MySQL多表查询实例详解【链接查询、子查询等】(2)-木庄网络博客

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语法：select * from employee left join department on employee.dep_id = department.id
union all
select * from employee right join department on employee.dep_id = department.id;

 mysql> select * from employee left join department on employee.dep_id = department.id
     union
    select * from employee right join department on employee.dep_id = department.id
      ;
+------+----------+--------+------+--------+------+--------------+
| id  | name   | sex  | age | dep_id | id  | name     |
+------+----------+--------+------+--------+------+--------------+
|  1 | egon   | male  |  18 |  200 | 200 | 技术     |
|  5 | nvshen  | male  |  18 |  200 | 200 | 技术     |
|  2 | alex   | female |  48 |  201 | 201 | 人力资源   |
|  3 | wupeiqi | male  |  38 |  201 | 201 | 人力资源   |
|  4 | yuanhao | female |  28 |  202 | 202 | 销售     |
|  6 | xiaomage | female |  18 |  204 | NULL | NULL     |
| NULL | NULL   | NULL  | NULL |  NULL | 203 | 运营     |
+------+----------+--------+------+--------+------+--------------+
7 rows in set (0.01 sec)

#注意 union与union all的区别：union会去掉相同的纪录

二、符合条件连接查询

以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25，即找出年龄大于25岁的员工以及员工所在的部门

select employee.name,department.name from employee inner join department
　　on employee.dep_id = department.id
　　where age > 25;

三、子查询

#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

（1）带in关键字的子查询

#查询平均年龄在25岁以上的部门名
select id,name from department
  where id in
    (select dep_id from employee group by dep_id having avg(age) > 25);
# 查看技术部员工姓名
select name from employee
  where dep_id in
    (select id from department where name='技术');
#查看不足1人的部门名
select name from department
  where id not in
    (select dep_id from employee group by dep_id);

（2）带比较运算符的子查询

#比较运算符：=、!=、>、>=、<、<=、<>
#查询大于所有人平均年龄的员工名与年龄
mysql> select name,age from employee where age > (select avg(age) from employee);
+---------+------+
| name  | age |
+---------+------+
| alex  |  48 |
| wupeiqi |  38 |
+---------+------+
#查询大于部门内平均年龄的员工名、年龄

思路：

（1）先对员工表(employee)中的人员分组(group by)，查询出dep_id以及平均年龄。
(2)将查出的结果作为临时表,再对根据临时表的dep_id和employee的dep_id作为筛选条件将employee表和临时表进行内连接。
(3)最后再将employee员工的年龄是大于平均年龄的员工名字和年龄筛选。

mysql> select t1.name,t1.age from employee as t1
       inner join
      (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2
      on t1.dep_id = t2.dep_id
      where t1.age > t2.avg_age;
+------+------+
| name | age |
+------+------+
| alex |  48 |

(3)带EXISTS关键字的子查询

#EXISTS关字键字表示存在。在使用EXISTS关键字时，内层查询语句不返回查询的记录。而是返回一个真假值。True或False
#当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询
#department表中存在dept_id=203，Ture
mysql> select * from employee where exists (select id from department where id=200);
+----+----------+--------+------+--------+
| id | name   | sex  | age | dep_id |
+----+----------+--------+------+--------+
| 1 | egon   | male  |  18 |  200 |
| 2 | alex   | female |  48 |  201 |
| 3 | wupeiqi | male  |  38 |  201 |
| 4 | yuanhao | female |  28 |  202 |
| 5 | nvshen  | male  |  18 |  200 |
| 6 | xiaomage | female |  18 |  204 |
+----+----------+--------+------+--------+
#department表中存在dept_id=205，False
mysql> select * from employee where exists (select id from department where id=204);
Empty set (0.00 sec)

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