SELECT u.userprovince, u.localarea, u.username, 0, sysdate, sysdate, 0
FROM user_info u LEFT OUTER JOIN shareaccess_stat stat ON u.username=stat.username AND stat.username IS NULL在连接条件中加stat.username is null 的作用是什么?
FROM user_info u LEFT OUTER JOIN shareaccess_stat stat ON u.username=stat.username AND stat.username IS NULL在连接条件中加stat.username is null 的作用是什么?
stat.username IS NULL 表示: 查询条件中stat表的username必须为空
0, sysdate, sysdate, 0 -----sysdate是系统时间,0只是占位列。
FROM user_info u,shareaccess_stat stat
where u.username=stat.username AND stat.username IS NULL