准备                                                           

建表和数据

#建表

create table department(

id int,

name varchar(20)

);

create table employee(

id int primary key auto_increment,

name varchar(20),

sex enum('male','female') not null default 'male',

age int,

dep_id int

);

#插入数据

insert into department values

(200,'技术'),

(201,'人力资源'),

(202,'销售'),

(203,'运营');

insert into employee(name,sex,age,dep_id) values

('egon','male',18,200),

('alex','female',48,201),

('wupeiqi','male',38,201),

('yuanhao','female',28,202),

('liwenzhou','male',18,200),

('jingliyang','female',18,204)

;

#查看表结构和数据

mysql> desc department;

+-------+-------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+-------+-------------+------+-----+---------+-------+

| id | int(11) | YES | | NULL | |

| name | varchar(20) | YES | | NULL | |

+-------+-------------+------+-----+---------+-------+

mysql> desc employee;

+--------+-----------------------+------+-----+---------+----------------+

| Field | Type | Null | Key | Default | Extra |

+--------+-----------------------+------+-----+---------+----------------+

| id | int(11) | NO | PRI | NULL | auto_increment |

| name | varchar(20) | YES | | NULL | |

| sex | enum('male','female') | NO | | male | |

| age | int(11) | YES | | NULL | |

| dep_id | int(11) | YES | | NULL | |

+--------+-----------------------+------+-----+---------+----------------+

mysql> select * from department;

+------+--------------+

| id | name |

+------+--------------+

| 200 | 技术 |

| 201 | 人力资源 |

| 202 | 销售 |

| 203 | 运营 |

+------+--------------+

mysql> select * from employee;

+----+------------+--------+------+--------+

| 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 | liwenzhou | male | 18 | 200 |

| 6 | jingliyang | female | 18 | 204 |

+----+------------+--------+------+--------+

表department与employee

内容

                                                                  多表连接查询                                                   

#重点:外连接语句

select 字段列表 from 表一 inner|left|right join 表二 on 表一.字段=表二.字段;

1.交叉连接: 不适用任何匹配条件.生成笛卡尔积

mysql> select * from employee,department;

+----+------------+--------+------+--------+------+--------------+

| id | name       | sex    | age  | dep_id | id   | name         |

+----+------------+--------+------+--------+------+--------------+

|  1 | egon       | male   |   18 |    200 |  200 | 技术         |

|  1 | egon       | male   |   18 |    200 |  201 | 人力资源     |

|  1 | egon       | male   |   18 |    200 |  202 | 销售         |

|  1 | egon       | male   |   18 |    200 |  203 | 运营         |

|  2 | alex       | female |   48 |    201 |  200 | 技术         |

|  2 | alex       | female |   48 |    201 |  201 | 人力资源     |

|  2 | alex       | female |   48 |    201 |  202 | 销售         |

|  2 | alex       | female |   48 |    201 |  203 | 运营         |

|  3 | wupeiqi    | male   |   38 |    201 |  200 | 技术         |

|  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |

|  3 | wupeiqi    | male   |   38 |    201 |  202 | 销售         |

|  3 | wupeiqi    | male   |   38 |    201 |  203 | 运营         |

|  4 | yuanhao    | female |   28 |    202 |  200 | 技术         |

|  4 | yuanhao    | female |   28 |    202 |  201 | 人力资源     |

|  4 | yuanhao    | female |   28 |    202 |  202 | 销售         |

|  4 | yuanhao    | female |   28 |    202 |  203 | 运营         |

|  5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |

|  5 | liwenzhou  | male   |   18 |    200 |  201 | 人力资源     |

|  5 | liwenzhou  | male   |   18 |    200 |  202 | 销售         |

|  5 | liwenzhou  | male   |   18 |    200 |  203 | 运营         |

|  6 | jingliyang | female |   18 |    204 |  200 | 技术         |

|  6 | jingliyang | female |   18 |    204 |  201 | 人力资源     |

|  6 | jingliyang | female |   18 |    204 |  202 | 销售         |

|  6 | jingliyang | female |   18 |    204 |  203 | 运营         |

+----+------------+--------+------+--------+------+--------------+

交叉连接

2.内连接:只连接匹配的行

#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 employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;

+----+-----------+------+--------+--------------+

| id | name      | age  | sex    | name         |

+----+-----------+------+--------+--------------+

|  1 | egon      |   18 | male   | 技术         |

|  2 | alex      |   48 | female | 人力资源     |

|  3 | wupeiqi   |   38 | male   | 人力资源     |

|  4 | yuanhao   |   28 | female | 销售         |

|  5 | liwenzhou |   18 | male   | 技术         |

+----+-----------+------+--------+--------------+

#上述sql等同于

mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

内连接

3.外连接之左右连接

#优先显示左表全部记录

#以左表为准，即找出所有员工信息，当然包括没有部门的员工

#本质就是：在内连接的基础上增加左边有右边没有的结果

mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;

+----+------------+--------------+

| id | name       | depart_name  |

+----+------------+--------------+

|  1 | egon       | 技术         |

|  5 | liwenzhou  | 技术         |

|  2 | alex       | 人力资源     |

|  3 | wupeiqi    | 人力资源     |

|  4 | yuanhao    | 销售         |

|  6 | jingliyang | NULL         |

+----+------------+--------------+

---------------------------------------------------------------------------

#优先显示右表全部记录

#以右表为准，即找出所有部门信息，包括没有员工的部门

#本质就是：在内连接的基础上增加右边有左边没有的结果

mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;

+------+-----------+--------------+

| id   | name      | depart_name  |

+------+-----------+--------------+

|    1 | egon      | 技术         |

|    2 | alex      | 人力资源     |

|    3 | wupeiqi   | 人力资源     |

|    4 | yuanhao   | 销售         |

|    5 | liwenzhou | 技术         |

| NULL | NULL      | 运营         |

+------+-----------+--------------+

外连接.左右

4.全外连接:显示左右两个表全部记录

全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果

#注意：mysql不支持全外连接 full JOIN

#强调：mysql可以使用此种方式间接实现全外连接

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

;

#查看结果

+------+------------+--------+------+--------+------+--------------+

| id   | name       | sex    | age  | dep_id | id   | name         |

+------+------------+--------+------+--------+------+--------------+

|    1 | egon       | male   |   18 |    200 |  200 | 技术         |

|    5 | liwenzhou  | 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 | jingliyang | female |   18 |    204 | NULL | NULL        |

| NULL | NULL       | NULL   | NULL |   NULL |  203 | 运营         |

+------+------------+--------+------+--------+------+--------------+

#注意 union与union all的区别：union会去掉相同的纪录

全外连接

                                                               符合条件连接查询                                              

#示例1：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;

#示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示

select employee.id,employee.name,employee.age,department.name from employee,department

    where employee.dep_id = department.id

    and age > 25

    order by age asc;

                                                                       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(子查询得到的是有人的部门id)

select name from department where id not in (select distinct dep_id from employee);

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 emp where age > (select avg(age) from emp);

+---------+------+

| name    | age  |

+---------+------+

| alex    | 48   |

| wupeiqi | 38   |

+---------+------+

2 rows in set (0.00 sec)

#查询大于部门内平均年龄的员工名、年龄

select t1.name,t1.age from emp t1

inner join

(select dep_id,avg(age) avg_age from emp group by dep_id) t2

on t1.dep_id = t2.dep_id

where t1.age > t2.avg_age;

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 | liwenzhou  | male   |   18 |    200 |

|  6 | jingliyang | 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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