day--38 mysql表的完整性约束总结

                                        表的简单查询
参考：https://www.cnblogs.com/clschao/articles/9995531.html

一：单表的查询：

查询数据的本质：mysql会去本地的硬盘上面找到对应的文件，然后打开文件，按照你的查询条件找到你需要的数据。

    查询的方法

　　01：select * from ,      这个select * 指的是要查询所有字段的数据
    02：select distinct 字段1，字段2.... from 库名.表名       #from后面是说从库中的某个表中去查找，mysql会自动去这个库对应的文件夹
        下去找和表对应的那个数据文件，找不到就报错，找点了就继续后续的操作。
    03：where 条件         #从表中找符合条件的数据记录，where后面跟的是你的查询条件
    04：group by field （字段）      #分组
    05：having 筛选(having 是对组进行删选)        #过滤，过滤之后执行select后面的字段筛选，就是说我要先确定一下需要哪个字段的数据，
            你查询的字段数据进行去重，然后再进行下面的操作。
    06：order by field（字段）       #将结果按照后面的字段进行排序
    07：limit  限制条数          #将最后的结果加一个限制条数，就是说我要将锅炉或则说限制查询出来的数据记录的条数

例子：创建一个员工表：
    01：创建表employee 表：

　　mysql> create table employee(
            -> id int not null unique auto_increment,
            -> name varchar(20) not null,
            -> sex 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
            -> );
        Query OK, 0 rows affected (0.15 sec)

    02：查看employee表的结构

mysql> desc employee;
        +--------------+-----------------------+------+-----+---------+----------------+

        | Field        | Type                  | Null | Key | Default | Extra          |

        +--------------+-----------------------+------+-----+---------+----------------+

        | id           | int(11)               | NO   | PRI | NULL    | auto_increment |

        | 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    |                |

        | salary       | double(15,2)          | YES  |     | NULL    |                |

        | office       | int(11)               | YES  |     | NULL    |                |

        | depart_id    | int(11)               | YES  |     | NULL    |                |

        +--------------+-----------------------+------+-----+---------+----------------+

        10 rows in set (0.00 sec)

    03：往表里添加数据：
    如果在windows系统中，插入中文字符，select的结果为空白，可以将所有字符编码统一设置成gbk

mysql> insert into employee(name,sex,age,hire_date,post,salary,office,depart_id) values
            -> ("egon","male",18,"","包子店",7300.33,401,1),
            -> ("教师02","male",78,"","teacher",10010.31,401,1),
            -> ("销售01","female",46,20190909,"sale",3000.13,402,2),
            -> ("销售02","female",23,201909010,"sale",3000.14,402,2),
            -> ("运营01","male",26,20150505,"operation",1000.13,403,3),
            -> ("运营02","male",26,20150506,"operation",1000.23,403,3);
        Query OK, 6 rows affected, 1 warning (0.07 sec)
        Records: 6  Duplicates: 0  Warnings: 1

    04：查看数据：

ps:也可以用select * from employee  进行查询，但是这样查找的效率会比较低。

        mysql> select id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id from employee;
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        | id | name     | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        |  1 | 教师01   | male   |  18 | 2017-07-07 | 包子店     | NULL         |  7300.33 |    401 |         1 |
        |  2 | 教师02   | male   |  78 | 2018-08-08 | teacher   | NULL         | 10010.31 |    401 |         1 |
        |  3 | 销售01   | female |  46 | 2019-09-09 | sale      | NULL         |  3000.13 |    402 |         2 |
        |  4 | 销售02   | female |  23 | 0000-00-00 | sale      | NULL         |  3000.14 |    402 |         2 |
        |  5 | 运营01   | male   |  26 | 2015-05-05 | operation | NULL         |  1000.13 |    403 |         3 |
        |  6 | 运营02   | male   |  26 | 2015-05-06 | operation | NULL         |  1000.23 |    403 |         3 |
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        6 rows in set (0.00 sec)

数据去重：
    05:我们需要查询post，用select post from employee,但是我只想查询有几种职业。下面这种显示不是很方便

mysql> select post from employee;
        +-----------+
        | post      |
        +-----------+
        | 包子店    |
        | teacher   |
        | sale      |
        | sale      |
        | operation |
        | operation |
        +-----------+
        6 rows in set (0.00 sec)

    06：但是上面这种select post from employee  查询职业，会有重复的职业。所以我们需要去重。
        用 distinct 即可。

mysql> select distinct post from employee;
        +-----------+
        | post      |
        +-----------+
        | 包子店    |
        | teacher   |
        | sale      |
        | operation |
        +-----------+
        4 rows in set (0.02 sec)

        显示结果（结果已经去重了）：
        mysql> select distinct post from employee;
        +-----------+
        | post      |
        +-----------+
        | 包子店    |
        | teacher   |
        | sale      |
        | operation |
        +-----------+
        4 rows in set (0.02 sec)
        注意：注意一点，使用distinct对记录进行去重的时候，distinct必须写在所有查询字段的前面，不然会报错，当然有些特别的用法可以结合着写到字段的中间或者后面，
            select post，sex from employee；
            select distinct post，sex from employee；

数据进行四则运算查询：
    07：将月薪进行四则运算：
        先查月薪：

mysql> select salary from employee;
        +----------+
        | salary   |
        +----------+
        |  7300.33 |
        | 10010.31 |
        |  3000.13 |
        |  3000.14 |
        |  1000.13 |
        |  1000.23 |
        +----------+
        6 rows in set (0.00 sec)

        将月薪进行四则运算，算成年薪：
        mysql> select name,salary*12 from employee;
        +----------+-----------+
        | name     | salary*12 |
        +----------+-----------+
        | egon     |  87603.96 |
        | 教师02   | 120123.72 |
        | 销售01   |  36001.56 |
        | 销售02   |  36001.68 |
        | 运营01   |  12001.56 |
        | 运营02   |  12002.76 |
        +----------+-----------+
        6 rows in set (0.04 sec)

        注意，将月薪进行运算后得到后，原来的字段salary变成了  salary*12  ，salary*12其实也是一个别名，是mysql自动给加上的。
        我们也可以给其用as命名，例如：salary*12 字段就变成  nianxin  了,但是这个新的字段只是在内存中，并不是我们存在硬盘中的。
        mysql> select name,salary*12 as nianxinn from employee;
        +----------+-----------+
        | name     | nianxinn  |
        +----------+-----------+
        | egon     |  87603.96 |
        | 教师02   | 120123.72 |
        | 销售01   |  36001.56 |
        | 销售02   |  36001.68 |
        | 运营01   |  12001.56 |
        | 运营02   |  12002.76 |
        +----------+-----------+
        6 rows in set (0.03 sec)

自定义显示格式，自己规定查询结果的显示格式（  concat（））：
        concat（） 函数用于连接字符串。
        例子：

mysql> select concat("姓名:,name","年薪：salary") as Annual_salary from employee;
        +-----------------------------+
        | Annual_salary               |
        +-----------------------------+
        | 姓名:,name年薪：salary      |
        | 姓名:,name年薪：salary      |
        | 姓名:,name年薪：salary      |
        | 姓名:,name年薪：salary      |
        | 姓名:,name年薪：salary      |
        | 姓名:,name年薪：salary      |
        +-----------------------------+
        6 rows in set (0.04 sec)

    分成两列查询：还可以分成两列来查询，例如：
        mysql> select concat("姓名:,name","年薪:,salary*12") as annual_salary,concat("性别:sex") from employee;
        +-------------------------------+----------------------+
        | annual_salary                 | concat("性别:sex")   |
        +-------------------------------+----------------------+
        | 姓名:,name年薪:,salary*12     | 性别:sex             |
        | 姓名:,name年薪:,salary*12     | 性别:sex             |
        | 姓名:,name年薪:,salary*12     | 性别:sex             |
        | 姓名:,name年薪:,salary*12     | 性别:sex             |
        | 姓名:,name年薪:,salary*12     | 性别:sex             |
        | 姓名:,name年薪:,salary*12     | 性别:sex             |
        +-------------------------------+----------------------+
        6 rows in set (0.00 sec)

    字符串拼接查询：cancat_ws() 第一个参数为分隔符来进行字符串拼接
        mysql> select concat_ws("___",name,salary*12) as annual_salary from employee;
        +----------------------+
        | annual_salary        |
        +----------------------+
        | egon___87603.96      |
        | 教师02___120123.72   |
        | 销售01___36001.56    |
        | 销售02___36001.68    |
        | 运营01___12001.56    |
        | 运营02___12002.76    |
        +----------------------+
        6 rows in set (0.00 sec)

二：where约束

 1：比较运算：〉，〈 ， 〉= ，〈=，〈〉，！=
    2：between 80 and 100  :值在80到100之间
    3：in(80,90,100)  值是80.或则90.或则100
    4：like "tom%":  pattern 可以是%或则_,%表示任意多字符，_表示一个字符
    5：逻辑运算：在多个条件下可以直接使用逻辑运算符 and or  not

条件查询：
01：单条件查询

还是以employee为例子：
    查询月薪大约10000的teacher下的名字和月薪
    mysql> select name,salary from employee where post="teacher" and salary>10000;
    +----------+----------+
    | name     | salary   |
    +----------+----------+
    | 教师02   | 10010.31 |
    +----------+----------+
    1 row in set (0.04 sec)

02：多条件查询：

mysql> select name,salary from employee where post="teacher" and salary>10000;
    +----------+----------+
    | name     | salary   |
    +----------+----------+
    | 教师02   | 10010.31 |
    +----------+----------+
    1 row in set (0.00 sec)

03:关键字  between  查询：（在--到--之间的）
    #  between

mysql> select name,salary from employee where salary between 10000 and 20000;
    +----------+----------+
    | name     | salary   |
    +----------+----------+
    | 教师02   | 10010.31 |
    +----------+----------+
    1 row in set (0.03 sec)

    # not between

mysql> select name,salary from employee where salary not between 10000 and 20000;
    +----------+---------+
    | name     | salary  |
    +----------+---------+
    | egon     | 7300.33 |
    | 销售01   | 3000.13 |
    | 销售02   | 3000.14 |
    | 运营01   | 1000.13 |
    | 运营02   | 1000.23 |
    +----------+---------+
    5 rows in set (0.00 sec)

04:关键字is null  (关键字IS NULL(判断某个字段是否为NULL不能用等号，需要用IS) 判断null只能用is)

mysql> select name,post_comment from employee
        -> where post_comment is null;
    +----------+--------------+
    | name     | post_comment |
    +----------+--------------+
    | egon     | NULL         |
    | 教师02   | NULL         |
    | 销售01   | NULL         |
    | 销售02   | NULL         |
    | 运营01   | NULL         |
    | 运营02   | NULL         |
    +----------+--------------+
    6 rows in set (0.00 sec)

05:查询空字符串：

注意''是空字符串，不是null，两个是不同的东西，null是啥也没有，''是空的字符串的意思，是一种数据类型，null是另外一种数据类型
        mysql> select name,post_comment from employee where post_comment="";
        Empty set (0.00 sec)

06：关键字in集合查询(判断是我们要找数据的是否在这个集合中）

mysql> select name,salary from employee where salary in(3000.14,3500,4000,9000);
    +----------+---------+
    | name     | salary  |
    +----------+---------+
    | 销售02   | 3000.14 |
    +----------+---------+
    1 row in set (0.00 sec)

07：关键字like 模糊查询：

 　　通配符 *  ：#匹配任意所有字符
    select * from employee

    通配符 _  : #匹配任意一个字符
    select * from employee where name like "ag_";

    这样会找不到数据，因为我只写了一个 _ ,egon后面还有两个字符，后面的没有匹配到，所以需要些两个_
    错误示范：

　　 mysql> select * from employee where name like "eg_";
    Empty set (0.00 sec)

    正确示范：

mysql> select * from employee where name like "eg__";
    +----+------+------+-----+------------+-----------+--------------+---------+--------+-----------+
    | id | name | sex  | age | hire_date  | post      | post_comment | salary  | office | depart_id |
    +----+------+------+-----+------------+-----------+--------------+---------+--------+-----------+
    |  1 | egon | male |  18 | 2017-07-07 | 包子店    | NULL         | 7300.33 |    401 |         1 |
    +----+------+------+-----+------------+-----------+--------------+---------+--------+-----------+
    1 row in set (0.00 sec)

    mysql> select * from employee where name like "eg%";
    +----+------+------+-----+------------+-----------+--------------+---------+--------+-----------+
    | id | name | sex  | age | hire_date  | post      | post_comment | salary  | office | depart_id |
    +----+------+------+-----+------------+-----------+--------------+---------+--------+-----------+
    |  1 | egon | male |  18 | 2017-07-07 | 包子店    | NULL         | 7300.33 |    401 |         1 |
    +----+------+------+-----+------------+-----------+--------------+---------+--------+-----------+
    1 row in set (0.00 sec)

总结：where条件运行的时候，我们以select id,name,age from employee where id>7;这个语句来说，首先先找到employee表，找到这个表
    之后，mysql会拿着where后面的约束条件去表里面找符合条件的数据，然后遍历你表中所有的数据，查看一下id是否大于7，逐条的对比，然后只
    要发现id比7大的，它就会把这一整条记录给select，但是select说我只拿id、name、age这个三个字段里面的数据，然后就打印了这三个字
    段的数据，然后where继续往下过滤，看看id是不是还有大于7的，然后发现一个符合条件的就给select一个，然后重复这样的事情，直到把数据
    全部过滤一遍才会结束。这就是where条件的一个工作方式。

三：分组查询：（group by)

 什么是分组：分组时在where之后得到的记录生成的，指的是将所有记录按照某个相同的字段进行分类，比如针对员工信息表的职位分类。或则按照性别分类等等。
    为啥要分组：因为我们需要对数据以组为单位来统计一些数据或则计算。比如我们取工资：where salary 〉1000  group by post

 01:group by   例子：

 mysql> select * from employee group by post;
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        | id | name     | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        |  5 | 运营01   | male   |  26 | 2015-05-05 | operation | NULL         |  1000.13 |    403 |         3 |
        |  3 | 销售01   | female |  46 | 2019-09-09 | sale      | NULL         |  3000.13 |    402 |         2 |
        |  2 | 教师02   | male   |  78 | 2018-08-08 | teacher   | NULL         | 10010.31 |    401 |         1 |
        |  1 | egon     | male   |  18 | 2017-07-07 | 包子店    | NULL         |  7300.33 |    401 |         1 |
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        4 rows in set (0.00 sec)

 02:only_full_group_by
    01:先查询mysql默认的sql_mode:

mysql> select @@global.sql_mode;
        +------------------------+
        | @@global.sql_mode      |
        +------------------------+
        | NO_ENGINE_SUBSTITUTION |
        +------------------------+
        1 row in set (0.02 sec)

        mysql> select * from employee group by post;
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        | id | name     | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        |  5 | 运营01   | male   |  26 | 2015-05-05 | operation | NULL         |  1000.13 |    403 |         3 |
        |  3 | 销售01   | female |  46 | 2019-09-09 | sale      | NULL         |  3000.13 |    402 |         2 |
        |  2 | 教师02   | male   |  78 | 2018-08-08 | teacher   | NULL         | 10010.31 |    401 |         1 |
        |  1 | egon     | male   |  18 | 2017-07-07 | 包子店    | NULL         |  7300.33 |    401 |         1 |
        +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
        4 rows in set (0.00 sec)

        设置only_full_group_by
        mysql> set global sql_mode="only_full_group_by";
        Query OK, 0 rows affected (0.03 sec)

        设置后一定要推出，重新进入数据库，找到那个表：
        #退出：
        mysql> quit
        Bye
        #重新进入：
        mysql> use day;
        Database changed

        #报错：意思是告诉我，select后面取的字段必须在你的group by后面的字段里面才行
        mysql> select * from employee group by post;
        ERROR 1055 (42000): 'day.employee.id' isn't in GROUP BY

        #正确的写法：（id  在group by 后面的额post 的字段里面。
        mysql> select post,count(id) from employee group by post;  #count 做统计用的
        +-----------+-----------+
        | post      | count(id) |
        +-----------+-----------+
        | operation |         2 |
        | sale      |         2 |
        | teacher   |         1 |
        | 包子店    |         1 |
        +-----------+-----------+
        4 rows in set (0.01 sec)

 02:按每个部门进行分类，并显示每个部门的员工信息。
 我们显示每个部门的员工名字和薪资(按岗位分类，并显示每个部门的员工名字和他的薪资，并用逗号 ， 分割。

mysql> select post,group_concat(name,salary) from employee group by post;
    +-----------+---------------------------------+
    | post      | group_concat(name,salary)       |
    +-----------+---------------------------------+
    | operation | 运营011000.13,运营021000.23     |
    | sale      | 销售013000.13,销售023000.14     |
    | teacher   | 教师0210010.31                  |
    | 包子店    | egon7300.33                     |
    +-----------+---------------------------------+
    4 rows in set (0.00 sec)

 03：聚合函数（一般group by  都会和聚合函数一起使用，聚合就是将分组的数据聚集在一起，合并起来拿到一个最后的结果。
     例子：

mysql> select post,count(id) as count from employee group by post;
    +-----------+-------+
    | post      | count |
    +-----------+-------+
    | operation |     2 |   #这个部门的人数出现了2次
    | sale      |     2 |
    | teacher   |     1 |
    | 包子店    |     1 |
    +-----------+-------+
    4 rows in set (0.00 sec)

    查看以下employee的数据：
    mysql> select * from employee;
    +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
    | id | name     | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
    +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
    |  1 | egon     | male   |  18 | 2017-07-07 | 包子店    | NULL         |  7300.33 |    401 |         1 |
    |  2 | 教师02   | male   |  78 | 2018-08-08 | teacher   | NULL         | 10010.31 |    401 |         1 |
    |  3 | 销售01   | female |  46 | 2019-09-09 | sale      | NULL         |  3000.13 |    402 |         2 |
    |  4 | 销售02   | female |  23 | 0000-00-00 | sale      | NULL         |  3000.14 |    402 |         2 |
    |  5 | 运营01   | male   |  26 | 2015-05-05 | operation | NULL         |  1000.13 |    403 |         3 |
    |  6 | 运营02   | male   |  26 | 2015-05-06 | operation | NULL         |  1000.23 |    403 |         3 |
    +----+----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
    6 rows in set (0.00 sec)

关于集合函数，mysql提供了以下几种聚合函数：count、max、min、avg、sum等，上面的group_concat也算是一个聚合函数了，做字符串拼接的操作

ps聚合函数补充点：

01:计数使用count(*)
    mysql> select count(*) from employee;
    +----------+
    | count(*) |
    +----------+
    |        6 |
    +----------+
    1 row in set (0.03 sec)

02:#后面跟where条件的意思是统计一下满足depart_id=1这个的所有记录的个数
    mysql> select count(*) from employee where depart_id=1;
    +----------+
    | count(*) |
    +----------+
    |        2 |
    +----------+
    1 row in set (0.00 sec)

 03: #max（）统计分组后每组的最大值，这里没有写group by，那么就是统计整个表中所有记录中薪资最大的，薪资的值
    mysql> select max(salary) from employee;
    +-------------+
    | max(salary) |
    +-------------+
    |    10010.31 |
    +-------------+
    1 row in set (0.03 sec)

 04:统计分组后的最小值
    mysql> select min(salary) from employee;
    +-------------+
    | min(salary) |
    +-------------+
    |     1000.13 |
    +-------------+
    1 row in set (0.02 sec)

 05：计算统计后的薪资的平均值。
    mysql> select avg(salary) from employee;
    +-------------+
    | avg(salary) |
    +-------------+
    | 4218.545000 |
    +-------------+
    1 row in set (0.00 sec)

 06：计算统计后薪资总和
    mysql> select sum(salary) from employee;
    +-------------+
    | sum(salary) |
    +-------------+
    |    25311.27 |
    +-------------+
    1 row in set (0.00 sec)

 07：计算满足where后面要求的薪资的总和
    mysql> select sum(salary) from employee where depart_id=3;
    +-------------+
    | sum(salary) |
    +-------------+
    |     2000.36 |
    +-------------+
    1 row in set (0.00 sec)

四：having 过滤
    01：以下这种查找方法也是可以的（和select id,name from employee;  )是一样的。

mysql> select employee.id,employee.name from employee;
        +----+----------+
        | id | name     |
        +----+----------+
        |  1 | egon     |
        |  2 | 教师02   |
        |  3 | 销售01   |
        |  4 | 销售02   |
        |  5 | 运营01   |
        |  6 | 运营02   |
        +----+----------+
        6 rows in set (0.00 sec)

    02：但是当我们给新查找的结果起一个新的表名时候，会发现报错

　　mysql> select employee.id,employee.name from employee as biaoming;
        ERROR 1054 (42S22): Unknown column 'employee.id' in 'field list'
        mysql>
    原因：因为这个语句先执行的是from，那么后面的as也是比select要先执行的，所以你先将表employee起了个新名字叫做tb1，
         然后在tb1里面取查询数据，那么tb1里面找不到employee.id这个字段，就会报错，如果我们查询的
         时候不带表名，你as来起一个新的表名也是没问题的。

    03：having和where的区别。

　　having

五：limit 限制查询（ 经常做分页处理）

01：select * from 表名 order by salary
        limit 3；  #从0开始取3个数。
    02：select * from 表名 order by salary
        limit 5，5  #从第5个开始，即先查询第6个数