MySQL 'on duplicate key update' 用法

If you specify ON DUPLICATE KEY UPDATE, and a row is inserted that would cause a duplicate value in a UNIQUE index or PRIMARY KEY, MySQL performs an UPDATE of the old row. For example, if column a is declared as UNIQUE and contains the value 1, the following two statements have similar effect:

INSERT INTO table (a,b,c) VALUES (1,2,3) 
   ON DUPLICATE KEY UPDATE c=c+1;

UPDATE table SET c=c+1 WHERE a=1;

如果在INSERT中指定ON DUPLICATE KEY UPDATE,并且插入将在UNIQUE索引或PRIMARY KEY中导致重复值的行,MySQL执行旧行的更新。比如，如果列a被声明为UNIQUE并且包含值1,以下两句话有相同的效果。