假如有表a 、表b、表c:
a left join b on 条件1 以a为主表
a left join b on 条件1 left join c on 条件2 执行顺序是:
先a和b进行连接,生成一张中间表d,然后再和c进行条件连接生成最后的表的数据
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;