MySQL怎么完成多表查询？MySQL多表查询的语句

# 找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果 #department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来 mysql> select * from employee1,department where employee1.dep_id=department.id; #上面用where表示的可以用下面的内连接表示，建议使用下面的那种方法 mysql> select * from employee1 inner join department on employee1.dep_id=department.id; # 也可以这样表示哈 mysql> select employee1.id,employee1.name,employee1.age,employee1.sex,department.name from employee1,department where employee1.dep_id=department.id;

#左链接：在按照on的条件取到两张表共同部分的基础上，保留左表的记录 mysql> select * from employee1 left join department on department.id=employee1.dep_id; mysql> select * from department left join employee1 on department.id=employee1.dep_id;

#右链接：在按照on的条件取到两张表共同部分的基础上，保留右表的记录 mysql> select * from employee1 right join department on department.id=employee1.dep_id; mysql> select * from department right join employee1 on department.id=employee1.dep_id;

mysql> select * from department full join employee1;

mysql> select * from employee1 inner join department on employee1.dep_id=department.id and age>25;

mysql> select * from employee1 inner join department on employee1.dep_id=department.id and age>25 and age>25 order by age asc;

#1：子查询是将一个查询语句嵌套在另一个查询语句中。 #2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。 #3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字 #4：还可以包含比较运算符：= 、 !=、> 、<等

# 查询平均年龄在25岁以上的部门名 mysql> select name from department where id in ( select dep_id from employee1 group by dep_id having avg(age) > 25 ); # 查看技术部员工姓名 mysql> select name from employee1 where dep_id = (select id from department where name='技术'); # 查看小于2人的部门名 mysql> select name from department where id in (select dep_id from employee1 group by dep_id having count(id) < 2) union select name from department where id not in (select distinct dep_id from employee1); # 提取空部门 #有人的部门 mysql> select * from department where id not in (select distinct dep_id from employee1);