保存或更新

在MySQL数据库中，如果在insert语句后面带上ON DUPLICATE KEY UPDATE 子句，而要插入的行与表中现有记录的惟一索引或主键中产生重复值，那么就会发生旧行的更新；如果插入的行数据与现有表中记录的唯一索引或者主键不重复，则执行新纪录插入操作。另外，ON DUPLICATE KEY UPDATE不能写where条件。

示例：

create table kid_score(
id tinyint unsigned not null,
birth_day date not null,
score int unsigned not null,
primary key(id, birth_day)  --唯一索引是由 id + birth_day 两个字段组成
) engine = InnoDB;

--初始化数据
insert into kid_score(id, birth_day, score) values (1,'2019-01-15',10),(2,'2019-01-16',20);

下面开始验证执行INSERT ··· ON DUPLICATE KEY UPDATE语法的规则：如果你插入的记录导致一个UNIQUE索引或者primary key(主键)出现重复，那么就会认为该条记录存在，则执行update语句而不是insert语句，反之，则执行insert语句而不是更新语句。

1. 唯一索引重复

insert into kid_score(id, birth_day, score) values (1,'2019-01-15',30) ON DUPLICATE KEY UPDATE score = score + 50;

结果：

2. 唯一索引不重复

insert into kid_score(id, birth_day, score) values (2,'2019-01-15',30) ON DUPLICATE KEY UPDATE score = score + 50;

结果：

3. 唯一索引重复，插入完全相同数据

insert into kid_score(id, birth_day, score) values (2,'2019-01-16',20) ON DUPLICATE KEY UPDATE score = 20;

结果：

4. 影响行数

需要注意的是：如果行作为新记录被插入，则受影响行的值为1；如果原有的记录被更新，则受影响行的值为2，如果更新的数据和已有的数据一模一样，则受影响的行数是0。

mysql> select * from kid_score;
+----+------------+-------+
| id | birth_day  | score |
+----+------------+-------+
|  1 | 2019-01-15 |    10 |
|  2 | 2019-01-16 |    20 |
+----+------------+-------+
2 rows in set

-- 唯一索引重复，执行更新
mysql> insert into kid_score(id, birth_day, score) values (1,'2019-01-15',30) ON DUPLICATE KEY UPDATE score = score + 50;
Query OK, 2 rows affected

mysql> select * from kid_score;
+----+------------+-------+
| id | birth_day  | score |
+----+------------+-------+
|  1 | 2019-01-15 |    60 |
|  2 | 2019-01-16 |    20 |
+----+------------+-------+
2 rows in set

-- 唯一索引不重复，执行插入
mysql> insert into kid_score(id, birth_day, score) values (2,'2019-01-15',30) ON DUPLICATE KEY UPDATE score = score + 50;

Query OK, 1 row affected

mysql> select * from kid_score;
+----+------------+-------+
| id | birth_day  | score |
+----+------------+-------+
|  1 | 2019-01-15 |    60 |
|  2 | 2019-01-15 |    30 |
|  2 | 2019-01-16 |    20 |
+----+------------+-------+
3 rows in set

-- 唯一索引重复，应该执行更新，但更新值与原值相同
mysql> insert into kid_score(id, birth_day, score) values (2,'2019-01-16',20) ON DUPLICATE KEY UPDATE score = 20;

Query OK, 0 rows affected

mysql> select * from kid_score;
+----+------------+-------+
| id | birth_day  | score |
+----+------------+-------+
|  1 | 2019-01-15 |    60 |
|  2 | 2019-01-15 |    30 |
|  2 | 2019-01-16 |    20 |
+----+------------+-------+
3 rows in set