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    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

     三,简单查询

    1,准备表和记录

    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
    create table employee(
    id int not null unique 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, #一个部门一个屋子
    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 |
    | 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    |                |
    +--------------+-----------------------+------+-----+---------+----------------+
    #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)
    ;
     
    #ps: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

    2,简单查询

    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;   
     
    通过四则运算查询
        SELECT name, salary*12 FROM employee;
        SELECT name, salary*12 AS Annual_salary FROM employee;
        SELECT name, salary*12 Annual_salary FROM employee;
     
    定义显示格式
       CONCAT() 函数用于连接字符串
       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
       SELECT CONCAT_WS(':',name,salary*12)  AS Annual_salary
       FROM employee;
     
       结合CASE语句:
       SELECT
           (
               CASE
               WHEN NAME = 'egon' THEN
                   NAME
               WHEN NAME = 'alex' THEN
                   CONCAT(name,'_BIGSB')
               ELSE
                   concat(NAME, 'SB')
               END
           ) as new_name
       FROM
           emp; 

    四,WHERE约束

    1,where字句中可以使用:

    • 1. 比较运算符:> < >= <= <> !=
    • 2. between 80 and 100 值在10到20之间
    • 3. in(80,90,100) 值是10或20或30
    • 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__';
    

    五,分组查询:GROUP BY

    1,什么是分组,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
    

    2,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!!!
    

      

    #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!!!
      
    
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    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      

    3,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

    4,聚合函数

    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
     
    示例:
        SELECT COUNT(*) FROM employee;
        SELECT COUNT(*) FROM employee WHERE depart_id=1;
        SELECT MAX(salary) FROM employee;
        SELECT MIN(salary) FROM employee;
        SELECT AVG(salary) FROM employee;
        SELECT SUM(salary) FROM employee;
        SELECT SUM(salary) FROM employee WHERE depart_id=3;
    !!!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
    

      

    六,HAVING过滤

    1,HAVING与WHERE不一样的地方在于下面

    我是Edison_chen 一个在自学道路上摸爬滚打的人!
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  • 原文地址:https://www.cnblogs.com/edison-chen/p/9796787.html
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