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  • 数据库:数据操作-单表查询

    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)

    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.限制结果的显式条数

    company.employee
        员工id      id                  int             
        姓名        emp_name            varchar
        性别        sex                 enum
        年龄        age                 int
        入职日期     hire_date           date
        岗位        post                varchar
        职位描述     post_comment        varchar
        薪水        salary              double
        办公室       office              int
        部门编号     depart_id           int
    
    
    
    
    # 创建表
    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_comment varchar(100),
        -> salary double(15,2),
        -> office int,
        -> depart_id int
        -> );
    Query OK, 0 rows affected (0.04 sec)
    
    # 查看表结构
    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_comment | varchar(100)          | YES  |     | NULL    |                |
    | salary       | double(15,2)          | YES  |     | NULL    |                |
    | office       | int(11)               | YES  |     | NULL    |                |
    | depart_id    | int(11)               | YES  |     | NULL    |                |
    +--------------+-----------------------+------+-----+---------+----------------+
    9 rows in set (0.01 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),
    ('成龙','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)
    ;
    

      

    #简单查询
        SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
        FROM employee;
    
        SELECT * FROM employee;
    
        SELECT name,salary FROM employee;
    
    # 避免重复distinct
    select distinct post from employee;
    
    # 通过四则运算
    select name,salary*12 from employee;
    
    select name,salary*12 as Annual_salary from employee;
    
    select name,salary*12 Annual_salary from employee;
    
    
    # 定义显式格式
    concat()函数用于连接字符串
    
    
    mysql> select concat('姓名',name,' 年薪: ', salary*12) as Annual_salary from employee;
    
    concat_ws() : 第一个参数为分隔符
    mysql> select concat_ws(':',name,salary*12) as Annual_salary from employee;
    

      

    1、查出所有员工的名字,薪资,格式为
    
    <
    名字:egon
    >
    <
    薪资:3000
    >
    
    mysql> select concat('<名字:',name,'>  ','<薪资:',salary,'>') from employee;
    
    2、 查出所有的岗位(去掉重复)
    mysql> select distinct depart_id from employee;
    
    3 查出所有员工名字,以及他们的年薪,年薪的字段名为annual_year
    
    
    mysql> select name, salary*12 annual_salary from employee;
    

      

    四 where约束

    where字句中可以使用:

    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 ‘egon%’,pattern 可以是%或_,  %:表示任意多字符;  _: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
    #1:单条件查询
        SELECT name FROM employee
            WHERE post='sale';
    
    #2:多条件查询
        SELECT name,salary FROM employee
            WHERE post='teacher' AND salary>10000;
    
    #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;
    
    #4:关键字IS NULL(判断某个字段是否为NULL不能用等号,需要用IS)
        SELECT name,post_comment FROM employee 
            WHERE post_comment IS NULL;
    
        SELECT name,post_comment FROM employee 
            WHERE post_comment IS NOT NULL;
    
        SELECT name,post_comment FROM employee 
            WHERE post_comment=''; 注意''是空字符串,不是null
        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
        SELECT name,salary FROM employee 
            WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
    
        SELECT name,salary FROM employee 
            WHERE salary IN (3000,3500,4000,9000) ;
    
        SELECT name,salary FROM employee 
            WHERE salary NOT IN (3000,3500,4000,9000) ;
    
    #6:关键字LIKE模糊查询
        通配符’%’
        SELECT * FROM employee 
                WHERE name LIKE 'eg%';
    
        通配符’_’
        SELECT * FROM employee 
                WHERE name LIKE 'al__';
    

      

    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
    
    
    
    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%';
    

      

    五 分组查询:group by

    一 什么是分组?为什么要分组?

    #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
    

      

    二 ONLY_FULL_GROUP_BY

    #查看MySQL 5.7默认的sql_mode如下:
    mysql> select @@global.sql_mode;
    ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION
    
    #!!!注意
    ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义,简单的说来,在ONLY_FULL_GROUP_BY模式下,target list中的值要么是来自于聚集函数的结果,要么是来自于group by list中的表达式的值。
    
    
    #设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式):
    mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';
    
    !!!SQL_MODE设置!!!
    

      

    mysql> select @@global.sql_mode;
    +-------------------+
    | @@global.sql_mode |
    +-------------------+
    |                   |
    +-------------------+
    row in set (0.00 sec)
    
    mysql> select * from emp 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                    | NULL         | 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
    
    mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
    Query OK, 0 rows affected (0.00 sec)
    
    mysql> quit #设置成功后,一定要退出,然后重新登录方可生效
    Bye
    
    mysql> use db1;
    Database changed
    mysql> select * from emp group by post; #报错
    ERROR 1055 (42000): 'db1.emp.id' isn't in GROUP BY
    mysql> select post,count(id) from emp group by post; #只能查看分组依据和使用聚合函数
    +----------------------------+-----------+
    | post                       | count(id) |
    +----------------------------+-----------+
    | operation                  |         5 |
    | sale                       |         5 |
    | teacher                    |         7 |
    | 老男孩驻沙河办事处外交大使 |         1 |
    +----------------------------+-----------+
    rows in set (0.00 sec)
    

      

    三 GROUP BY

    单独使用GROUP BY关键字分组
        SELECT post FROM employee GROUP BY post;
        注意:我们按照post字段分组,那么select查询的字段只能是post,想要获取组内的其他相关信息,需要借助函数
    
    GROUP BY关键字和GROUP_CONCAT()函数一起使用
        SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照岗位分组,并查看组内成员名
        SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post;
    
    GROUP BY与聚合函数一起使用
        select post,count(id) as count from employee group by post;#按照岗位分组,并查看每个组有多少人
    

      强调

    如果我们用unique的字段作为分组的依据,则每一条记录自成一组,这种分组没有意义
    多条记录之间的某个字段值相同,该字段通常用来作为分组的依据
    

      

    五 小练习:

    1. 查询岗位名以及岗位包含的所有员工名字
    mysql> select post,group_concat(name) from employee group by post;   
    +-----------------------------------------+---------------------------------------------------------+
    | post                                    | group_concat(name)                                      |
    +-----------------------------------------+---------------------------------------------------------+
    | operation                               | 程咬铁,程咬铜,程咬银,程咬金,张野          |
    | sale                                    | 格格,星星,丁丁,丫丫,歪歪                      |
    | teacher                                 | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
    | 老男孩驻沙河办事处外交大使 | egon                                                    |
    +-----------------------------------------+---------------------------------------------------------+
    
    2. 查询岗位名以及各岗位内包含的员工个数
    mysql> select post,count(id) from employee group by post;
    +-----------------------------------------+-----------+
    | post                                    | count(id) |
    +-----------------------------------------+-----------+
    | operation                               |         5 |
    | sale                                    |         5 |
    | teacher                                 |         7 |
    | 老男孩驻沙河办事处外交大使 |         1 |
    +-----------------------------------------+-----------+
    
    3. 查询公司内男员工和女员工的个数
    
    mysql> select sex, count(id) from employee group by sex;
    +--------+-----------+
    | sex    | count(id) |
    +--------+-----------+
    | male   |        10 |
    | female |         8 |
    +--------+-----------+
    
    4. 查询岗位名以及各岗位的平均薪资
    mysql> select post,avg(salary) from employee group by post;
    +-----------------------------------------+---------------+
    | post                                    | avg(salary)   |
    +-----------------------------------------+---------------+
    | operation                               |  16800.026000 |
    | sale                                    |   2600.294000 |
    | teacher                                 | 151842.901429 |
    | 老男孩驻沙河办事处外交大使 |   7300.330000 |
    +-----------------------------------------+---------------+
    
    5. 查询岗位名以及各岗位的最高薪资
    mysql> select post,max(salary) from employee group by post;   
    +-----------------------------------------+-------------+
    | post                                    | max(salary) |
    +-----------------------------------------+-------------+
    | operation                               |    20000.00 |
    | sale                                    |     4000.33 |
    | teacher                                 |  1000000.31 |
    | 老男孩驻沙河办事处外交大使 |     7300.33 |
    +-----------------------------------------+-------------+
    
    6. 查询岗位名以及各岗位的最低薪资
    mysql> select post,min(salary) from employee group by post;  
    +-----------------------------------------+-------------+
    | post                                    | min(salary) |
    +-----------------------------------------+-------------+
    | operation                               |    10000.13 |
    | sale                                    |     1000.37 |
    | teacher                                 |     2100.00 |
    | 老男孩驻沙河办事处外交大使 |     7300.33 |
    +-----------------------------------------+-------------+
    7. 查询男员工与男员工的平均薪资,女员工与女员工的平均薪资
    
    mysql> select sex,avg(salary) from employee group by sex;        
    +--------+---------------+
    | sex    | avg(salary)   |
    +--------+---------------+
    | male   | 110920.077000 |
    | female |   7250.183750 |
    +--------+---------------+
    

     

    六 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,但是均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,无法直接取到其它字段,可以使用聚合函数
    mysql> select @@sql_mode;
    +--------------------+
    | @@sql_mode         |
    +--------------------+
    | ONLY_FULL_GROUP_BY |
    +--------------------+
    row in set (0.00 sec)
    
    mysql> select * from emp where salary > 100000;
    +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
    | id | name | sex  | age | hire_date  | post    | post_comment | salary     | office | depart_id |
    +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
    |  2 | alex | male |  78 | 2015-03-02 | teacher | NULL         | 1000000.31 |    401 |         1 |
    +----+------+------+-----+------------+---------+--------------+------------+--------+-----------+
    row in set (0.00 sec)
    
    mysql> select * from emp having salary > 100000;
    ERROR 1463 (42000): Non-grouping field 'salary' is used in HAVING clause
    
    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 'salary' in 'having clause'
    mysql> select post,group_concat(name) from emp group by post having avg(salary) > 10000;
    +-----------+-------------------------------------------------------+
    | post | group_concat(name) |
    +-----------+-------------------------------------------------------+
    | operation | 程咬铁,程咬铜,程咬银,程咬金,张野 |
    | teacher | 成龙,jinxin,jingliyang,liwenzhou,yuanhao,wupeiqi,alex |
    +-----------+-------------------------------------------------------+
    rows in set (0.00 sec)
    

      小练习

    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
    mysql> select post,group_concat(name),count(id) from employee group by post having count(id) < 2;
    +-----------------------------------------+--------------------+-----------+
    | post                                    | group_concat(name) | count(id) |
    +-----------------------------------------+--------------------+-----------+
    | 老男孩驻沙河办事处外交大使              | egon               |         1 |
    +-----------------------------------------+--------------------+-----------+
    
    2. 查询各岗位平均薪资大于10000的岗位名、平均工资
    mysql> select post,avg(salary) from employee  group by post having avg(salary)>10000;
    +-----------+---------------+
    | post      | avg(salary)   |
    +-----------+---------------+
    | operation |  16800.026000 |
    | teacher   | 151842.901429 |
    +-----------+---------------+
    
    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
    mysql> select post,avg(salary) from employee  group by post having avg(salary) between 10000 and 20000;
    +-----------+--------------+
    | post      | avg(salary)  |
    +-----------+--------------+
    | operation | 16800.026000 |
    +-----------+--------------+
    

      

    七 查询排序:order by

    按单列排序
        SELECT * FROM employee ORDER BY salary;
        SELECT * FROM employee ORDER BY salary ASC;
        SELECT * FROM employee ORDER BY salary DESC;
    
    按多列排序:先按照age排序,如果年纪相同,则按照薪资排序
        SELECT * from employee
            ORDER BY age,
            salary DESC;
    

      小练习

    1. 查询所有员工信息,先按照age升序排序,如果age相同则按照hire_date降序排序
    mysql> select * from employee order by age,hire_date desc;
    
    2. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资升序排列
    
    mysql> select post,avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary) asc; 
    +-----------+---------------+
    | post      | avg(salary)   |
    +-----------+---------------+
    | operation |  16800.026000 |
    | teacher   | 151842.901429 |
    +-----------+---------------+
    
    3. 查询各岗位平均薪资大于10000的岗位名、平均工资,结果按平均薪资降序排列
    mysql> select post,avg(salary) from employee group by post having avg(salary)>10000 order by avg(salary) desc;
    +-----------+---------------+
    | post      | avg(salary)   |
    +-----------+---------------+
    | teacher   | 151842.901429 |
    | operation |  16800.026000 |
    +-----------+---------------+
    

      

     八 限制查询的记录数:limit

    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
    

      

    1. 分页显示,每页5条
    mysql> select * from  employee limit 0,5;
    +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
    | id | name      | sex  | age | hire_date  | post                                    | post_comment | salary     | office | depart_id |
    +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
    |  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 |
    |  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 |
    |  4 | yuanhao   | male |  73 | 2014-07-01 | teacher                                 | NULL         |    3500.00 |    401 |         1 |
    |  5 | liwenzhou | male |  28 | 2012-11-01 | teacher                                 | NULL         |    2100.00 |    401 |         1 |
    +----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
    rows in set (0.00 sec)
    
    mysql> select * from  employee limit 5,5;
    +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
    | 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 |
    |  8 | 成龙       | male   |  48 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |
    |  9 | 歪歪       | female |  48 | 2015-03-11 | sale    | NULL         |  3000.13 |    402 |         2 |
    | 10 | 丫丫       | female |  38 | 2010-11-01 | sale    | NULL         |  2000.35 |    402 |         2 |
    +----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
    rows in set (0.00 sec)
    
    mysql> select * from  employee limit 10,5;
    +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
    | id | name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
    +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
    | 11 | 丁丁      | female |  18 | 2011-03-12 | sale      | NULL         |  1000.37 |    402 |         2 |
    | 12 | 星星      | female |  18 | 2016-05-13 | sale      | NULL         |  3000.29 |    402 |         2 |
    | 13 | 格格      | female |  28 | 2017-01-27 | sale      | NULL         |  4000.33 |    402 |         2 |
    | 14 | 张野      | male   |  28 | 2016-03-11 | operation | NULL         | 10000.13 |    403 |         3 |
    | 15 | 程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |
    +----+-----------+--------+-----+------------+-----------+--------------+----------+--------+-----------+
    rows in set (0.00 sec)
    

      

    九 使用正则表达式查询

    SELECT * FROM employee WHERE name REGEXP '^ale';
    
    SELECT * FROM employee WHERE name REGEXP 'on$';
    
    SELECT * FROM employee WHERE name REGEXP 'm{2}';
    
    
    小结:对字符串匹配的方式
    WHERE name = 'egon';
    WHERE name LIKE 'yua%';
    WHERE name REGEXP 'on$';
    

      小练习

    查看所有员工中名字是jin开头,n或者g结果的员工信息
    select * from employee where name regexp '^jin.*[gn]$';
    

      

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  • 原文地址:https://www.cnblogs.com/mike-liu/p/9347175.html
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