mysql 이건 오류인가요?.

mysql> select *  from id;
+------+---------+--------+
| NUM  | ID_NAME | NATION |
+------+---------+--------+
|    1 | kim     | korea  |
|    2 | park    | jappan |
|    2 | hong    | china  |
|    3 | yang    | spain  |
|    3 | hoog    | korea  |
+------+---------+--------+
5 rows in set (0.03 sec)

mysql> select * from id2;
+------+------+--------+
| sub  | juso | NATION |
+------+------+--------+
| kor  | kkk  | korea  |
| eng  | jjj  | hoko   |
| mat  | ooo  | china  |
| mat  | ooo  | usa    |
| kor  | ooo  | china  |
+------+------+--------+
5 rows in set (0.03 sec)

mysql> select * from id3;
+------+---------+------------+
| NUM  | sub     | food       |
+------+---------+------------+
|    1 | eng     | apple      |
|    2 | kor     | strawberry |
|    3 | mat     | kewii      |
|    4 | pysical | orange     |
|    2 | mat     | apple      |
+------+---------+------------+
5 rows in set (0.05 sec)

mysql> select id_name, juso, food
    -> from id as a, id2 as b, id3 as c
    -> where a.num=c.num
    -> and a.nation=b.nation;
+---------+------+------------+
| id_name | juso | food       |
+---------+------+------------+
| kim     | kkk  | apple      |
| hong    | ooo  | strawberry |
| hong    | ooo  | strawberry |
| hoog    | kkk  | kewii      |
| hong    | ooo  | apple      |
| hong    | ooo  | apple      |
+---------+------+------------+
6 rows in set (0.00 sec)

mysql>

이 출력결과는 정말 오류같내요.
결과 몇번이고 봤는데, 도저히나올수 없는결과에요.  확인부탁드립니다.