首先看个例子(主键是自增长)：
mysql> insert into bankaccount(name,balance) values('123', 1000);
Query OK, 1 row affected (0.06 sec)
mysql> insert into bankstatement(action, txdate, amt, toaccno, fromaccno) values
('122', curdate(), 1000, 1, 2);
Query OK, 1 row affected (0.00 sec)
mysql> select last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
| 7 |
+------------------+
1 row in set (0.00 sec)
mysql> select * from bankaccount;
+-------+------+---------+
| accno | name | balance |
+-------+------+---------+
| 1 | 张三 | 200 |
| 2 | 李四 | 900 |
| 3 | 123 | 1000 |
| 4 | 123 | 1000 |
+-------+------+---------+
4 rows in set (0.00 sec)
mysql> select * from bankstatement;
+----+--------------+------------+------+---------+-----------+
| id | action | txdate | amt | toaccno | fromaccno |
+----+--------------+------------+------+---------+-----------+
| 1 | 开户 | 2012-10-14 | 100 | NULL | 1 |
| 2 | 开户 | 2012-10-14 | 1000 | NULL | 2 |
| 3 | 查找账户信息 | 2012-10-14 | 0 | NULL | 2 |
| 4 | 查找账户信息 | 2012-10-14 | 0 | NULL | 1 |
| 5 | 转账 | 2012-10-14 | 100 | 1 | 2 |
| 6 | 122 | 2012-10-14 | 1000 | 1 | 2 |
| 7 | 122 | 2012-10-14 | 1000 | 1 | 2 |
+----+--------------+------------+------+---------+-----------+
7 rows in set (0.00 sec)
总结：LAST_INSERT_ID()返回最后一个INSERT或UPDATE语句中AUTO_INCREMENT列的值。
参考资料：
http://zhaohe162.blog.163.com/blog/static/38216797201122411193745/
http://it.100xuexi.com/view/otdetail/20120619/73a6cc8f-36b8-4b70-8904-57c18d3ab385.html
http://blog.csdn.net/wh62592855/article/details/6722298