Question

Given are two tables with with a common id and integer values:

table1:            table2:
| id   | val  |    | id   | val  |
+------+------+    +------+------+
| 1    |   0  |    | 1    |   1  |
| 2    |   1  |    | 1    |   1  |
| 3    |   1  |    |  2   |   2  |
| 4    |   2  |    |  2   |   2  |
| 5    |   2  |    |   3  |   2  |
                   |   3  |   2  |
                   |   3  |   2  |
                   |    4 |   6  |  <id shifted for readability>

How to archive this result in one query:

result-table:
| id   | val  |
+------+------+
| 1    |   2  |
| 2    |   5  |
| 3    |   7  |
| 4    |   8  |
| 5    |   2  |

EDIT:

Following some answers I got:

SELECT t1.id, t1.val + SUM(t2.val) AS val
FROM table1 t1
LEFT JOIN table2 t2 ON (t1.id = t2.id)
GROUP BY t1.id

what gives me:

id  val
1   2
2   5
3   7
4   8
5   NULL

But 2 plus nothing = 2 and not NULL. Is there a different way?

Answer 1

You forgot to group by:

SELECT t1.id, t1.val - SUM(t2.val) AS val
FROM t1
JOIN table2 t2 ON (t1.id = t2.id)
GROUP BY t1.id

of course, I’m not sure why you’re trying to subtract the values in your sample query. maybe you meant + instead?