mysql 去重方式

1. DISTINCT
2. GROUP BY（不能与ORDER BY一起使用，细节看执行计划）
3. EXISTS（某些地方推荐，数据越多越好用）

对下面这条语句去重，查询数据多时优化，数据多不推荐

SELECT 1 FROM a_r a  
left join t_a_m b on a.id = b.pid
left join t_a_m c on a.id = c.pid
where a.status = '1'   and ((b.tId = 55 AND c.tId = 55) OR (b.tId = 38 AND c.tId = 38)) order by seq desc LIMIT 9

 

SELECT ar.id FROM a_r ar WHERE EXISTS (SELECT 1 FROM a_r a  
left join t_a_m b on a.id = b.pid
left join t_a_m c on a.id = c.pid
where a.status = '1'   and ((b.tId = 55 AND c.tId = 55) OR (b.tId = 38 AND c.tId = 38)) AND a.id = ar.id) order by seq desc LIMIT 9

 

1. UNION（不推荐使用 使用 UNION ALL）