Something

mysql>  select * from kupcija, trgovec where trgovec.tid=kupcija.tid and kid='K2';
+------+------+------+-------+------+-----+------+-----------+
| tid  | aid  | kid  | kosov | kpid | tid | oid  | provizija |
+------+------+------+-------+------+-----+------+-----------+
| T2   | A3   | K2   |   200 | KP04 | T2  | O2   |      10.0 |
| T2   | A5   | K2   |   100 | KP10 | T2  | O2   |      10.0 |
| T3   | A4   | K2   |   500 | KP12 | T3  | O3   |      30.0 |
| T5   | A2   | K2   |   200 | KP15 | T5  | O5   |      30.0 |
| T5   | A6   | K2   |   200 | KP19 | T5  | O5   |      30.0 |
+------+------+------+-------+------+-----+------+-----------+
5 rows in set (0.00 sec)

mysql>  select * from kupcija, trgovec where trgovec.tid=kupcija.tid and kid='K1';
+------+------+------+-------+------+-----+------+-----------+
| tid  | aid  | kid  | kosov | kpid | tid | oid  | provizija |
+------+------+------+-------+------+-----+------+-----------+
| T1   | A1   | K1   |   200 | KP01 | T1  | O1   |      20.0 |
| T2   | A3   | K1   |   400 | KP03 | T2  | O2   |      10.0 |
| T3   | A3   | K1   |   200 | KP11 | T3  | O3   |      30.0 |
+------+------+------+-------+------+-----+------+-----------+
3 rows in set (0.00 sec)