两个多对多关系的表：employer与employee 还有一个中间连接表：employment
mysql> desc employer;
+------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+----------------+
| employerid | int(11) | | PRI | NULL | auto_increment |
| name | varchar(255) | YES | | NULL | |
+------------+--------------+------+-----+---------+----------------+
mysql> desc employment;
+--------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+----------+------+-----+---------+----------------+
| employmentid | int(11) | | PRI | NULL | auto_increment |
| startDate | datetime | YES | | NULL | |
| endDate | datetime | YES | | NULL | |
| employerid | int(11) | | MUL | 0 | |
| employeeid | int(11) | | MUL | 0 | |
+--------------+----------+------+-----+---------+----------------+
mysql> desc employee;
+---------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+----------------+
| employeeid | int(11) | | PRI | NULL | auto_increment |