root@localhost : test 04:47:33>select * from ab;
+------------+--------------+-----------------+
| product_id | product_name | product_type_id |
+------------+--------------+-----------------+
|          1 | 产品A      | 1,2             |
|          2 | 产品B      | 2,3             |
+------------+--------------+-----------------+
2 rows in set (0.00 sec)

root@localhost : test 04:47:42>select * from ac;
+-----------------+-------------------+
| product_type_id | product_type_name |
+-----------------+-------------------+
|               1 | 类别1           |
|               2 | 类别2           |
|               3 | 类别3           |
+-----------------+-------------------+
3 rows in set (0.00 sec)

root@localhost : test 04:47:44>select product_id,product_name,group_concat(product_type_name) as type from ab,ac where instr(ab.product_type_id,ac.product_type_id) group by product_id,product_name;
+------------+--------------+-----------------+
| product_id | product_name | type            |
+------------+--------------+-----------------+
|          1 | 产品A      | 类别1,类别2 |
|          2 | 产品B      | 类别2,类别3 |
+------------+--------------+-----------------+
2 rows in set (0.00 sec)

------解决方案--------------------
SQL code
select product_id,product_name,ab.product_type_id,group_concat(product_type_name) as typename from ab,ac where instr(ab.product_type_id,ac.product_type_id) group by product_id,product_name;
+------------+--------------+-----------------+-----------------+
| product_id | product_name | product_type_id | typename        |
+------------+--------------+-----------------+-----------------+
|          1 | 产品A      | 1,2             | 类别1,类别2 |
|          2 | 产品B      | 2,3             | 类别3,类别2 |
+------------+--------------+-----------------+-----------------+

------解决方案--------------------
用GROUP_CONCAT即可

------解决方案--------------------
select a.*,group_concat(b.product_type_name)
from 表1 a, 表2 b
where find_in_set(b.product_type_id, a.product_type_id)
group by a.product_id