely speaking, the results of a query need to (a) have the same straucture, (b) be somehow related. The resultset you appear to be trying to build has neither property (at the very least, all pieces of your querys should return the same number of rows)It appears that you need to gain better understanding of sql, besides explaining a little better what you need to do downstream.