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)
    



posted @ 2019-08-08 00:01  与鹿逐秋  阅读(555)  评论(0编辑  收藏  举报