MySQL:多表查詢

一，介紹

本節主題

• 多表連接查詢
• 復合條件連接查詢
• 子查詢

準備工作

#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
);
 
#插入數據
insert into department values
(200,‘技術‘),
(201,‘人力資源‘),
(202,‘銷售‘),
(203,‘運營‘);
 
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)
;
 
 
#查看表結構和數據
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 | 技術 |
| 201 | 人力資源 |
| 202 | 銷售 |
| 203 | 運營 |
+------+--------------+
 
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與employee　　

二，多表連接查詢

重點：外鏈接語法

SELECT 字段列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.字段 = 表2.字段;

1，交叉連接：不適用任何匹配條件。生成笛卡兒積

2，內連接：只連接匹配的行

#找兩張表共有的部分，相當於利用條件從笛卡爾積結果中篩選出了正確的結果
#department沒有204這個部門，因而employee表中關於204這條員工信息沒有匹配出來
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   | 技術         |
|  2 | alex      |   48 | female | 人力資源     |
|  3 | wupeiqi   |   38 | male   | 人力資源     |
|  4 | yuanhao   |   28 | female | 銷售         |
|  5 | liwenzhou |   18 | male   | 技術         |
+----+-----------+------+--------+--------------+
 
#上述sql等同於
mysql> select employee.id,employee.name,employee.age,
employee.sex,department.name from employee,department
where employee.dep_id=department.id;

3 外鏈接之左連接：優先顯示左表全部記錄

#以左表為準，即找出所有員工信息，當然包括沒有部門的員工
#本質就是：在內連接的基礎上增加左邊有右邊沒有的結果
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       | 技術         |
|  5 | liwenzhou  | 技術         |
|  2 | alex       | 人力資源     |
|  3 | wupeiqi    | 人力資源     |
|  4 | yuanhao    | 銷售         |
|  6 | jingliyang | NULL         |
+----+------------+--------------+

4 外鏈接之右連接：優先顯示右表全部記錄

#以右表為準，即找出所有部門信息，包括沒有員工的部門
#本質就是：在內連接的基礎上增加右邊有左邊沒有的結果
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      | 技術         |
|    2 | alex      | 人力資源     |
|    3 | wupeiqi   | 人力資源     |
|    4 | yuanhao   | 銷售         |
|    5 | liwenzhou | 技術         |
| NULL | NULL      | 運營         |
+------+-----------+--------------+

5 全外連接：顯示左右兩個表的全部記錄

全外連接：在內連接的基礎上增加左邊有右邊沒有的和右邊有左邊沒有的結果
#註意：mysql不支持全外連接 full JOIN
#強調：mysql可以使用此種方式間接實現全外連接
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
;
#查看結果
+------+------------+--------+------+--------+------+--------------+
| id   | name       | sex    | age  | dep_id | id   | name         |
+------+------------+--------+------+--------+------+--------------+
|    1 | egon       | male   |   18 |    200 |  200 | 技術         |
|    5 | liwenzhou  | male   |   18 |    200 |  200 | 技術         |
|    2 | alex       | female |   48 |    201 |  201 | 人力資源     |
|    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力資源     |
|    4 | yuanhao    | female |   28 |    202 |  202 | 銷售         |
|    6 | jingliyang | female |   18 |    204 | NULL | NULL         |
| NULL | NULL       | NULL   | NULL |   NULL |  203 | 運營         |
+------+------------+--------+------+--------+------+--------------+
 
#註意 union與union all的區別：union會去掉相同的紀錄

三，符合條件連接查詢

示例1：以內連接的方式查詢employee和department表，
並且employee表中的age字段值必須大於25,
即找出年齡大於25歲的員工以及員工所在的部門
 
select employee.name,department.name
     from employee inner join department
    on employee.dep_id = department.id
    where age > 25;
 
示例2：以內連接的方式查詢employee和department表，並且以age字段的升序方式顯示
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;

四，子查詢

• 1：子查詢是將一個查詢語句嵌套在另一個查詢語句中。

• 2：內層查詢語句的查詢結果，可以為外層查詢語句提供查詢條件。

• 3：子查詢中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字

• 4：還可以包含比較運算符：= 、 !=、> 、<等

1，帶IN關鍵字的子查詢

#查詢平均年齡在25歲以上的部門名
select id,name from department
    where id in
        (select dep_id from employee group by dep_id having avg(age) > 25);
 
#查看技術部員工姓名
select name from employee
    where dep_id in
        (select id from department where name=‘技術‘);
 
#查看不足1人的部門名(子查詢得到的是有人的部門id)
select name from department where id not in (select distinct dep_id from employee);

2，帶比較運算符的子查詢

#比較運算符：=、!=、>、>=、<、<=、<>
#查詢大於所有人平均年齡的員工名與年齡
mysql> select name,age from emp where age > (select avg(age) from emp);
+---------+------+
| name | age |
+---------+------+
| alex | 48 |
| wupeiqi | 38 |
+---------+------+
rows in set (0.00 sec)
 
 
#查詢大於部門內平均年齡的員工名、年齡
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，帶exists關鍵字的子查詢

EXISTS關字鍵字表示存在。在使用EXISTS關鍵字時，內層查詢語句不返回查詢的記錄。
　　而是返回一個真假值。True或False
　　當返回True時，外層查詢語句將進行查詢；當返回值為False時，外層查詢語句不進行查詢

#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)

4 - MySQL:多表查詢