tab1
+----+------+
| id | dane |
+----+------+
|  1 |  ala |
+----+------+
|  2 |  ola |
+----+------+

tab2
+----+------+---------+--------------+
| id | cena | data_id | id_przedmiot |
+----+------+---------+--------------+
|  1 |   20 |       1 |            1 |
+----+------+---------+--------------+
|  2 |   10 |       1 |            2 |
+----+------+---------+--------------+
|  3 |   20 |       2 |            1 |
+----+------+---------+--------------+

tab3
+----+-----------+----------+
| id | przedmiot |   nazwa  |
+----+-----------+----------+
|  1 |         1 | długopis |
+----+-----------+----------+
|  2 |         2 |   ołówek |
+----+-----------+----------+

[SQL] pobierz, plaintext 
SELECT
           tab1.id
           tab1.dane
           SUM(tab2.cena) AS cena 
           tab3.przedmiot 
FROM tab1
LEFT JOIN tab2 ON tab1.id = tab2.data_id
LEFT JOIN tab3 ON tab2.id_przedmiot = tab3.przedmiot
GROUP BY (tab2.data_id)
[SQL] pobierz, plaintext 

+---------+-----------+------+----------------+
| tab1.id | tab1.dane | cena | tab3.przedmiot |
+---------+-----------+------+----------------+
|       1 |       ala |   30 |       długopis |
+---------+-----------+------+----------------+
|       2 |       ola |   20 |       długopis |
+---------+-----------+------+----------------+

+---------+-----------+------+----------------+
| tab1.id | tab1.dane | cena | tab3.przedmiot |
+---------+-----------+------+----------------+
|       1 |       ala |   30 |       długopis |
+---------+-----------+------+----------------+
|         |           |      |         ołówek |
+---------+-----------+------+----------------+
|       2 |       ola |   20 |       długopis |
+---------+-----------+------+----------------+