多表查询
SELECT
a.user_uid_type,
a.user_uid,
c.user_id,
c.user_type
FROM
mytable1 a
JOIN mytable2 b
ON(a.user_uid_type = b.user_type AND a.user_uid = b.user_id)
JOIN mytable3 c
on(b.device_id = c.device_id)
WHERE a.date='' AND b.date='' AND c.date='' SORT BY rand() LIMIT
给列起别名需要加AS
SELECT xxx_id AS id FROM yourtable
多列去重选择
SELECT DISTINCE col1, col2 FROM yourtable
left join 和 left outer join区别
这两个其实是一样的，left join是 left outer join的简称
join和left join的区别 比如有两个表 table1 name age
jack
bob
kate table age career student
doctor 那么table1 LEFT JOIN table2就是 name age career
jack student
bob NULL
kate NULL 如果table1 JOIN table2 就是 name age career
jack student