MySQL数据库3分组与单表、多表查询

• 2019 年 12 月 16 日
• 笔记

浮华褪尽，人比烟花寂寞…… ——张爱玲

一、表操作的补充

select * from 表名 where 列名 in （值1，值2,。。。）；

查出对应值的数据。

1.1null 和 not null

使用null的时候：

当创建的表中有null时我们如果需要查询出来null所对应的信息，需要用select * from 表名 where 字段名 is null;

mysql> create table v1(id int auto_increment primary key,      -> name varchar(32),email varchar(32))charset=utf8;  Query OK, 0 rows affected (0.70 sec)    mysql> insert into v1(email) values('xxx');  Query OK, 1 row affected (0.07 sec)    mysql> select * from v1;  +----+------+-------+  | id | name | email |  +----+------+-------+  |  1 | NULL | xxx   |  +----+------+-------+  1 row in set (0.00 sec)    mysql> select * from v1 where name is null;  +----+------+-------+  | id | name | email |  +----+------+-------+  |  1 | NULL | xxx   |  +----+------+-------+  1 row in set (0.01 sec)    mysql> select *from v1 where name='';  Empty set (0.00 sec)

使用null会降低数据的查询效率，不推荐使用，在创建表的时候建议把值默认为空。

1.2使用not null的时候

mysql> create table v2(id int auto_increment primary key,      -> name varchar(32) not null default '',email varchar(32)not null default '')charset=utf8;  Query OK, 0 rows affected (0.44 sec)      mysql> insert into v2(email) values('xxx');  Query OK, 1 row affected (0.06 sec)    mysql> select *from v2 where name='';  +----+------+-------+  | id | name | email |  +----+------+-------+  |  1 |      | xxx   |  +----+------+-------+  1 row in set (0.00 sec)

二、单表的操作（import）

2.1分组

分组：将所标记的某个相同字段进行归类，比如员工信息表的职位分组，或者按照性别进行分组等。

2.1.1聚合函数

max（列）求出列中的最大值

min（列）求出列中的最小值

sum（列）对列中的数据求和

count（列）对列中的数据计数

avg（列）对列中的数据计算平均数

例子见group by

2.1.2group by

用法：

select 聚合函数, 选取的字段 from employee group by 分组的字段;

group by 是分组的关键词，group by 必须和聚合函数（count）一块出现。count（字段名），按照条件对字段中的数据进行计数。

例子：

1.以性别为例， 进行分组， 统计一下男生和女生的人数是多少个。

mysql> create table employee(      -> id int not null unique auto_increment primary key,      -> name varchar(20) not null,      -> gender enum('male','female') not null default 'male',      -> age int(3) unsigned not null default 28,      -> hire_date date not null,      -> post varchar(50),      -> post_comment varchar(100),      -> salary double(15,2),      -> office int,      -> depart_id int      -> )charset=utf8;  Query OK, 0 rows affected (0.61 sec)    mysql> insert into employee(name,gender,age,hire_date,post,salary,office,depart_id) values      -> ('小张','male',73,'20140701','研发部',3500,401,1),      -> ('小李','male',28,'20121101','研发部',2100,401,1),      -> ('小赵','female',18,'20150411','研发部',18000,403,3),      -> ('歪歪','female',48,'20150311','销售部',3000.13,402,2),      -> ('丫丫','female',38,'20101101','销售部',2000.35,402,2),      -> ('丁丁','female',18,'20110312','销售部',1000.37,402,2),      -> ('小明','male',28,'20160311','运营部',10000.13,403,3),      -> ('小华','male',18,'19970312','运营部',20000,403,3),      -> ('小王','female',18,'20130311','运营部',19000,403,3);  Query OK, 9 rows affected (0.09 sec)  Records: 9  Duplicates: 0  Warnings: 0    mysql> select count(id),gender from employee group by gender;  +-----------+--------+  | count(id) | gender |  +-----------+--------+  |         4 | male   |  |         5 | female |  +-----------+--------+  2 rows in set (0.10 sec)    mysql> select gender,count(id) as total from employee group by gender;  #这里可以用as重命名显示的列名  +--------+-------+  | gender | total |  +--------+-------+  | male   |     4 |  | female |     5 |  +--------+-------+  2 rows in set (0.00 sec)

2.对部门进行分组， 求出每个部门年龄最大的那个人。

mysql> select depart_id , max(age) from employee group by depart_id;  +-----------+----------+  | depart_id | max(age) |  +-----------+----------+  |         1 |       73 |  |         2 |       48 |  |         3 |       28 |  +-----------+----------+  3 rows in set (0.04 sec)

3.对部门进行分组， 求出每个部门年龄求和。

mysql> select depart_id,sum(age) from employee group by depart_id;  +-----------+----------+  | depart_id | sum(age) |  +-----------+----------+  |         1 |      101 |  |         2 |      104 |  |         3 |       82 |  +-----------+----------+  3 rows in set (0.00 sec)

4.对部门进行分组， 求出每个部门年龄求平均数。

mysql> select depart_id,avg(age) from employee group by depart_id;  +-----------+----------+  | depart_id | avg(age) |  +-----------+----------+  |         1 |  50.5000 |  |         2 |  34.6667 |  |         3 |  20.5000 |  +-----------+----------+  3 rows in set (0.02 sec)

2.1.3having

对group by 之后的数据进行二次筛选

例子

5.对部门进行分组， 求出每个部门年龄求平均数,选出平均数最大的部门。

mysql> select depart_id,avg(age) from employee group by depart_id having avg(age)>35;  +-----------+----------+  | depart_id | avg(age) |  +-----------+----------+  |         1 |  50.5000 |  +-----------+----------+  1 row in set (0.00 sec)      mysql> select depart_id,avg(age)as pj from employee group by depart_id having pj>35;  +-----------+---------+  | depart_id | pj      |  +-----------+---------+  |         1 | 50.5000 |  +-----------+---------+  1 row in set (0.00 sec)

2.1.4升序和降序

order by 字段名 asc（升序）desc(降序)

升序和降序可以同时使用如age desc, id asc; 表示： 先对age进行降序， 如果age有相同的行， 则对id进行升序。

例子

mysql> select * from employee order by age desc,id desc;  +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+  | id | name   | gender | age | hire_date  | post      | post_comment | salary   | office | depart_id |  +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+  |  1 | 小张   | male   |  73 | 2014-07-01 | 研发部    | NULL         |  3500.00 |    401 |         1 |  |  4 | 歪歪   | female |  48 | 2015-03-11 | 销售部    | NULL         |  3000.13 |    402 |         2 |  |  5 | 丫丫   | female |  38 | 2010-11-01 | 销售部    | NULL         |  2000.35 |    402 |         2 |  |  7 | 小明   | male   |  28 | 2016-03-11 | 运营部    | NULL         | 10000.13 |    403 |         3 |  |  2 | 小李   | male   |  28 | 2012-11-01 | 研发部    | NULL         |  2100.00 |    401 |         1 |  |  9 | 小王   | female |  18 | 2013-03-11 | 运营部    | NULL         | 19000.00 |    403 |         3 |  |  8 | 小华   | male   |  18 | 1997-03-12 | 运营部    | NULL         | 20000.00 |    403 |         3 |  |  6 | 丁丁   | female |  18 | 2011-03-12 | 销售部    | NULL         |  1000.37 |    402 |         2 |  |  3 | 小赵   | female |  18 | 2015-04-11 | 研发部    | NULL         | 18000.00 |    403 |         3 |  +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+  9 rows in set (0.00 sec)

2.1.5limit限制输出

limit offset ，size

limit 起始行索引，向下查询的长度(索引为0代表第一行)

例子

mysql> select * from employee limit 0,3;  +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+  | id | name   | gender | age | hire_date  | post      | post_comment | salary   | office | depart_id |  +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+  |  1 | 小张   | male   |  73 | 2014-07-01 | 研发部    | NULL         |  3500.00 |    401 |         1 |  |  2 | 小李   | male   |  28 | 2012-11-01 | 研发部    | NULL         |  2100.00 |    401 |         1 |  |  3 | 小赵   | female |  18 | 2015-04-11 | 研发部    | NULL         | 18000.00 |    403 |         3 |  +----+--------+--------+-----+------------+-----------+--------------+----------+--------+-----------+  3 rows in set (0.00 sec)

2.1.6查询表示需要遵循的顺序（important）

select * from 表名 where 条件 group by 条件 having 条件 order by 条件 limit 条件;

where > group by > having > order by > limit

三、多表操作

外键

主关键字(primary key)是表中的一个或多个字段，它的值用于唯一地标识表中的某一条记录。

公共关键字（Common Key）在关系数据库中，关系之间的联系是通过相容或相同的属性或属性组来表示的。如果两个关系中具有相容或相同的属性或属性组，那么这个属性或属性组被称为这两个关系的公共关键字。

如果公共关键字在一个关系中是主关键字，那么这个公共关键字被称为另一个关系的外键。由此可见，外键表示了两个关系之间的相关联系。以另一个关系的外键作主关键字的表被称为主表，具有此外键的表被称为主表的从表。外键又称作外关键字。

使用外键的原因：

1.减少占用的内存空间

2.只需要修改主表的数据，从表的数据也会相应的跟着修改

3.1一对多

一对多指一个主表中的数据和从表中的数据是一对多的关系，如下例，一个部门可以有多个员工。

使用方法：

constraint 外键名 foreign key (被约束的字段) references 约束的表(约束的字段)

mysql> create table department(      -> id int auto_increment primary key,      -> name varchar(32) not null default '')      -> charset utf8;  Query OK, 0 rows affected (0.42 sec)    mysql> insert into department(name) values('研发部'),('运维部'),('前台部'),('小卖部');  Query OK, 4 rows affected (0.06 sec)  Records: 4  Duplicates: 0  Warnings: 0    mysql> create table userinfo(id int auto_increment primary key,      -> name varchar(32) not null default '',      -> depart_id int not null default 1,      -> constraint fk_user_depart foreign key (depart_id) references department(id))      -> charset utf8;  Query OK, 0 rows affected (0.39 sec)    mysql> insert into userinfo (name,depart_id) values('xiaozhu',1),('xiaoyu',1),      -> ('laohe',2),('longge',2),('ludi',3),('xiaoguo',4);  Query OK, 6 rows affected (0.21 sec)  Records: 6  Duplicates: 0  Warnings: 0    mysql> select * from userinfo;  +----+---------+-----------+  | id | name    | depart_id |  +----+---------+-----------+  |  1 | xiaozhu |         1 |  |  2 | xiaoyu  |         1 |  |  3 | laohe   |         2 |  |  4 | longge  |         2 |  |  5 | ludi    |         3 |  |  6 | xiaoguo |         4 |  +----+---------+-----------+  6 rows in set (0.00 sec)    mysql> select * from department;  +----+-----------+  | id | name      |  +----+-----------+  |  1 | 研发部    |  |  2 | 运维部    |  |  3 | 前台部    |  |  4 | 小卖部    |  +----+-----------+  4 rows in set (0.00 sec)    mysql> insert into userinfo(name,depart_id) values('xiaozhang',5);#depart_id受department.id的约束  ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`test2`.`userinfo`, CONSTRAINT `fk_user_depart` FOREIGN KEY (`depart_id`) REFERENCES `department` (`id`))    #联表查询  mysql> select userinfo.name as uname,department.name as dname from userinfo left      -> join department on depart_id = department.id;  +---------+-----------+  | uname   | dname     |  +---------+-----------+  | xiaozhu | 研发部    |  | xiaoyu  | 研发部    |  | laohe   | 运维部    |  | longge  | 运维部    |  | ludi    | 前台部    |  | xiaoguo | 小卖部    |  +---------+-----------+  6 rows in set (0.00 sec)

3.2多对多

多对多指当一个主表有多个从表时，从表之间的每个数据之间的关系就是多对多，如下图，一个boy可以和多个girl约会，一个girl也可以和多个boy约会。

mysql> create table boy(id int auto_increment primary key,      -> bname varchar(32) not null default'')charset utf8;  Query OK, 0 rows affected (0.36 sec)    mysql> insert into boy(bname) values('zhangsan'),('lisi'),('wangwu');  Query OK, 3 rows affected (0.09 sec)  Records: 3  Duplicates: 0  Warnings: 0    mysql> create table girl(id int auto_increment primary key,      -> gname varchar(32) not null default'')charset utf8;  Query OK, 0 rows affected (0.33 sec)    mysql> insert into girl(gname) values('xiaoli'),('xiaohua'),('xiaomei');  Query OK, 3 rows affected (0.06 sec)  Records: 3  Duplicates: 0  Warnings: 0    mysql> create table boy_girl(id int auto_increment primary key,      -> bid int not null default 1,      -> gid int not null default 1,      -> constraint fk_boy_girl_boy foreign key(bid) references boy(id),      -> constraint fk_boy_girl_girl foreign key(gid) references girl(id)      -> )charset utf8;  Query OK, 0 rows affected (0.42 sec)    mysql> insert into boy_girl(bid,gid) values(1,1),(2,2),(3,3),(3,1),(2,1),(1,2);  Query OK, 6 rows affected (0.04 sec)  Records: 6  Duplicates: 0  Warnings: 0    mysql> select * from boy left join boy_girl on boy.id = boy_girl.bid left join girl      -> on girl.id = boy_girl.gid;  +----+----------+------+------+------+------+---------+  | id | bname    | id   | bid  | gid  | id   | gname   |  +----+----------+------+------+------+------+---------+  |  1 | zhangsan |    1 |    1 |    1 |    1 | xiaoli  |  |  2 | lisi     |    5 |    2 |    1 |    1 | xiaoli  |  |  3 | wangwu   |    4 |    3 |    1 |    1 | xiaoli  |  |  1 | zhangsan |    6 |    1 |    2 |    2 | xiaohua |  |  2 | lisi     |    2 |    2 |    2 |    2 | xiaohua |  |  3 | wangwu   |    3 |    3 |    3 |    3 | xiaomei |  +----+----------+------+------+------+------+---------+  6 rows in set (0.03 sec)    mysql> select bname,gname from boy left join boy_girl on boy.id = boy_girl.bid      -> left join girl on girl.id = boy_girl.gid;  +----------+---------+  | bname    | gname   |  +----------+---------+  | zhangsan | xiaoli  |  | lisi     | xiaoli  |  | wangwu   | xiaoli  |  | zhangsan | xiaohua |  | lisi     | xiaohua |  | wangwu   | xiaomei |  +----------+---------+  6 rows in set (0.00 sec)

3.3一对一

一对一指的是两个表中的数据是一对一的关系，使用unique（字段名）来约束这种关系。如下例，由于工资属于员工的敏感信息，用单独的表去存储，这时工资和员工信息就是一对一的关系。

mysql> create table priv(      -> id int auto_increment primary key,      -> salary int not null default 0,      -> uid int not null default 1,      -> constraint fk_priv_user foreign key (uid) references userinfo(id),      -> unique(uid)) charset=utf8;  Query OK, 0 rows affected (0.52 sec)    mysql> insert into priv(salary,uid) values (10000,1),(12000,2),(15000,3),(8000,4),(9000,5),(9900,6);  Query OK, 6 rows affected (0.16 sec)  Records: 6  Duplicates: 0  Warnings: 0    mysql> select name,salary from userinfo left join priv on priv.uid = userinfo.id;  +---------+--------+  | name    | salary |  +---------+--------+  | xiaozhu |  10000 |  | xiaoyu  |  12000 |  | laohe   |  15000 |  | longge  |   8000 |  | ludi    |   9000 |  | xiaoguo |   9900 |  +---------+--------+  6 rows in set (0.00 sec)

3.4多表联查

多表联查就是将多个有关系的表放在一起查，使用的语句有：

left join……on查询时以左边的数据为主

right join ……on查询时以右边的数据为主

mysql> insert into department(name) values('业务部');  Query OK, 1 row affected (0.12 sec)    mysql> select userinfo.name as uname,department.name as dname from userinfo left      -> join department on depart_id = department.id;  +---------+-----------+  | uname   | dname     |  +---------+-----------+  | xiaozhu | 研发部    |  | xiaoyu  | 研发部    |  | laohe   | 运维部    |  | longge  | 运维部    |  | ludi    | 前台部    |  | xiaoguo | 小卖部    |  +---------+-----------+  6 rows in set (0.00 sec)    mysql> select userinfo.name as uname,department.name as dname from userinfo right      -> join department on depart_id = department.id;  +---------+-----------+  | uname   | dname     |  +---------+-----------+  | xiaozhu | 研发部    |  | xiaoyu  | 研发部    |  | laohe   | 运维部    |  | longge  | 运维部    |  | ludi    | 前台部    |  | xiaoguo | 小卖部    |  | NULL    | 业务部    |  +---------+-----------+  7 rows in set (0.00 sec)