摘要：UNION:操作符用于合并两个或多个 SELECT 语句的结果集。SELECT id, name FROM user1UNIONSELECT id, name FROM user2如果user1id name1 ok2 bee3 colluser2id name3 coll4 new结果为id name1 ok2 bee3 coll4 newUNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL,如下：SELECT id, name FROM user1UNION ALLSELECT id, name FROM user2结果为i... 阅读全文

摘要：举个例子说明：select d.id, d.uid,d.username,d.dateline, d.message,d.replynum, c.picid, c.filename from doing d left join( select a.*, b.filename from picture a left join my_picture b on (a.id = b.id and a.mid = b.mid)) con c.doid = d.doid limit 1, 20;解析： 左连接是左边的表数据全部保留，右边的表选择符合条件的数据，拼接成一个数据集（表）。select a... 阅读全文