INSERT INTO video_tag_all(tagname,ctime)      SELECT ‘利物浦‘,1413954816 FROM video_tag_all      WHERE (SELECT last_insert_id(id) FROM video_tag_all WHERE tagname=‘利物浦‘) IS NULLLIMIT 1;SELECT LAST_INSERT_ID();

背景：使用insert ignore或insert ... on duplicate key update，都会使自增字段加1，造成不必要的数据空洞，上面的SQL避免了该问题并能够在任何时候都拿到记录ID

缺陷：必须保证表初始化时有一条记录，否则select .... limit 1查不出数据来，就无法insert

Mysql insert without auto-increase when duplicate