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  • MySQL查询

    一、单表查询

    语法:

    一、单表查询的语法
       SELECT 字段1,字段2... FROM 表名
                      WHERE 条件
                      GROUP BY field
                      HAVING 筛选
                      ORDER BY field
                      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
    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
    

    1. where约束

      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10或20或30
      4.like 'xiaomagepattern': pattern可以是%或者_。%小时任意多字符,_表示一个字符
      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
      

    2. 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
      

      当执行以下sql语句的时候,是以post字段查询了组中的第一条数据,没有任何意义,因为我们现在想查出当前组的多条记

      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 | dogfa   | male   |  78 | 2015-03-02 | teacher                                 |              | 1000000.31 |    401 |         1 |
      |  1 | oldniu   | male   |  18 | 2017-03-01 | student              | NULL         |    7300.33 |    401 |         1 |
      +----+--------+--------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
      4 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> select @@global.sql_mode;
      +--------------------+
      | @@global.sql_mode  |
      +--------------------+
      | ONLY_FULL_GROUP_BY |
      +--------------------+
      1 row in set (0.00 sec)
      
      mysql> exit;#设置成功后,一定要退出,然后重新登录方可生效
      Bye
      

      继续验证通过group by分组之后,只能查看当前字段,如果想查看组内信息,需要借助于聚合函数

      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                                 |
      | student                                 |
      +-----------------------------------------+
      4 rows in set (0.00 sec)
      

    3. 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;
      

    4. 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
      

    5. order by 查询排序

      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;
      

    6. limit 限制查询的记录数

      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
      

    二、多表查询

    准备工作:准备两张表,部门表(department)、员工表(employee)

    create table department(
    id int,
    name varchar(20) 
    );
    
    create table employee(
    id int primary key auto_increment,
    name varchar(20),
    sex enum('male','female') not null default 'male',
    age int,
    dep_id int
    );
    
    #插入数据
    insert into department values
    (200,'技术'),
    (201,'人力资源'),
    (202,'销售'),
    (203,'运营');
    
    insert into employee(name,sex,age,dep_id) values
    ('egon','male',18,200),
    ('alex','female',48,201),
    ('wupeiqi','male',38,201),
    ('yuanhao','female',28,202),
    ('nvshen','male',18,200),
    ('xiaomage','female',18,204)
    ;
    
    # 查看表结构和数据
    mysql> desc department;
    +-------+-------------+------+-----+---------+-------+
    | Field | Type        | Null | Key | Default | Extra |
    +-------+-------------+------+-----+---------+-------+
    | id    | int(11)     | YES  |     | NULL    |       |
    | name  | varchar(20) | YES  |     | NULL    |       |
    +-------+-------------+------+-----+---------+-------+
    rows in set (0.19 sec)
    
    mysql> desc employee;
    +--------+-----------------------+------+-----+---------+----------------+
    | 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    |                |
    +--------+-----------------------+------+-----+---------+----------------+
    rows in set (0.01 sec)
    
    mysql> select * from department;
    +------+--------------+
    | id   | name         |
    +------+--------------+
    |  200 | 技术         |
    |  201 | 人力资源     |
    |  202 | 销售         |
    |  203 | 运营         |
    +------+--------------+
    rows in set (0.02 sec)
    
    mysql> select * from employee;
    +----+----------+--------+------+--------+
    | id | name     | sex    | age  | dep_id |
    +----+----------+--------+------+--------+
    |  1 | egon     | male   |   18 |    200 |
    |  2 | alex     | female |   48 |    201 |
    |  3 | wupeiqi  | male   |   38 |    201 |
    |  4 | yuanhao  | female |   28 |    202 |
    |  5 | nvshen   | male   |   18 |    200 |
    |  6 | xiaomage | female |   18 |    204 |
    +----+----------+--------+------+--------+
    rows in set (0.00 sec)
    

    ps:观察两张表,发现department表中id=203部门在employee中没有对应的员工,发现employee中id=6的员工在department表中没有对应关系。


    1. 多表连接查询

      两张表的准备工作已完成,比如现在我要查询的员工信息以及该员工所在的部门。从该题中,我们看出既要查员工又要查该员工的部门,肯定要将两张表进行连接查询,多表连接查询。

      重点:外链接语法

      语法:

      SELECT 字段列表
          FROM 表1 INNER|LEFT|RIGHT JOIN 表2
          ON 表1.字段 = 表2.字段;
      

      (1)先看第一种情况交叉连接:不适用任何匹配条件。生成笛卡尔积

      mysql> select * from employee,department;
      +----+----------+--------+------+--------+------+--------------+
      | id | name     | sex    | age  | dep_id | id   | name         |
      +----+----------+--------+------+--------+------+--------------+
      |  1 | egon     | male   |   18 |    200 |  200 | 技术         |
      |  1 | egon     | male   |   18 |    200 |  201 | 人力资源     |
      |  1 | egon     | male   |   18 |    200 |  202 | 销售         |
      |  1 | egon     | male   |   18 |    200 |  203 | 运营         |
      |  2 | alex     | female |   48 |    201 |  200 | 技术         |
      |  2 | alex     | female |   48 |    201 |  201 | 人力资源     |
      |  2 | alex     | female |   48 |    201 |  202 | 销售         |
      |  2 | alex     | female |   48 |    201 |  203 | 运营         |
      |  3 | wupeiqi  | male   |   38 |    201 |  200 | 技术         |
      |  3 | wupeiqi  | male   |   38 |    201 |  201 | 人力资源     |
      |  3 | wupeiqi  | male   |   38 |    201 |  202 | 销售         |
      |  3 | wupeiqi  | male   |   38 |    201 |  203 | 运营         |
      |  4 | yuanhao  | female |   28 |    202 |  200 | 技术         |
      |  4 | yuanhao  | female |   28 |    202 |  201 | 人力资源     |
      |  4 | yuanhao  | female |   28 |    202 |  202 | 销售         |
      |  4 | yuanhao  | female |   28 |    202 |  203 | 运营         |
      |  5 | nvshen   | male   |   18 |    200 |  200 | 技术         |
      |  5 | nvshen   | male   |   18 |    200 |  201 | 人力资源     |
      |  5 | nvshen   | male   |   18 |    200 |  202 | 销售         |
      |  5 | nvshen   | male   |   18 |    200 |  203 | 运营         |
      |  6 | xiaomage | female |   18 |    204 |  200 | 技术         |
      |  6 | xiaomage | female |   18 |    204 |  201 | 人力资源     |
      |  6 | xiaomage | female |   18 |    204 |  202 | 销售         |
      |  6 | xiaomage | female |   18 |    204 |  203 | 运营         |
      

      (2)内连接:只连接匹配的行

      #找两张表共有的部分,相当于利用条件从笛卡尔积结果中筛选出了匹配的结果
      #department没有204这个部门,因而employee表中关于204这条员工信息没有匹配出来
      mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id;
      +----+---------+------+--------+--------------+
      | id | name    | age  | sex    | name         |
      +----+---------+------+--------+--------------+
      |  1 | egon    |   18 | male   | 技术         |
      |  2 | alex    |   48 | female | 人力资源     |
      |  3 | wupeiqi |   38 | male   | 人力资源     |
      |  4 | yuanhao |   28 | female | 销售         |
      |  5 | nvshen  |   18 | male   | 技术         |
      +----+---------+------+--------+--------------+
      rows in set (0.00 sec)
      
      #上述sql等同于
      mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;
      

      (3)外链接之左连接:优先显示左表全部记录

      #以左表为准,即找出所有员工信息,当然包括没有部门的员工
      #本质就是:在内连接的基础上增加左边有,右边没有的结果
      mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
      +----+----------+--------------+
      | id | name     | depart_name  |
      +----+----------+--------------+
      |  1 | egon     | 技术         |
      |  5 | nvshen   | 技术         |
      |  2 | alex     | 人力资源     |
      |  3 | wupeiqi  | 人力资源     |
      |  4 | yuanhao  | 销售         |
      |  6 | xiaomage | NULL         |
      +----+----------+--------------+
      6 rows in set (0.00 sec)
      

      (4) 外链接之右连接:优先显示右表全部记录

      #以右表为准,即找出所有部门信息,包括没有员工的部门
      #本质就是:在内连接的基础上增加右边有,左边没有的结果
      mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;
      +------+---------+--------------+
      | id   | name    | depart_name  |
      +------+---------+--------------+
      |    1 | egon    | 技术         |
      |    2 | alex    | 人力资源     |
      |    3 | wupeiqi | 人力资源     |
      |    4 | yuanhao | 销售         |
      |    5 | nvshen  | 技术         |
      | NULL | NULL    | 运营         |
      +------+---------+--------------+
      rows in set (0.00 sec)
      

      (5) 全外连接:显示左右两个表全部记录

      #外连接:在内连接的基础上增加左边有右边没有的和右边有左边没有的结果
      #注意:mysql不支持全外连接 full JOIN
      #强调:mysql可以使用此种方式间接实现全外连接
      语法:select * from employee left join department on employee.dep_id = department.id 
             union all
            select * from employee right join department on employee.dep_id = department.id;
      
       mysql> select * from employee left join department on employee.dep_id = department.id
                union
              select * from employee right join department on employee.dep_id = department.id
                 ;
      +------+----------+--------+------+--------+------+--------------+
      | id   | name     | sex    | age  | dep_id | id   | name         |
      +------+----------+--------+------+--------+------+--------------+
      |    1 | egon     | male   |   18 |    200 |  200 | 技术         |
      |    5 | nvshen   | male   |   18 |    200 |  200 | 技术         |
      |    2 | alex     | female |   48 |    201 |  201 | 人力资源     |
      |    3 | wupeiqi  | male   |   38 |    201 |  201 | 人力资源     |
      |    4 | yuanhao  | female |   28 |    202 |  202 | 销售         |
      |    6 | xiaomage | female |   18 |    204 | NULL | NULL         |
      | NULL | NULL     | NULL   | NULL |   NULL |  203 | 运营         |
      +------+----------+--------+------+--------+------+--------------+
      rows in set (0.01 sec)
      
      #注意 union与union all的区别:union会去掉相同的纪录
      

    2. 符合条件连接查询

      示例1:以内连接的方式查询employee和department表,并且employee表中的age字段值必须大于25,即找出年龄大于25岁的员工以及员工所在的部门

      select employee.name,department.name from employee inner join department
        on employee.dep_id = department.id
        where age > 25;
      

      示例2:以内连接的方式查询employee和department表,并且以age字段的升序方式显示。

      select employee.id,employee.name,employee.age,department.name from employee,department
          where employee.dep_id = department.id
          and age > 25
          order by age asc;
      

    3. 子查询

      #1:子查询是将一个查询语句嵌套在另一个查询语句中。
      #2:内层查询语句的查询结果,可以为外层查询语句提供查询条件。
      #3:子查询中可以包含:IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
      #4:还可以包含比较运算符:= 、 !=、> 、<等
      

      例子:

      (1)带in关键字的子查询

      #查询平均年龄在25岁以上的部门名
      select id,name from department
          where id in 
              (select dep_id from employee group by dep_id having avg(age) > 25);
      # 查看技术部员工姓名
      select name from employee
          where dep_id in 
              (select id from department where name='技术');
      #查看不足1人的部门名
      select name from department
          where id not in 
              (select dep_id from employee group by dep_id);
      

      (2)带比较运算符的子查询

      #比较运算符:=、!=、>、>=、<、<=、<>
      #查询大于所有人平均年龄的员工名与年龄
      mysql> select name,age from employee where age > (select avg(age) from employee);
      +---------+------+
      | name    | age  |
      +---------+------+
      | alex    |   48 |
      | wupeiqi |   38 |
      +---------+------+
      
      #查询大于部门内平均年龄的员工名、年龄
      思路:
            (1)先对员工表(employee)中的人员分组(group by),查询出dep_id以及平均年龄。
             (2)将查出的结果作为临时表,再对根据临时表的dep_id和employee的dep_id作为筛选条件将employee表和临时表进行内连接。
             (3)最后再将employee员工的年龄是大于平均年龄的员工名字和年龄筛选。
      
      
      
      mysql> select t1.name,t1.age from employee as t1
                   inner join
                  (select dep_id,avg(age) as avg_age from employee group by dep_id) as t2
                  on t1.dep_id = t2.dep_id
                  where t1.age > t2.avg_age;
      +------+------+
      | name | age  |
      +------+------+
      | alex |   48 |
      

      (3) 带EXISTS关键字的子查询

      #EXISTS关字键字表示存在。在使用EXISTS关键字时,内层查询语句不返回查询的记录。而是返回一个真假值。True或False
      #当返回True时,外层查询语句将进行查询;当返回值为False时,外层查询语句不进行查询
      #department表中存在dept_id=203,Ture
      mysql> select * from employee  where exists (select id from department where id=200);
      +----+----------+--------+------+--------+
      | id | name     | sex    | age  | dep_id |
      +----+----------+--------+------+--------+
      |  1 | egon     | male   |   18 |    200 |
      |  2 | alex     | female |   48 |    201 |
      |  3 | wupeiqi  | male   |   38 |    201 |
      |  4 | yuanhao  | female |   28 |    202 |
      |  5 | nvshen   | male   |   18 |    200 |
      |  6 | xiaomage | female |   18 |    204 |
      +----+----------+--------+------+--------+
      #department表中存在dept_id=205,False
      mysql> select * from employee  where exists (select id from department where id=204);
      Empty set (0.00 sec)
      



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  • 原文地址:https://www.cnblogs.com/wangyueping/p/11257985.html
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