执行INSERT ... ON DUPLICATE KEY UPDATE语句，如果INSERT的语句插入的值和已有的UNIQUE索引或主键重复的话，MySQL会更新已存在的行。 测试没有主键和UNIQUE索引的表

mysql> select * from dept2;
+--------+-------------+-------------+
| deptno | dname| report_date |
+--------+-------------+-------------+
| 10 | Research| 2016-06-03 |
| 20 | Maintenance | 2016-06-03 |
| 30 | Leader | 2016-06-03 |
| 40 | Market | 2015-08-02 |
+--------+-------------+-------------+
4 rows in set (0.00 sec)

mysql> desc dept2;
+-------------+-------------+------+-----+---------+-------+
| Field| Type| Null | Key | Default | Extra |
+-------------+-------------+------+-----+---------+-------+
| deptno | int(5) | NO| MUL | NULL||
| dname| varchar(14) | YES | | NULL||
| report_date | date| YES | MUL | NULL||
+-------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> INSERT INTO dept2 (deptno,dname,report_date) VALUES (20,'Development','2010-10-30')
->ON DUPLICATE KEY UPDATE report_date='2010-10-30';
Query OK, 1 row affected (0.01 sec)

mysql> select * from dept2;
+--------+-------------+-------------+
| deptno | dname| report_date |
+--------+-------------+-------------+
| 10 | Research| 2016-06-03 |
| 20 | Maintenance | 2016-06-03 |
| 30 | Leader | 2016-06-03 |
| 40 | Market | 2015-08-02 |
| 20 | Development | 2010-10-30 |
+--------+-------------+-------------+
5 rows in set (0.00 sec)

mysql> delete from dept2 where deptno=20 and report_date=date'2010-10-30';
Query OK, 1 row affected (0.01 sec)

mysql> select * from dept2;
+--------+-------------+-------------+
| deptno | dname| report_date |
+--------+-------------+-------------+
| 10 | Research| 2016-06-03 |
| 20 | Maintenance | 2016-06-03 |
| 30 | Leader | 2016-06-03 |
| 40 | Market | 2015-08-02 |
+--------+-------------+-------------+
4 rows in set (0.00 sec)

增加主键，再进行测试

mysql> alter table dept2 add primary key(deptno);
Query OK, 0 rows affected (0.28 sec)
Records: 0 Duplicates: 0 Warnings: 0

mysql> INSERT INTO dept2 (deptno,dname,report_date) VALUES (20,'Development','2010-10-30')
->ON DUPLICATE KEY UPDATE report_date='2010-10-30';
Query OK, 2 rows affected (0.14 sec)

mysql> select * from dept2;
+--------+-------------+-------------+
| deptno | dname| report_date |
+--------+-------------+-------------+
| 10 | Research| 2016-06-03 |
| 20 | Maintenance | 2010-10-30 |
| 30 | Leader | 2016-06-03 |
| 40 | Market | 2015-08-02 |
+--------+-------------+-------------+
4 rows in set (0.00 sec)