ON DUPLICATE KEY UPDATE

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 value1, 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;

(The effects are not identical for an InnoDB table where a is an auto-increment column. With an auto-increment column, an INSERT statement increases the auto-increment value but UPDATE does not.)

The ON DUPLICATE KEY UPDATE clause can contain multiple column assignments, separated by commas.

ON DUPLICATE KEY UPDATE

原文：http://www.cnblogs.com/studyNT/p/6054947.html