假如有表a 、表b、表c:
a left join b on 條件1 以a爲主表
a left join b on 條件1 left join c on 條件2 執行順序是:
先a和b進行連接,生成一張中間表d,然後再和c進行條件連接生成最後的表的數據
示例:select u.user_id,u.dept_id,d.dept_name from sys_user u left join sys_dept d on u.dept_id = d.dept_id;
select u.user_id,u.dept_id,d.dept_name from sys_user u left join sys_dept d on u.dept_id = d.dept_id and d.dept_id > 103;