mysql联表查询聚合_MySQL--04(聚合函数表连接查询)
聚合函数
统计函数 count()
select count(要统计的字段) from 表名 where [条件]
MariaDB [employe]> select count(*) fromdepartment;+----------+
| count(*) |
+----------+
| 6 |
+----------+
1 row in set (0.00sec)
MariaDB [employe]> select count(id) fromdepartment;+-----------+
| count(id) |
+-----------+
| 6 |
+-----------+
1 row in set (0.00sec)
MariaDB [employe]> select count(id) fromdepartment;+-----------+
| count(id) |
+-----------+
| 6 |
+-----------+
1 row in set (0.00sec)
MariaDB [employe]> select count(id) from department where groups='一分司';+-----------+
| count(id) |
+-----------+
| 2 |
+-----------+
1 row in set (0.00 sec)
View Code
求最大值
select max(求最大值的字段) from 表名;
--注意，max和其它字段不一定是同一条记录
MariaDB [employe]> select max(amount),name fromdepartment;
ERROR1054 (42S22): Unknown column 'name' in 'field list'MariaDB [employe]> select max(amount),master fromdepartment;+-------------+--------+
| max(amount) | master |
+-------------+--------+
| 10 | 张三丰 |
+-------------+--------+
1 row in set (0.00sec)--下面的语句求最大值的部门主管
MariaDB [employe]> select master from department order by amount desc limit 1;+--------+
| master |
+--------+
| 黄山 |
+--------+
1 row in set (0.00sec)
MariaDB [employe]>
View Code
求最小值
select min(要计算最小值的字段) from 表名;
MariaDB [employe]> select min(kpi) fromdepartment;+----------+
| min(kpi) |
+----------+
| 8.00 |
+----------+
1 row in set (0.00sec)
MariaDB [employe]>
View Code
求和 sum
select sum(要求和/总数的字段) from 表名;
MariaDB [employe]> select sum(amount) fromdepartment;+-------------+
| sum(amount) |
+-------------+
| 54 |
+-------------+
1 row in set (0.00 sec)
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求平均值
select avg(要求平均值字段) from 表名;
MariaDB [employe]> select avg(kpi) fromdepartment;+----------+
| avg(kpi) |
+----------+
| 8.663333 |
+----------+
1 row in set (0.00 sec)
View Code
having
MariaDB [employe]> select master from department having amount>8;
ERROR1054 (42S22): Unknown column 'amount' in 'having clause'MariaDB [employe]> select master,amount from department having amount>8;+-----------------+--------+
| master | amount |
+-----------------+--------+
| 黄山 | 10 |
| 黄玉石 | 10 |
| 黄尼古拉斯*赵四 | 10 |
+-----------------+--------+
3 rows in set (0.00sec)
MariaDB [employe]> select master,amount from department where amount>8;+-----------------+--------+
| master | amount |
+-----------------+--------+
| 黄山 | 10 |
| 黄玉石 | 10 |
| 黄尼古拉斯*赵四 | 10 |
+-----------------+--------+
3 rows in set (0.00sec)
MariaDB [employe]> select master from department where amount>8;+-----------------+
| master |
+-----------------+
| 黄山 |
| 黄玉石 |
| 黄尼古拉斯*赵四 |
+-----------------+
3 rows in set (0.00 sec)
View Code
适合用 having,但不适合用where
MariaDB [employe]> select count(id),bumen fromdepartment group by bumen;+-----------+--------+
| count(id) | bumen |
+-----------+--------+
| 1 | NULL |
| 3 | 总部 |
| 2 | 一分司 |
+-----------+--------+
3 rows in set (0.00sec)
MariaDB [employe]> select count(id),bumen from department group by bumen having count(id)>2;+-----------+-------+
| count(id) | bumen |
+-----------+-------+
| 3 | 总部 |
+-----------+-------+
1 row in set (0.00sec)
MariaDB [employe]> select count(id),bumen from department group by bumen where count(id)>2;
ERROR1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'where count(id)>2' at line 1MariaDB [employe]>
View Code
having 和 where 都 可以对记录进行筛选；但是having跟在group by 后面，group by 跟在 where 后面；having 后面条件必须在 select 字段中出现，没有，就会报错；where 是必须是表中字段
where ... group by ... having
表连接
数据准备
create table student(
id int primary key auto_increment,
sname varchar(50),
cls_id int
)engine=innodb default charset=utf8;
create tableclass(
id int primary key auto_increment,
cname varchar(10),
teacher varchar(20)
)engine=innodb default charset=utf8;
insert intoclass (cname,teacher)values('1811a','孙静香');
insert intoclass (cname,teacher)values('1812a','王三多');
insert intoclass (cname,teacher)values('1813a','王多鱼');
insert into student(sname,cls_id)values('谢逊',1);
insert into student(sname,cls_id)values('周大福',1);
insert into student(sname,cls_id)values('高程远',1);
View Code
1. 内联接查询
select [字段] from 表名1 inner join 表名2 on 表名1.连接字段=表名2.连接字段
MariaDB [shop]> select * from class c inner join student s on s.cls_id=c.id;+----+-------+---------+----+--------+--------+
| id | cname | teacher | id | sname | cls_id |
+----+-------+---------+----+--------+--------+
| 1 | 1811a | 孙静香 | 1 | 谢逊 | 1 |
| 1 | 1811a | 孙静香 | 2 | 周大福 | 1 |
| 1 | 1811a | 孙静香 | 3 | 高程远 | 1 |
+----+-------+---------+----+--------+--------+
3 rows in set (0.00sec)
insert into student(sname,cls_id)values('谢三逊',5);
View Code
2. 右联接
select * from student s right join class c on s.cls_id=c.id;
MariaDB [shop]> select * from student s right join class c on s.cls_id=c.id;+------+--------+--------+----+-------+---------+
| id | sname | cls_id | id | cname | teacher |
+------+--------+--------+----+-------+---------+
| 1 | 谢逊 | 1 | 1 | 1811a | 孙静香 |
| 2 | 周大福 | 1 | 1 | 1811a | 孙静香 |
| 3 | 高程远 | 1 | 1 | 1811a | 孙静香 |
| NULL | NULL | NULL | 2 | 1812a | 王三多 |
| NULL | NULL | NULL | 3 | 1813a | 王多鱼 |
+------+--------+--------+----+-------+---------+
5 rows in set (0.00 sec)
View Code
3. 左联接
select * from class c left join student s on s.cls_id=c.id;
MariaDB [shop]> select * from class c left join student s on s.cls_id=c.id;+----+-------+---------+------+--------+--------+
| id | cname | teacher | id | sname | cls_id |
+----+-------+---------+------+--------+--------+
| 1 | 1811a | 孙静香 | 1 | 谢逊 | 1 |
| 1 | 1811a | 孙静香 | 2 | 周大福 | 1 |
| 1 | 1811a | 孙静香 | 3 | 高程远 | 1 |
| 2 | 1812a | 王三多 | NULL | NULL | NULL |
| 3 | 1813a | 王多鱼 | NULL | NULL | NULL |
+----+-------+---------+------+--------+--------+
5 rows in set (0.00 sec)
View Code
4. 笛卡尔积
select * from class c join student s;
MariaDB [shop]> select * from classc join student s;+----+-------+---------+----+--------+--------+
| id | cname | teacher | id | sname | cls_id |
+----+-------+---------+----+--------+--------+
| 1 | 1811a | 孙静香 | 1 | 谢逊 | 1 |
| 2 | 1812a | 王三多 | 1 | 谢逊 | 1 |
| 3 | 1813a | 王多鱼 | 1 | 谢逊 | 1 |
| 1 | 1811a | 孙静香 | 2 | 周大福 | 1 |
| 2 | 1812a | 王三多 | 2 | 周大福 | 1 |
| 3 | 1813a | 王多鱼 | 2 | 周大福 | 1 |
| 1 | 1811a | 孙静香 | 3 | 高程远 | 1 |
| 2 | 1812a | 王三多 | 3 | 高程远 | 1 |
| 3 | 1813a | 王多鱼 | 3 | 高程远 | 1 |
| 1 | 1811a | 孙静香 | 4 | 谢三逊 | 5 |
| 2 | 1812a | 王三多 | 4 | 谢三逊 | 5 |
| 3 | 1813a | 王多鱼 | 4 | 谢三逊 | 5 |
+----+-------+---------+----+--------+--------+
12 rows in set (0.00 sec)
View Code
5. 左表独有
select * from class c left join student s on c.id=s.cls_id where s.id is null;
MariaDB [shop]> select * from class c left join student s on c.id=s.cls_id where s.id isnull;+----+-------+---------+------+-------+--------+
| id | cname | teacher | id | sname | cls_id |
+----+-------+---------+------+-------+--------+
| 2 | 1812a | 王三多 | NULL | NULL | NULL |
| 3 | 1813a | 王多鱼 | NULL | NULL | NULL |
+----+-------+---------+------+-------+--------+
2 rows in set (0.00 sec)
View Code
6. 右表独有
select * from student s right join class c on s.cls_id=c.id where s.id is null;
MariaDB [shop]> select * from student s right join class c on s.cls_id=c.id where s.id isnull;+------+-------+--------+----+-------+---------+
| id | sname | cls_id | id | cname | teacher |
+------+-------+--------+----+-------+---------+
| NULL | NULL | NULL | 2 | 1812a | 王三多 |
| NULL | NULL | NULL | 3 | 1813a | 王多鱼 |
+------+-------+--------+----+-------+---------+
2 rows in set (0.00 sec)
View Code
7. 全联接
MariaDB [shop]> select * from class c left join student s on c.id=s.cls_id->union-> select * from class c right join student s on c.id=s.cls_id;+------+-------+---------+------+--------+--------+
| id | cname | teacher | id | sname | cls_id |
+------+-------+---------+------+--------+--------+
| 1 | 1811a | 孙静香 | 1 | 谢逊 | 1 |
| 1 | 1811a | 孙静香 | 2 | 周大福 | 1 |
| 1 | 1811a | 孙静香 | 3 | 高程远 | 1 |
| 2 | 1812a | 王三多 | NULL | NULL | NULL |
| 3 | 1813a | 王多鱼 | NULL | NULL | NULL |
| NULL | NULL | NULL | 4 | 谢三逊 | 5 |
+------+-------+---------+------+--------+--------+
6 rows in set (0.00 sec)
View Code
# # mysql 本身没有全联接，但是我们可以模拟出来
8. 并集去交集
MariaDB [shop]> select * from class c left join student s on s.cls_id=c.id where s.id isnull->union-> select * from class c right join student s on c.id=s.cls_id where c.id isnull;+------+-------+---------+------+--------+--------+
| id | cname | teacher | id | sname | cls_id |
+------+-------+---------+------+--------+--------+
| 2 | 1812a | 王三多 | NULL | NULL | NULL |
| 3 | 1813a | 王多鱼 | NULL | NULL | NULL |
| NULL | NULL | NULL | 4 | 谢三逊 | 5 |
+------+-------+---------+------+--------+--------+
3 rows in set (0.00 sec)
View Code
9. 自联接(自查询)
表和自身的连接，使用 inner join 来完成
MariaDB [shop]>create table areas(
id int primary key auto_increment,
area varchar(10),
pid int
)engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.02 sec)
View Code
测试数据
INSERT INTO `areas` VALUES (1,'北京',NULL),
(2,'上海',NULL),
(3,'广东',NULL),
(4,'天津',NULL),
(5,'重庆',NULL),
(6,'昌平',1),
(7,'顺义',1),
(8,'海淀',1),
(9,'朝阳',1),
(10,'河北',NULL),
(11,'河南',NULL),
(12,'山西',NULL),
(13,'山东',NULL),
(98,'广州市',3),
(103,'大同',12),
(104,'朔州',12),
(105,'忻州',12),
(106,'阳泉',12),
(107,'吕梁',12),
(108,'晋中',12),
(109,'长治',12),
(110,'晋城',12),
(111,'临汾',12),
(112,'运城',12),
(230,'越秀区',98),
(231,'荔湾区',98),
(232,'海珠区',98),
(233,'天河区',98),
(234,'白云区',98),
(235,'黄埔区',98),
(236,'番禺区',98),
(237,'花都区',98),
(238,'南沙区',98),
(239,'增城区',98),
(240,'从化区',98);
View Code
查询1：所有一级地区
MariaDB [shop]> select * from areas where pid isnull;+----+------+------+
| id | area | pid |
+----+------+------+
| 1 | 北京 | NULL |
| 2 | 上海 | NULL |
| 3 | 广东 | NULL |
| 4 | 天津 | NULL |
| 5 | 重庆 | NULL |
| 10 | 河北 | NULL |
| 11 | 河南 | NULL |
| 12 | 山西 | NULL |
| 13 | 山东 | NULL |
+----+------+------+
9 rows in set (0.00 sec)
View Code
查询2：查询省的名称为“山西”的所有城市
MariaDB [shop]> select * from areas as p inner join areas c on c.pid=p.id where p.area='山西';+----+------+------+-----+------+------+
| id | area | pid | id | area | pid |
+----+------+------+-----+------+------+
| 12 | 山西 | NULL | 103 | 大同 | 12 |
| 12 | 山西 | NULL | 104 | 朔州 | 12 |
| 12 | 山西 | NULL | 105 | 忻州 | 12 |
| 12 | 山西 | NULL | 106 | 阳泉 | 12 |
| 12 | 山西 | NULL | 107 | 吕梁 | 12 |
| 12 | 山西 | NULL | 108 | 晋中 | 12 |
| 12 | 山西 | NULL | 109 | 长治 | 12 |
| 12 | 山西 | NULL | 110 | 晋城 | 12 |
| 12 | 山西 | NULL | 111 | 临汾 | 12 |
| 12 | 山西 | NULL | 112 | 运城 | 12 |
+----+------+------+-----+------+------+
View Code
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