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  • 单表查询

    先创建表

    复制代码
    #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','teacher',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)
    ;

    复制代码
    1 1.注意:                                                                                        
    2     select * from t1 where 条件 group by 分组字段                                                  
    3     1.分组只能查询分组字段,要想查看其余的利用聚合函数                                                               
    4     2.聚合函数的分类:count,min,max,avg,group_concat,sum等。                                           
    5     3.模糊匹配:用like关键字。                                                                         
    6       select * from t1 where name like '%eg%'; #%表示任意字符                                      
    7       select * from t1 where name like 'd__l'; #一个下划线表示一个字符,两个下划线就表示两个字符                     
    8     4.拷贝表 :create table t2 select * from t1;                                                 
    9                create table t2 select * from t1 where 1=2 ;                                  
    一点小知识复习

    一、查询语法

    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、简单查询

    复制代码
    #简单查询
        SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
        FROM employee;
    
    SELECT </span>*<span style="color: #000000;"> 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, salary12 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;

    复制代码

    小练习:

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

    三、where约束

    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80或90或100
    4. like 'eg%'
        可以是%或_,
        %表示任意多字符
        _表示一个字符

     like 'e__n' :
    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 

    复制代码
    #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 </span>10000 AND 20000<span style="color: #000000;">;
    

    #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</span>=<span style="color: #800000;">''</span>; 注意<span style="color: #800000;">''</span><span style="color: #000000;">是空字符串,不是null
    ps:
        执行
        update employee set post_comment</span>=<span style="color: #800000;">''</span> where id=2<span style="color: #000000;">;
        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 (</span>3000,3500,4000,9000<span style="color: #000000;">) ;
    
    SELECT name,salary FROM employee 
        WHERE salary NOT IN (</span>3000,3500,4000,9000<span style="color: #000000;">) ;
    

    #6:关键字LIKE模糊查询
    通配符’%
    SELECT
    * FROM employee
    WHERE name LIKE
    'eg%';

    通配符’_’
    SELECT </span>*<span style="color: #000000;"> FROM employee 
            WHERE name LIKE </span><span style="color: #800000;">'</span><span style="color: #800000;">al__</span><span style="color: #800000;">'</span>;</pre>
    
    复制代码

    四、having过滤

    having和where语法上是一样的。

    select * from employee where id>15;    
    select * from employee having id>15;   

    但是having和where不一样的地方在于以下几点!!!

    复制代码
    #!!!执行优先级从高到低:where > group by > 聚合函数 > having >order by
    1.where和having的区别                                                                                
         1. Where 是一个约束声明,使用Where约束来自数据库的数据,Where是在结果返回之前起作用的                                        
         (先找到表,按照where的约束条件,从表(文件)中取出数据),Where中不能使用聚合函数                                              
         2.Having是一个过滤声明,是在查询返回结果集以后对查询结果进行的过滤操作                                                     
         (先找到表,按照where的约束条件,从表(文件)中取出数据,然后group by分组,                                                
          如果没有group by则所有记录整体为一组,然后执行聚合函数,然后使用having对聚合的结果进行过滤),                                     
          在Having中可以使用聚合函数。                                                                          
         3.where的优先级比having的优先级高                                                                     
         4.having可以放到group by之后,而where只能放到group by 之前。                                               
    复制代码

    验证不同之处:

    复制代码
    1.查看员工的id>15的有多少个
    select count(id) from employee where id>15;#正确,分析:where先执行,后执行聚合count(id),
                                                然后select出结果
    select count(id) from employee having id>15; #报错,分析:先执行聚合count(id),后执行having过滤,
                                                #无法对id进行id>15的过滤
    #以上两条sql的顺序是
    1:找到表employee--->用where过滤---->没有分组则默认一组执行聚合count(id)--->select执行查看组内id数目
    2:找到表employee--->没有分组则默认一组执行聚合count(id)---->having 基于上一步聚合的结果(此时只有count(id)字段了)
    进行id>15的过滤,很明显,根本无法获取到id字段
    复制代码
    1 ------having-----------
    2 select depart_id,count(id) from employee group by depart_id;
    3 select depart_id,count(id) from employee group by depart_id having depart_id = 3;
    4 select depart_id,count(id) from employee group by depart_id having count(id)>7;
    5 select max(salary) 最大工资 from employee where id>2 group by depart_id having count(id)>3;
    6 select * from employee where id>7; #查看所有id>7的员工信息
    having 举例

    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
    select post,group_concat(name) 员工姓名,count(id) 个数 from employee group by post having count(id)<2;
    2. 查询各岗位平均薪资大于10000的岗位名、平均工资
    select post,avg(salary) from employee group by post having avg(salary)>10000;
    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
    select post,avg(salary)  from employee group by post having  avg(salary) between 10000 and 20000;

    五、分组查询 group by

    大前提:可以按照任意字段分组,但分完组后,只能查看分组的那个字段,要想取的组内的其他字段信息,需要借助函数

    复制代码
    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

    复制代码

    强调:

    分组:一般相同的多的话就可以分成一组(一定是有重复的字段)
    小练习:
    复制代码
    1. 查询岗位名以及岗位包含的所有员工名字
    select post,group_concat(name) from employee group by post;
    2. 查询岗位名以及各岗位内包含的员工个数 select post,count(id) from employee group by post;
    3. 查询公司内男员工和女员工的个数 select sex,count(id) from employee group by sex;
    4. 查询岗位名以及各岗位的平均薪资 select post,max(salary) from employee group by post;
    5. 查询岗位名以及各岗位的最高薪资 select post,max(salary) from employee group by post;
    6. 查询岗位名以及各岗位的最低薪资 select post,min(salary) from employee group by post;
    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 select sex,avg(salary) from employee group by sex;
    复制代码

    六、关键字的执行优先级(重点)

    复制代码
    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
    复制代码

    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.如果有聚合函数,则将组进行聚合

    5.将4的结果过滤:having

    6.查出结果:select

    7.去重

    8.将6的结果按条件排序:order by

    9.将7的结果限制显示条数

    七、查询排序order by 

    复制代码
    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;



    ===========order by========== 1.select * from employee order by salary;#如果不指定,默认就是升序 2.select * from employee order by salary asc; 3.select * from employee order by salary desc; #先按照年龄升序,当年龄相同的太多,分不清大小时,在按照工资降序 4.select * from employee order by age asc, salary desc;
    复制代码

    小例子:

    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
    select * form 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
    select post ,avg(salary) from employee group by post having avg(salary)>10000;
    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
    select post ,avg(salary) from employee group by post having avg(salary)>10000 desc;

    八、使用聚合函数查询

    先from找到表

    再用where的条件约束去表中取出记录

    然后进行分组group by,没有分组则默认一组

    然后进行聚合

    最后select出结果

    复制代码
    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) form employee WHERE depart_id=3;
    复制代码

    九、where的补充(使用正则表达式查询)

    复制代码
    1.select * from employee where name regexp '^ale';  #匹配以ale开头的员工信息
    2.select * from employee where name regexp 'on$'; #匹配以on结尾的员工信息
    3.select * from employee where name regexp 'n{1,2}'; #匹配name里面包含1到2个n的员工信息
    小结:对字符串匹配的方式
    where name = 'egon';
    where name like 'yua%';
    where name regexp 'on$';
    复制代码

    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.*[ng]$';

    十、限制查询的记录数:LIMIT

    复制代码
    =========limit:限制打印几条=========
    1.select * from employee limit 3;#打印前三条
    2.像这样表示的:指的是从哪开始,往后取几条 (这样的操作一般用来分页)
    select * from employee limit 0,3;
    select * from employee limit 3,4;
    select * from employee limit 6,3;
    select * from employee limit 9,3;
    3.select * from employee order by id desc limit 3; #查看后三条
    复制代码

    小练习

    1. 分页显示,每页5条
    select * from employee limit 0,5;
    select * from employee limit 5,5;
    select * from employee limit 10,5;
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  • 原文地址:https://www.cnblogs.com/maaosheng/p/11619205.html
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