MySQL多表查詢建表與數據準備多表連接查詢 1 交叉連接：不適用任何匹配條件。生成笛卡爾積 2 內連接：只連接匹配的行 3 外鏈接之左連接：優先顯示左表全部記錄 4 外鏈接之右連接：優先顯示右表全部記錄 5 全外連接：顯示左右兩個表全部記錄符合條件連接查詢子查詢 2 帶比較運算符的子查詢 ...

MySQL多表查詢

建表與數據準備

#建表
create table department(    #部門表
id int,
name varchar(20) 
);

create table employee(      #員工表
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
('daxia','female',18,200),
('anwen','male',23,201),
('wudi','male',38,201),
('yage','female',23,202),
('dafei','male',18,200),
('jingyu','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 | daxia | female | 18 | 200 |
| 2 | anwen | male | 23 | 201 |
| 3 | wudi | male | 38 | 201 |
| 4 | yage | female | 23 | 202 |
| 5 | dafei | male | 18 | 200 |
| 6 | jingyu | female | 18 | 204 |
+----+------------+--------+------+--------+

多表連接查詢

#重點：外鏈接語法

SELECT 欄位列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.欄位 = 表2.欄位;

1 交叉連接：不適用任何匹配條件。生成笛卡爾積

mysql> select * from employee,department;
+----+------------+--------+------+--------+------+--------------+
| id | name       | sex    | age  | dep_id | id   | name         |
+----+------------+--------+------+--------+------+--------------+
|  1 | daxia       | female   |   18 |    200 |  200 | 技術         |
|  1 | daxia       | female   |   18 |    200 |  201 | 人力資源     |
|  1 | daxia       | female   |   18 |    200 |  202 | 銷售         |
|  1 | daxia       | female   |   18 |    200 |  203 | 運營         |
|  2 | anwen       | male     |   48 |    201 |  200 | 技術         |
|  2 | anwen       | male     |   48 |    201 |  201 | 人力資源     |
|  2 | anwen       | male     |   48 |    201 |  202 | 銷售         |
|  2 | anwen       | male     |   48 |    201 |  203 | 運營         |
|  3 | wudi        | male   |   38 |    201 |  200 | 技術         |
|  3 | wudi        | male   |   38 |    201 |  201 | 人力資源     |
|  3 | wudi        | male   |   38 |    201 |  202 | 銷售         |
|  3 | wudi        | male   |   38 |    201 |  203 | 運營         |
|  4 | yage        | female |   28 |    202 |  200 | 技術         |
|  4 | yage        | female |   28 |    202 |  201 | 人力資源     |
|  4 | yage        | female |   28 |    202 |  202 | 銷售         |
|  4 | yage        | female |   28 |    202 |  203 | 運營         |
|  5 | dafei       | male   |   18 |    200 |  200 | 技術         |
|  5 | dafei       | male   |   18 |    200 |  201 | 人力資源     |
|  5 | dafei       | male   |   18 |    200 |  202 | 銷售         |
|  5 | dafei       | male   |   18 |    200 |  203 | 運營         |
|  6 | jingyu      | female |   18 |    204 |  200 | 技術         |
|  6 | jingyu      | female |   18 |    204 |  201 | 人力資源     |
|  6 | jingyu      | female |   18 |    204 |  202 | 銷售         |
|  6 | jingyu      | female |   18 |    204 |  203 | 運營         |
+----+------------+--------+------+--------+------+--------------+

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 | daxia      |   18 | female   | 技術       |
|  2 | anwen      |   48 | male     | 人力資源    |
|  3 | wudi       |   38 | male     | 人力資源    |
|  4 | yage       |   28 | female   | 銷售       |
|  5 | dafei      |   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 | daxia       | 技術         |
|  5 | dafei       | 技術         |
|  2 | anwen       | 人力資源     |
|  3 | wudi        | 人力資源     |
|  4 | dafei       | 銷售         |
|  6 | jingyu      | 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 | daxia      | 技術         |
|    2 | anwen      | 人力資源     |
|    3 | wudi       | 人力資源     |
|    4 | yage      | 銷售         |
|    5 | dafei      | 技術         |
| NULL | NULL       | 運營         |
+------+-----------+--------------+

5 全外連接：顯示左右兩個表全部記錄

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 | daxia       | female   |   18 |    200 |  200 | 技術         |
|    5 | dafei       | male   |   18 |    200 |  200 | 技術         |
|    2 | anwen       | male |   48 |    201 |  201 | 人力資源     |
|    3 | wudi        | male   |   38 |    201 |  201 | 人力資源     |
|    4 | yage        | female |   28 |    202 |  202 | 銷售         |
|    6 | jingyu      | 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

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欄位值必須大於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欄位的升序方式顯示
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;

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  |
+---------+------+
| anwen    | 48   |
| wudi     | 38   |
+---------+------+
2 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 | daxia       | female   |   18 |    200 |
|  2 | anwen       | male |   48 |    201 |
|  3 | wudi        | male   |   38 |    201 |
|  4 | yage        | female |   28 |    202 |
|  5 | dafei       | male   |   18 |    200 |
|  6 | jingyu      | 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)