[SQL]如果Key不存在就新增记录，存在就更新记录

需求：

根据某个主键查询，如果存在某条记录，就更新这条记录，否则新增一条记录

假设student表，结构如下

+-------------+--------------+------+-----+---------+-------+
| Field       | Type         | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+-------+
| id          | int          | NO   | PRI | NULL    |       |
| age         | int          | YES  |     | NULL    |       |
| name        | varchar(255) | YES  |     | NULL    |       |
+-------------+--------------+------+-----+---------+-------+

假设记录是id=1, name='Terry', age=20

--MySQL，ID是主键，如果存在1就更新，否则新建

INSERT INTO student (id, name, age) VALUES(1, "Terry", 20) ON DUPLICATE KEY UPDATE    
name="Terry", age=20

insert into STUDENT (ID, NAME, AGE) select 1, 'Terry', 20 from dual where not exists (select * from STUDENT where ID=1)