mysql 多表查詢

時間  2019-12-10
標籤 mysql 查詢 欄目 MySQL 简体版
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目錄python

mysql 多表查詢

數據準備

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

create table staff(
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,'挖礦小分隊'),
(201,'人力資源'),
(202,'銷售'),
(203,'運營');

insert into staff(name,sex,age,dep_id) values
('程咬金','male',38,200),
('露娜','female',26,201),
('李白','male',38,201),
('王昭君','female',28,202),
('典韋','male',118,200),
('小喬','female',16,204)
;


#查看錶結構和數據
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.06 sec)

mysql> desc staff;
+--------+-----------------------+------+-----+---------+----------------+
| 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    |                |
+--------+-----------------------+------+-----+---------+----------------+
5 rows in set (0.04 sec)

#表department與staff
mysql> select * from department;
+------+-----------------+
| id   | name            |
+------+-----------------+
|  200 | 挖礦小分隊      |
|  201 | 人力資源        |
|  202 | 銷售            |
|  203 | 運營            |
+------+-----------------+
4 rows in set (0.00 sec)

mysql> select * from staff;
+----+-----------+--------+------+--------+
| id | name      | sex    | age  | dep_id |
+----+-----------+--------+------+--------+
|  1 | 程咬金    | male   |   38 |    200 |
|  2 | 露娜      | female |   26 |    201 |
|  3 | 李白      | male   |   38 |    201 |
|  4 | 王昭君    | female |   28 |    202 |
|  5 | 典韋      | male   |  118 |    200 |
|  6 | 小喬      | female |   16 |    204 |
+----+-----------+--------+------+--------+
6 rows in set (0.00 sec)

連表查

  • 交叉鏈接 不適用任何匹配條件,生成笛卡爾積

  • select * from 表1,表2;

  • 內連接 (經常使用) 只鏈接匹配的行

  • select * from staff inner join department on 條件(表1.字段=表2.字段)

  • 小結: 找兩張表共有的部分，利用條件從笛卡爾積結果中篩選出了正確的結果mysql

  • 外鏈接

  • 左外鏈接(經常使用) 優先顯示左表所有記錄 left join

  • select * from staff left join department on 條件(表1.字段=表2.字段)

  • 右外連接 優先顯示右表所有記錄 right join

  • select * from staff right join department on 條件(表1.字段=表2.字段)

  • 全外鏈接 顯示左右兩個表所有記錄

  • 查詢語句 (mysql沒有full join,能夠有左外鏈接+右外鏈接來實現全外鏈接)
select * from 表1  left join 表2 on 條件((表1.字段=表2.字段))
union
select * from 表1  right join 表2 on 條件((表1.字段=表2.字段));

#全外鏈接：在內鏈接的基礎上增長左邊有右邊沒有的和右邊有左邊沒有的結果
#注意：mysql不支持全外鏈接 full join
#強調：mysql能夠使用此種方式間接實現全外鏈接
#注意 union與union all的區別：union會去掉相同的紀錄

  • 練習

1.找挖礦小分隊的全部員工的信息
#答:
mysql> select * from staff inner join department on department.id=staff.dep_id where department.name='挖礦小分隊';
+----+-----------+------+------+--------+------+-----------------+
| id | name      | sex  | age  | dep_id | id   | name            |
+----+-----------+------+------+--------+------+-----------------+
|  1 | 程咬金    | male |   38 |    200 |  200 | 挖礦小分隊      |
|  5 | 典韋      | male |  118 |    200 |  200 | 挖礦小分隊      |
+----+-----------+------+------+--------+------+-----------------+
2 rows in set (0.00 sec)

2.查找人力資源全部的員工名字
#答:  (名字太長能夠起別名)
mysql> select staff.name  from staff inner join department as dep on dep.id=staff.dep_id where dep.name='人力資源';
+--------+
| name   |
+--------+
| 露娜   |
| 李白   |
+--------+
2 rows in set (0.00 sec)

3.找出年齡大於38的員工的姓名,及其所在的部門名稱
#答:
mysql> select staff.name,dep.name  from staff  inner join department as dep on dep.id=staff.dep_id where age>38;
+--------+-----------------+
| name   | name            |
+--------+-----------------+
| 典韋   | 挖礦小分隊       |
+--------+-----------------+
1 row in set (0.00 sec)

4.之內鏈接的方式查詢 staff 和 department表，而且以age字段的升序方式顯示
答:
mysql> select *  from staff  inner join department as dep on dep.id=staff.dep_id order by age;
+----+-----------+--------+------+--------+------+-----------------+
| id | name      | sex    | age  | dep_id | id   | name            |
+----+-----------+--------+------+--------+------+-----------------+
|  2 | 露娜      | female |   26 |    201 |  201 | 人力資源        |
|  4 | 王昭君    | female |   28 |    202 |  202 | 銷售            |
|  1 | 程咬金    | male   |   38 |    200 |  200 | 挖礦小分隊      |
|  3 | 李白      | male   |   38 |    201 |  201 | 人力資源        |
|  5 | 典韋      | male   |  118 |    200 |  200 | 挖礦小分隊      |
+----+-----------+--------+------+--------+------+-----------------+
5 rows in set (0.05 sec)

5.找到部門爲 挖礦小分隊 和 人力資源 的全部員工的名字
#答: 
mysql> select staff.name from staff inner join department as dep on dep.id = staff.dep_id where dep.name in ('挖礦小分隊','人力資源');
+-----------+
| name      |
+-----------+
| 程咬金    |
| 露娜      |
| 李白      |
| 典韋      |
+-----------+
4 rows in set (0.00 sec)

子查詢

#1：子查詢是將一個查詢語句嵌套在另外一個查詢語句中。
#2：內層查詢語句的查詢結果，能夠爲外層查詢語句提供查詢條件。
#3：子查詢中能夠包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等關鍵字
#4：還能夠包含比較運算符：= 、 !=、> 、<等
#5: 多用連表查,由於連表查詢比子查詢效率高

1.用子查詢 找到部門是銷售的全部員工的姓名
#解題思路
①先找department表部門爲銷售的部門的id
mysql> select id from department where name = '銷售';
+------+
| id   |
+------+
|  202 |
+------+
1 row in set (0.00 sec)

②再找staff表中部門dep_id = 202
mysql> select name from staff where dep_id = 202;
+-----------+
| name      |
+-----------+
| 王昭君    |
+-----------+
1 row in set (0.00 sec)

③字表查
mysql> select name from staff where dep_id =(select id from department where name = '銷售');
+-----------+
| name      |
+-----------+
| 王昭君    |
+-----------+
1 row in set (0.00 sec)

2.用子查詢 找到部門爲 銷售 和 人力資源 的全部員工的名字
①先找department表部門爲銷售和人力資源的部門的id
mysql> select id from department where name = '銷售' or name = '人力資源';
+------+
| id   |
+------+
|  201 |
|  202 |
+------+
2 rows in set (0.00 sec)

②子查詢
mysql> select name from staff where dep_id in (select id from department where name = '銷售' or name = '人力資源');
+-----------+
| name      |
+-----------+
| 露娜      |
| 李白      |
| 王昭君    |
+-----------+
3 rows in set (0.00 sec)

1. 帶IN關鍵字的子查詢
①查詢平均年齡在28歲以上的部門名
select id,name from department
    where id in 
        (select dep_id from staff group by dep_id having avg(age) > 28);
#結果        
+------+-----------------+
| id   | name            |
+------+-----------------+
|  200 | 挖礦小分隊      |
|  201 | 人力資源        |
+------+-----------------+
2 rows in set (0.00 sec)

②查看部門是挖礦小分隊員工姓名
select name from staff
    where dep_id in 
        (select id from department where name='挖礦小分隊');
#結果
+-----------+
| name      |
+-----------+
| 程咬金    |
| 典韋      |
+-----------+
2 rows in set (0.00 sec)

③查看不足1人的部門名(子查詢獲得的是有人的部門id)
select name from department where id not in (select distinct dep_id from staff);
#結果
+--------+
| name   |
+--------+
| 運營   |
+--------+
1 row in set (0.02 sec)


2. 帶比較運算符的子查詢
#比較運算符：=、!=、>、>=、<、<=、<>

①查詢大於全部人平均年齡的員工名與年齡
mysql> select name,age from staff where age > (select avg(age) from staff);
+--------+------+
| name   | age  |
+--------+------+
| 典韋   |  118 |
+--------+------+
1 row in set (0.00 sec)


②查詢大於部門內平均年齡的員工名、年齡
select t1.name,t1.age from staff t1
inner join 
(select dep_id,avg(age) avg_age from staff group by dep_id) t2
on t1.dep_id = t2.dep_id
where t1.age > t2.avg_age; 
#結果
+--------+------+
| name   | age  |
+--------+------+
| 李白   |   38 |
| 典韋   |  118 |
+--------+------+
2 rows in set (0.04 sec)


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；當返回值爲False時，外層查詢語句不進行查詢

#department表中存在dept_id=203，Ture
select * from staff
    where exists
    (select id from department where id=200);
#結果
+----+-----------+--------+------+--------+
| id | name      | sex    | age  | dep_id |
+----+-----------+--------+------+--------+
|  1 | 程咬金    | male   |   38 |    200 |
|  2 | 露娜      | female |   26 |    201 |
|  3 | 李白      | male   |   38 |    201 |
|  4 | 王昭君    | female |   28 |    202 |
|  5 | 典韋      | male   |  118 |    200 |
|  6 | 小喬      | female |   16 |    204 |
+----+-----------+--------+------+--------+
6 rows in set (0.00 sec)



#department表中存在dept_id=205，False
mysql> select * from staff
    where exists
    (select id from department where id=204);
Empty set (0.00 sec)
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