多表查詢

準備工作：準備兩張表，部門表（department）、員工表（employee）

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
);

#插入資料
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),
('nvshen','male',18,200),
('xiaomage','female',18,204)
;

# 查看錶結構和資料
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
rows 
 
in set (0.19 sec)

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    |                |
+--------+-----------------------+------+-----+---------+----------------+
rows in set (0.01 sec)

mysql> select * from department;
+------+--------------+
| id   | name         |
+------+--------------+
|  200 | 技術         |
|  201 | 人力資源     |
|  202 | 銷售         |
|  203 | 運營         |
+------+--------------+
rows in set (0.02 sec)

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 | nvshen   | male   |   18 |    200 |
|  6 | xiaomage | female |   18 |    204 |
+----+----------+--------+------+--------+
rows in set (0.00 sec)

ps:觀察兩張表，發現department表中id=203部門在employee中沒有對應的員工，發現employee中id=6的員工在department表中沒有對應關係。

多表連線查詢

兩張表的準備工作已完成，比如現在我要查詢的員工資訊以及該員工所在的部門。從該題中，我們看出既要查員工又要查該員工的部門，肯定要將兩張表進行連線查詢，多表連線查詢。

重點：外連結語法

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.先看第一種情況交叉連線：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 | egon     | male   |   18 |    200 |  200 | 技術         |
|  1 | egon     | male   |   18 |    200 |  201 | 人力資源     |
|  1 | egon     | male   |   18 |    200 |  202 | 銷售         |
|  1 | egon     | male   |   18 |    200 |  203 | 運營         |
|  2 | alex     | female |   48 |    201 |  200 | 技術         |
|  2 | alex     | female |   48 |    201 |  201 | 人力資源     |
|  2 | alex     | female |   48 |    201 |  202 | 銷售         |
|  2 | alex     | female |   48 |    201 |  203 | 運營         |
|  3 | wupeiqi  | male   |   38 |    201 |  200 | 技術         |
|  3 | wupeiqi  | male   |   38 |    201 |  201 | 人力資源     |
|  3 | wupeiqi  | male   |   38 |    201 |  202 | 銷售         |
|  3 | wupeiqi  | male   |   38 |    201 |  203 | 運營         |
|  4 | yuanhao  | female |   28 |    202 |  200 | 技術         |
|  4 | yuanhao  | female |   28 |    202 |  201 | 人力資源     |
|  4 | yuanhao  | female |   28 |    202 |  202 | 銷售         |
|  4 | yuanhao  | female |   28 |    202 |  203 | 運營         |
|  5 | nvshen   | male   |   18 |    200 |  200 | 技術         |
|  5 | nvshen   | male   |   18 |    200 |  201 | 人力資源     |
|  5 | nvshen   | male   |   18 |    200 |  202 | 銷售         |
|  5 | nvshen   | male   |   18 |    200 |  203 | 運營         |
|  6 | xiaomage | female |   18 |    204 |  200 | 技術         |
|  6 | xiaomage | female |   18 |    204 |  201 | 人力資源     |
|  6 | xiaomage | female |   18 |    204 |  202 | 銷售         |
|  6 | xiaomage | female |   18 |    204 |  203 | 運營         |

2.內連線：只連線匹配的行

#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匹配的結果
#department沒有204這個部門，因而employee表中關於204這條員工資訊沒有匹配出來
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 | nvshen  |   18 | male   | 技術         |
+----+---------+------+--------+--------------+
rows in set (0.00 sec)

#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.外連結之左連線：優先顯示左表全部記錄

#以左表為準，即找出所有員工資訊，當然包括沒有部門的員工
#本質就是：在內連線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 | nvshen   | 技術         |
|  2 | alex     | 人力資源     |
|  3 | wupeiqi  | 人力資源     |
|  4 | yuanhao  | 銷售         |
|  6 | xiaomage | NULL         |
+----+----------+--------------+
rows in set (0.00 sec)

4.外連結之右連線：優先顯示右表全部記錄

#以右表為準，即找出所有部門資訊，包括沒有員工的部門
#本質就是：在內連線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 | nvshen  | 技術         |
| NULL | NULL    | 運營         |
+------+---------+--------------+
rows in set (0.00 sec)

5.全外連線：顯示左右兩個表全部記錄(瞭解)

#外連線：在內連線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
#注意：mysql不支援全外連線 full JOIN
#強調：mysql可以使用此種方式間接實現全外連線
語法：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 | 技術         |
|    5 | nvshen   | 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 | xiaomage | female |   18 |    204 | NULL | NULL         |
| NULL | NULL     | NULL   | NULL |   NULL |  203 | 運營         |
+------+----------+--------+------+--------+------+--------------+
rows in set (0.01 sec)

#注意 union與union all的區別：union會去掉相同的紀錄

符合條件連線查詢

示例1：以內連線的方式查詢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：以內連線的方式查詢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巢狀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子：= 、 !=、> 、<等

例子：

（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);
# 檢視技術部員工姓名
select name from employee
    where dep_id in 
        (select id from department where name='技術');
#檢視不足1人的部門名
select name from department
    where id not in 
        (select dep_id from employee group by dep_id);

（2）帶比較運算子的子查詢

#比較運算子：=、!=、>、>=、<、<=、<>
#查詢大於所有人平均年齡的員工名與年齡
mysql> select name,age from employee where age > (select avg(age) from employee);
+---------+------+
| name    | age  |
+---------+------+
| alex    |   48 |
| wupeiqi |   38 |
+---------+------+

#查詢大於部門內平均年齡的員工名、年齡
思路：
      （1）先對員工表(employee)中的人員分組(group by)，查詢出dep_id以及平均年齡。
       (2)將查出的結果作為臨時表,再對根據臨時表的dep_id和employee的dep_id作為篩選條件將employee表和臨時表進行內連線。
       (3)最後再將employee員工的年齡是大於平均年齡的員工名字和年齡篩選。



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)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 | 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)

(4) all 和 any

# all 滿足子查詢所有結果才為真

# 只有年齡大於所有部門平均年齡的記錄才會被查詢出來
select * from 表 where age > all(select avg(age) from 表 group by 部門欄位);

# any滿足子查詢中一個就為真

# 只要年齡大於任意一個部門的平均年齡就會被查詢出來
select * from 表 where age > any(select avg(age) from 表 group by 部門欄位);