mysql> select oseba.priimek, sum(kosov) from trgovec, oseba, kupcija where trgovec.oid=oseba.oid and trgovec.tid=kupcija.tid group by oseba.priimek;
+---------+------------+
| priimek | sum(kosov) |
+---------+------------+
| Adamic  |       5000 |
| Bajec   |        700 |
| Cokan   |        600 |
| Jazbec  |       3200 |
| Smolar  |        900 |
| Trcek   |        200 |
+---------+------------+
6 rows in set (0.00 sec)

mysql> select oseba.priimek, sum(kosov) from trgovec, oseba, kupcija where trgovec.oid=oseba.oid and trgovec.tid=kupcija.tid group by oseba.priimek and sum(kosov) < 2000;
ERROR 1111 (HY000): Invalid use of group function
mysql>