表的关系与查询

三种关系: 多对多　　;　　多对一　　;　　一对一 .

找出俩张表之间的关系

分析步骤：
#1、先站在左表的角度去找
是否左表的多条记录可以对应右表的一条记录，如果是，则证明左表的一个字段foreign key 右表一个字段（通常是id）

#2、再站在右表的角度去找
是否右表的多条记录可以对应左表的一条记录，如果是，则证明右表的一个字段foreign key 左表一个字段（通常是id）

#3、总结：
#多对一：
如果只有步骤1成立，则是左表多对一右表
如果只有步骤2成立，则是右表多对一左表

#多对多
如果步骤1和2同时成立，则证明这两张表时一个双向的多对一，即多对多,需要定义一个这两张表的关系表来专门存放二者的关系

#一对一:
如果1和2都不成立，而是左表的一条记录唯一对应右表的一条记录，反之亦然。这种情况很简单，就是在左表foreign key右表的基础上，将左表的外键字段设置成unique即可

表的三种关系

　　一对多(多对一) :例 书和出版社

　　关联方式:foreign key

create table press(
    id int primary key auto_increment,
    name varchar(20)
);

create table book(
    id int primary key auto_increment,
    name varchar(20),
    press_id int not null,
         constraint fk_book_press foreign key(press_id) references press(id)
    on delete cascade
    on update cascade
);

# 先往被关联表中插入记录
insert into press(name) values
('北京工业地雷出版社'),
('人民音乐不好听出版社'),
('知识产权没有用出版社')
;

# 再往关联表中插入记录
insert into book(name,press_id) values
('九阳神功',1),
('九阴真经',2),
('九阴白骨爪',2),
('独孤九剑',3),
('降龙十巴掌',2),
('葵花宝典',3)
;

查询结果：
mysql> select * from book;
+----+-----------------+----------+
| id | name            | press_id |
+----+-----------------+----------+
|  1 | 九阳神功        |        1 |
|  2 | 九阴真经        |        2 |
|  3 | 九阴白骨爪      |        2 |
|  4 | 独孤九剑        |        3 |
|  5 | 降龙十巴掌      |        2 |
|  6 | 葵花宝典        |        3 |
+----+-----------------+----------+
rows in set (0.00 sec)

mysql> select * from press;
+----+--------------------------------+
| id | name                           |
+----+--------------------------------+
|  1 | 北京工业地雷出版社             |
|  2 | 人民音乐不好听出版社           |
|  3 | 知识产权没有用出版社           |
+----+--------------------------------+
rows in set (0.00 sec)

　　多对多: 例 作者和书籍 (一个作者可以写多本书,一本书也可以有多个作者.)

　　关联方式: foreign key+一张新表

# 创建被关联表author表，在之前的book表的关系已创建的情况下:
create table author(
    id int primary key auto_increment,
    name varchar(20)
);
#这张表就存放了author表和book表的关系，即查询二者的关系查这表就可以了
create table author2book(
    id int not null unique auto_increment,
    author_id int not null,
    book_id int not null,
    constraint fk_author foreign key(author_id) references author(id)
    on delete cascade
    on update cascade,
    constraint fk_book foreign key(book_id) references book(id)
    on delete cascade
    on update cascade,
    primary key(author_id,book_id)
);
#插入四个作者，id依次排开
insert into author(name) values('egon'),('alex'),('wusir'),('yuanhao');

# 每个作者的代表作
egon: 九阳神功、九阴真经、九阴白骨爪、独孤九剑、降龙十巴掌、葵花宝典
alex: 九阳神功、葵花宝典
wusir:独孤九剑、降龙十巴掌、葵花宝典
yuanhao:九阳神功

# 在author2book表中插入相应的数据

insert into author2book(author_id,book_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(1,5),
(1,6),
(2,1),
(2,6),
(3,4),
(3,5),
(3,6),
(4,1)
;
# 现在就可以查author2book对应的作者和书的关系了
mysql> select * from author2book;
+----+-----------+---------+
| id | author_id | book_id |
+----+-----------+---------+
|  1 |         1 |       1 |
|  2 |         1 |       2 |
|  3 |         1 |       3 |
|  4 |         1 |       4 |
|  5 |         1 |       5 |
|  6 |         1 |       6 |
|  7 |         2 |       1 |
|  8 |         2 |       6 |
|  9 |         3 |       4 |
| 10 |         3 |       5 |
| 11 |         3 |       6 |
| 12 |         4 |       1 |
+----+-----------+---------+
rows in set (0.00 sec)

　　一对一  例 :用户和博客(一个用户只能注册一个博客.)

#例如： 一个用户只能注册一个博客

#两张表： 用户表 （user）和 博客表(blog)
# 创建用户表
create table user(
    id int primary key auto_increment,
    name varchar(20)
);
# 创建博客表
create table blog(
    id int primary key auto_increment,
    url varchar(100),
    user_id int unique,
    constraint fk_user foreign key(user_id) references user(id)
    on delete cascade
    on update cascade
);
#插入用户表中的记录
insert into user(name) values
('alex'),
('wusir'),
('egon'),
('xiaoma')
;
# 插入博客表的记录
insert into blog(url,user_id) values
('http://www.cnblog/alex',1),
('http://www.cnblog/wusir',2),
('http://www.cnblog/egon',3),
('http://www.cnblog/xiaoma',4)
;
# 查询wusir的博客地址
select url from blog where user_id=2;

单表查询:
　　语法:

一、单表查询的语法
   SELECT 字段1,字段2... FROM 表名(select * from 表名   #一般不用 * )
                  WHERE 条件(第一次筛选)
                  GROUP BY field(根据某条件分组)
                  HAVING 筛选(二次筛选,应在分组之后进行)
                  ORDER BY field(排序)
                  LIMIT 限制条数
二、关键字的执行优先级（重点）

重点中的重点：关键字的执行优先级
from
where
group by
having
select
distinct(去重)
order by
limit

1.找到表:from

2.拿着where指定的约束条件，去文件/表中取出一条条记录

3.将取出的一条条记录进行分组group by，如果没有group by，则整体作为一组

4.将分组的结果进行having过滤

5.执行select

6.去重

7.将结果按条件排序：order by

8.限制结果的显示条数

#创建表,设置字段的约束条件
create table employee(
    id int primary key auto_increment,
    name  varchar(20) not null,
    sex enum('male','female') not null default 'male', #大部分是男的
    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,#一个部门一个屋
    depart_id int
);
# 查看表结构
mysql> desc employee;
+--------------+-----------------------+------+-----+---------+----------------+
| Field                | Type                              | Null | Key     | Default | Extra          |
+--------------+-----------------------+------+-----+---------+----------------+
| id                      | int(11)                            | NO   | PRI     | NULL    | auto_increment |
| emp_name             | varchar(20)                   | NO   |             | NULL    |                |
| sex                  | enum('male','female')   | NO   |             | male    |                |
| age                  | int(3) unsigned               | NO   |             | 28         |                |
| hire_date        | date                              | NO   |             | NULL    |                |
| post                 | varchar(50)                   | YES  |         | NULL    |                |
| post_comment     | varchar(100)                  | YES  |         | NULL    |                |
| salart               | double(15,2)                  | YES  |         | NULL    |                |
| office              | int(11)                           | YES  |         | NULL    |                |
| depart_id        | int(11)                           | YES  |         | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+
rows in set (0.08 sec)

#插入记录
#三个部门：教学，销售，运营
insert into employee(name ,sex,age,hire_date,post,salary,office,depart_id) values
('egon','male',18,'20170301','老男孩驻沙河办事处外交大使',7300.33,401,1), #以下是教学部
('alex','male',78,'20150302','teacher',1000000.31,401,1),
('wupeiqi','male',81,'20130305','teacher',8300,401,1),
('yuanhao','male',73,'20140701','teacher',3500,401,1),
('liwenzhou','male',28,'20121101','teacher',2100,401,1),
('jingliyang','female',18,'20110211','teacher',9000,401,1),
('jinxin','male',18,'19000301','teacher',30000,401,1),
('xiaomage','male',48,'20101111','teacher',10000,401,1),

('歪歪','female',48,'20150311','sale',3000.13,402,2),#以下是销售部门
('丫丫','female',38,'20101101','sale',2000.35,402,2),
('丁丁','female',18,'20110312','sale',1000.37,402,2),
('星星','female',18,'20160513','sale',3000.29,402,2),
('格格','female',28,'20170127','sale',4000.33,402,2),

('张野','male',28,'20160311','operation',10000.13,403,3), #以下是运营部门
('程咬金','male',18,'19970312','operation',20000,403,3),
('程咬银','female',18,'20130311','operation',19000,403,3),
('程咬铜','male',18,'20150411','operation',18000,403,3),
('程咬铁','female',18,'20140512','operation',17000,403,3)
;

　　where 约束

where子句中可以使用
1.比较运算符：>、<、>=、<=、<>、!=
2.between 80 and 100 ：值在80到100之间
3.in(80,90,100)值是80或90或100
4.like 'xiaomagepattern': pattern可以是%或者_ %小时任意多字符，_表示一个字符
　　 'xaiomage%'　　'xiaomage_'　　'xiaomage'
5.逻辑运算符：在多个条件直接可以使用逻辑运算符 and / or / not

　　例:

#1 :单条件查询
mysql> select id,emp_name from employee where id > 5;
+----+------------+
| id | emp_name   |
+----+------------+
|  6 | jingliyang |
|  7 | jinxin     |
|  8 | xiaomage   |
|  9 | 歪歪       |
| 10 | 丫丫       |
| 11 | 丁丁       |
| 12 | 星星       |
| 13 | 格格       |
| 14 | 张野       |
| 15 | 程咬金     |
| 16 | 程咬银     |
| 17 | 程咬铜     |
| 18 | 程咬铁     |

#2 多条件查询
mysql> select emp_name from employee where post='teacher' and salary>10000;
+----------+
| emp_name |
+----------+
| alex         |
| jinxin     |
+----------+

#3.关键字BETWEEN AND
 SELECT name,salary FROM employee 
        WHERE salary BETWEEN 10000 AND 20000;

 SELECT name,salary FROM employee 
        WHERE salary NOT BETWEEN 10000 AND 20000;

#注意''是空字符串，不是null
 SELECT name,post_comment FROM employee WHERE post_comment='';
 ps：
        执行
        update employee set post_comment='' where id=2;
        再用上条查看，就会有结果了
#5:关键字IN集合查询
mysql>  SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
+------------+---------+
| name       | salary  |
+------------+---------+
| yuanhao    | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
rows in set (0.00 sec)

mysql>  SELECT name,salary FROM employee  WHERE salary IN (3000,3500,4000,9000) ;
+------------+---------+
| name       | salary  |
+------------+---------+
| yuanhao    | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
mysql>  SELECT name,salary FROM employee  WHERE salary NOT IN (3000,3500,4000,9000) ;
+-----------+------------+
| name      | salary     |
+-----------+------------+
| egon      |    7300.33 |
| alex      | 1000000.31 |
| wupeiqi   |    8300.00 |
| liwenzhou |    2100.00 |
| jinxin    |   30000.00 |
| xiaomage  |   10000.00 |
| 歪歪      |    3000.13 |
| 丫丫      |    2000.35 |
| 丁丁      |    1000.37 |
| 星星      |    3000.29 |
| 格格      |    4000.33 |
| 张野      |   10000.13 |
| 程咬金    |   20000.00 |
| 程咬银    |   19000.00 |
| 程咬铜    |   18000.00 |
| 程咬铁    |   17000.00 |
+-----------+------------+
rows in set (0.00 sec)

#6:关键字LIKE模糊查询
通配符’%’
mysql> SELECT * FROM employee WHERE name LIKE 'jin%';
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
rows in set (0.00 sec)


通配符'_'

mysql> SELECT  age FROM employee WHERE name LIKE 'ale_';
+-----+
| age |
+-----+
|  78 |
+-----+
row in set (0.00 sec)

练习：
1. 查看岗位是teacher的员工姓名、年龄
2. 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
3. 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
4. 查看岗位描述不为NULL的员工信息
5. 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
6. 查看岗位是teacher且薪资不是10000或9000或30000的员工姓名、年龄、薪资
7. 查看岗位是teacher且名字是jin开头的员工姓名、年薪

#对应的sql语句
select name,age from employee where post = 'teacher';
select name,age from employee where post='teacher' and age > 30; 
select name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select name,salary*12 from employee where post='teacher' and name like 'jin%';

　　group by 分组查询

#1、首先明确一点：分组发生在where之后，即分组是基于where之后得到的记录而进行的

#2、分组指的是：将所有记录按照某个相同字段进行归类，比如针对员工信息表的职位分组，或者按照性别进行分组等

#3、为何要分组呢？
    取每个部门的最高工资
    取每个部门的员工数
    取男人数和女人数

小窍门：‘每’这个字后面的字段，就是我们分组的依据

#4、大前提：
    可以按照任意字段分组，但是分组完毕后，比如group by post，只能查看post字段，如果想查看组内信息，需要借助于聚合函数

　　注意: 在使用group by 查询时,只是筛选组中的第一条数据,但是我们常常
　　想知道多条数据.

mysql> select * from employee group by post;
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name   | sex    | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 14 | 张野   | male   |  28 | 2016-03-11 | operation                               | NULL         |   10000.13 |    403 |         3 |
|  9 | 歪歪   | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 |
|  2 | alex   | male   |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
|  1 | egon   | male   |  18 | 2017-03-01 | 老男孩驻沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
+----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
rows in set (0.00 sec)

#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的
如果想分组，则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY
mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

#查看MySQL 5.7默认的sql_mode如下：

mysql> exit;#设置成功后，一定要退出，然后重新登录方可生效
Bye

 　　通过group by 分组后,只能查询当前字段,如果想查看组内信息,需要借助

　　聚合函数

mysql> select * from emp group by post;# 报错
ERROR 1054 (42S22): Unknown column 'post' in 'group statement'

mysql>  select post from employee group by post;
+-----------------------------------------+
| post                                    |
+-----------------------------------------+
| operation                               |
| sale                                    |
| teacher                                 |
| 老男孩驻沙河办事处外交大使              |
+-----------------------------------------+
rows in set (0.00 sec)

　　
　　聚合函数

max()求最大值
min()求最小值
avg()求平均值
sum() 求和
count() 求总个数

#强调：聚合函数聚合的是组的内容，若是没有分组，则默认一组
# 每个部门有多少个员工
select post,count(id) from employee group by post;
# 每个部门的最高薪水
select post,max(salary) from employee group by post;
# 每个部门的最低薪水
select post,min(salary) from employee group by post;
# 每个部门的平均薪水
select post,avg(salary) from employee group by post;
# 每个部门的所有薪水
select post,sum(age) from employee group by post;

　　having过滤

HAVING与WHERE不一样的地方在于

#！！！执行优先级从高到低：where > group by > having 
#1. Where 发生在分组group by之前，因而Where中可以有任意字段，但是绝对不能使用聚合函数。

#2. Having发生在分组group by之后，因而Having中可以使用分组的字段，无法直接取到其他字段,可以使用聚合函数

　　示例:

验证：
mysql> select * from employee where salary>1000000;
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
| id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
|  2 | alex | male |  78 | 2015-03-02 | teacher |              | 1000000.31 |    401 |         1 |
+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
row in set (0.00 sec)

mysql> select * from employee having salary>1000000;
ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause

# 必须使用group by才能使用group_concat()函数，将所有的name值连接
mysql> select post,group_concat(name) from emp group by post having salary > 10000; ##错误，分组后无法直接取到salary字段
ERROR 1054 (42S22): Unknown column 'post' in 'field list'

　　练习:

1. 查询各岗位内包含的员工个数小于2的岗位名、岗位内包含员工名字、个数
2. 查询各岗位平均薪资大于10000的岗位名、平均工资
3. 查询各岗位平均薪资大于10000且小于20000的岗位名、平均工资

答案:

# 题1：
mysql> select post,group_concat(name),count(id) from employee group by post;
+-----------------------------------------+-----------------------------------------------------------+-----------+
| post                                    | group_concat(name)                                        | count(id) |
+-----------------------------------------+-----------------------------------------------------------+-----------+
| operation                               | 程咬铁,程咬铜,程咬银,程咬金,张野                          |         5 |
| sale                                    | 格格,星星,丁丁,丫丫,歪歪                                  |         5 |
| teacher                                 | xiaomage,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |         7 |
| 沙河办事处外交大使              | egon                                                      |         1 |
+-----------------------------------------+-----------------------------------------------------------+-----------+
rows in set (0.00 sec)

mysql> select post,group_concat(name),count(id) from employee group by post having count(id)<2;
+-----------------------------------------+--------------------+-----------+
| post                                    | group_concat(name) | count(id) |
+-----------------------------------------+--------------------+-----------+
| 沙河办事处外交大使              | egon               |         1 |
+-----------------------------------------+--------------------+-----------+
row in set (0.00 sec)


#题2：
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
+-----------+---------------+
| post      | avg(salary)   |
+-----------+---------------+
| operation |  16800.026000 |
| teacher   | 151842.901429 |
+-----------+---------------+
rows in set (0.00 sec)

#题3：
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;
+-----------+--------------+
| post      | avg(salary)  |
+-----------+--------------+
| operation | 16800.026000 |
+-----------+--------------+
row in set (0.00 sec)

　　order by 查询顺序

按单列排序
    SELECT * FROM employee ORDER BY age;
    SELECT * FROM employee ORDER BY age ASC;
    SELECT * FROM employee ORDER BY age DESC;
按多列排序:先按照age升序排序，如果年纪相同，则按照id降序
    SELECT * from employee
        ORDER BY age ASC,
        id DESC;

　　示例:

验证多列排序：
SELECT * from employee ORDER BY age ASC,id DESC;
mysql> SELECT * from employee ORDER BY age ASC,id DESC;
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | name       | sex    | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 18 | 程咬铁     | female |  18 | 2014-05-12 | operation                               | NULL         |   17000.00 |    403 |         3 |
| 17 | 程咬铜     | male   |  18 | 2015-04-11 | operation                               | NULL         |   18000.00 |    403 |         3 |
| 16 | 程咬银     | female |  18 | 2013-03-11 | operation                               | NULL         |   19000.00 |    403 |         3 |
| 15 | 程咬金     | male   |  18 | 1997-03-12 | operation                               | NULL         |   20000.00 |    403 |         3 |
| 12 | 星星       | female |  18 | 2016-05-13 | sale                                    | NULL         |    3000.29 |    402 |         2 |
| 11 | 丁丁       | female |  18 | 2011-03-12 | sale                                    | NULL         |    1000.37 |    402 |         2 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher                                 | NULL         |   30000.00 |    401 |         1 |
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher                                 | NULL         |    9000.00 |    401 |         1 |
|  1 | egon       | male   |  18 | 2017-03-01 | 沙河办事处外交大使              | NULL         |    7300.33 |    401 |         1 |
| 14 | 张野       | male   |  28 | 2016-03-11 | operation                               | NULL         |   10000.13 |    403 |         3 |
| 13 | 格格       | female |  28 | 2017-01-27 | sale                                    | NULL         |    4000.33 |    402 |         2 |
|  5 | liwenzhou  | male   |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
| 10 | 丫丫       | female |  38 | 2010-11-01 | sale                                    | NULL         |    2000.35 |    402 |         2 |
|  9 | 歪歪       | female |  48 | 2015-03-11 | sale                                    | NULL         |    3000.13 |    402 |         2 |
|  8 | xiaomage   | male   |  48 | 2010-11-11 | teacher                                 | NULL         |   10000.00 |    401 |         1 |
|  4 | yuanhao    | male   |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
|  2 | alex       | male   |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
|  3 | wupeiqi    | male   |  81 | 2013-03-05 | teacher                                 | NULL         |    8300.00 |    401 |         1 |
+----+------------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
rows in set (0.01 sec)

mysql>

　　limit  限制查询的记录:

示例：
    SELECT * FROM employee ORDER BY salary DESC 
     LIMIT 3;                    #默认初始位置为0 

    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 0,5; #从第0开始，即先查询出第一条，然后包含这一条在内往后查5条

    SELECT * FROM employee ORDER BY salary DESC
        LIMIT 5,5; #从第5开始，即先查询出第6条，然后包含这一条在内往后查5条