mysql 多表查詢
數據准備
#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 | |
+-------+-------------+------+-----+---------+-------+
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
-
交叉連接 不適用任何匹配條件,生成笛卡爾積
-
select * from 表1,表2;
-
內鏈接 (常用) 只連接匹配的行
-
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
-
外連接
-
左外連接(常用) 優先顯示左表全部記錄 left join
-
select * from staff left join department on 條件(表1.字段=表2.字段)
-
右外鏈接 優先顯示右表全部記錄 right join
-
select * from staff right join department on 條件(表1.字段=表2.字段)
-
全外連接 顯示左右兩個表全部記錄
-
查詢語句 (mysql沒有full join,可以有左外連接+右外連接來實現全外連接)
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
#注意：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
#注意 union與union all的區別：union會去掉相同的紀錄
-
練習
1.找挖礦小分隊的所有員工的信息
#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.查找人力資源所有的員工名字
#答: (名字太長可以起別名)
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.以內連接的方式查詢 staff 和 department表，並且以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.找到部門為 挖礦小分隊 和 人力資源 的所有員工的名字
#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
#5: 多用連表查,因為連表查詢比子查詢效率高
1.用子查詢 找到部門是銷售的所有員工的姓名
#解題思路
①先找department表部門為銷售的部門的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 找到部門為 銷售 和 人力資源 的所有員工的名字
①先找department表部門為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(子查詢得到的是有人的部門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查詢大於所有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；當返回值為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)