Join problem

[gaz69a]

Hi,

Let’s say that we have three tables:
[CODE=mysql]
CREATE TABLE t_1 (id_x INT);
CREATE TABLE t_2 (id_x INT, value_2 INT);
CREATE TABLE t_3 (id_x INT, value_3 INT);
[/CODE]

And some data in these:
[code=mysql]
INSERT INTO t_1 VALUES(1);
INSERT INTO t_2 VALUES(1,10),(1 ,20),(1,5);
INSERT INTO t_3 VALUES(1,1),(1, 2);
[/code]

Now try to select data with this select:
[code=mysql]
SELECT `t1`.`id_x`,
SUM(`t2`.`value _2`) AS `value_2`,
SUM(`t3`.`value _3`) AS `value_3`
FROM `t_1` `t1`
LEFT JOIN `t_2` `t2` ON `t1`.`id_x` = `t2`.`id_x`
LEFT JOIN `t_3` `t3` ON `t1`.`id_x` = `t3`.`id_x`
WHERE `t1`.`id_x` = 1
GROUP BY `t1`.`id_x`;
[/code]

Result is (which is wrong):

+------+---------+---------+
| id_x | value_2 | value_3 |
+------+---------+---------+
|    1 |      70 |       9 |
+------+---------+---------+

How is possible to get correct result:

+------+---------+---------+
| id_x | value_2 | value_3 |
+------+---------+---------+
|    1 |      35 |       3 |
+------+---------+---------+

Question is - how I should change the join to get correct values?

[Atli]

Hi.

You could try using Correlated Subqueries.
Those are basically just subqueries that reference tables from the outer query.

For example:
[code=mysql]
SELECT
t1.ID,
(SELECT SUM(t2.value) FROM t2 WHERE t2.t1_ID_FK = t1.ID) AS 'Sum'
FROM t1
WHERE t1.ID = 1
[/code]

[gaz69a]

Thanks for your reply Atli!

You right – we can use subqueries but then we can’t select more than one field with one subquery…

One possible way to solve a problem is “union select” :
[code=mysql]
SELECT `x`.`id_x`, SUM(`x`.`value_ 2`) AS `value_2`, SUM(`x`.`value_ 3`) AS `value_3` FROM
(
SELECT
`t1`.`id_x`,
`t2`.`value_2`,
0 AS `value_3`
FROM `t_1` `t1`
LEFT JOIN `t_2` `t2` ON `t1`.`id_x` = `t2`.`id_x`
UNION
SELECT
`t1`.`id_x`,
0 AS `value_2`,
`t3`.`value_3`
FROM `t_1` `t1`
LEFT JOIN `t_3` `t3` ON `t1`.`id_x` = `t3`.`id_x`
) AS `x`
WHERE `x`.`id_x` = 1
GROUP BY `x`.`id_x`;
[/code]
Is it really so, that MySQL doesn’t have “normal” way to join multiple tables?