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MYsql数据库----之-----单表查询

单表查询

单表查询的语法和优先级

单表查询的顺序不能改变(有没有无所谓)

SELECT distinct(去重) 字段1,字段2... FROM 表名

　where 约束条件
　group by 分组的字段
　having 过滤条件 (是在分组之后进行过滤的)
　select 选择 (这个就是放在开头)
　distinct 去重
　order by 排序字段
　limit 限制条件;

select    选择字段
distinct  是否去重  
from      选择是哪张表
where     指定约束条件
group by 按照字段分组 不分组默认整体是一组
having    分组的结果可以进行 过滤
order by  进行条件排序 
limit     限制显示条数 
concat      聚合函数

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
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','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)
;

实例代码

单表查询简单用法

select post from employee;
去重:distinct
select distinct post from employee;
    mysql> select distinct post from employee;
    +----------------------------+
    | post                       |
    +----------------------------+
    | 老男孩驻沙河办事处外交大使 |
    | teacher                    |
    | sale                       |
    | operation                  |
    +----------------------------+
    4 rows in set (0.01 sec)
    
可以选择字段直接进行四则运算
select name,salary*12 from employee;
    mysql> select name,salary*12 from employee;
    +------------+-------------+
    | name       | salary*12   |
    +------------+-------------+
    | egon       |    87603.96 |
    | alex       | 12000003.72 |
    | 程咬银     |   228000.00 |
    | 程咬铜     |   216000.00 |
    | 程咬铁     |   204000.00 |
    +------------+-------------+
    18 rows in set (0.01 sec
    
select name,salary*12 as 年薪 from employee;
    查询并修改显示名称
    +------------+-------------+
    | name       | 年薪        |
    +------------+-------------+
    | egon       |    87603.96 |
    | alex       | 12000003.72 |
    | wupeiqi    |    99600.00 |
    +------------+-------------+    

定义显示格式

concat()函数用于连接字符串
select concat('名字: ',name,' ','年薪: ',salary*12 )from employee;
    直接小括号内进行字符串的查询和拼接
    +-----------------------------------------------+
    | concat('名字: ',name,' ','年薪: ',salary*12 ) |
    +-----------------------------------------------+
    | 名字: egon 年薪: 87603.96                     |
    | 名字: alex 年薪: 12000003.72                  |
    | 名字: wupeiqi 年薪: 99600.00                  |
        
        
select concat(name,':',salary*12 )from employee;
    直接名字加年薪
    | egon:87603.96               |
    | alex:12000003.72            |
    | wupeiqi:99600.00            |
        

select concat(name,':',salary*12,':',age,':',sex )from employee;
    添加直接往后加就可以了

concat_ws 第一个参数为分隔符
select concat_ws(':',name,salary*12)from employee;
    直接定义分割符

练习

1 查出所有员工的名字，薪资,格式为
    <名字:egon>    <薪资:3000>
2 查出所有的岗位（去掉重复）
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

select concat('<名字:',name,'>    ','<薪资:',salary,'>') from employee;
select distinct depart_id from employee;
select name,salary*12 annual_salary from employee;

练习

单表查询 where约束

where + 字段名 +比较运算符：> < >= <= <> != 和逻辑符 and or not
between 数值 and 数值范围查询多少到多少
in(10,20,30)值查找符合条件的显示
like 模糊查询以什么开头 + %代表任意字符或者 _ 表示一个字符

where 约束条件

select* from employee;
查出来老师都有哪些?
where 指定字段名进行查询
select* from employee where post='teacher';
只查老师的部分信息
select id,name from employee where post='teacher';
查出来id 3到5 之间的信息,指定约束条件即可
<比较运算符 <>= != 和 and or>
select id,name from employee where id>=3 and id<5;


between 数值 and 数值 选择范围进行查询
select id,name from employee where between 3 and 5;
选择一个字段的多条结果进行查询
select id,name,age from employee 
    where age=78 or age=18 or age=60;
查找字段多条结果的简便写法,只有符合条件的会显示出来
select id,name,age from employee 
    where age in (60,70,80,18,'aaa');
    
like 模糊查询匹配 % 或者_ (表示一个字符)
查询名字以e开头
select id,name,age from employee
    where name like 'e%';
查询名字是三个字符的
select id,name,age from employee
    where name like '___';
    
not 加在关键字前 即可

is 查询指定字段是否是空 
select * from employee where post_comment is null ;

#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__';


小练习：
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%';

详解练习

分组查询:GROUP BY 与聚合函数

没有分组默认整体一组

分组查询group by 分组查询 (先分完组才好统计数量)
group_concat()一起使用的函数
强调分组之后select只能查看分组的字段 ,要想查看组内内容不能直接查看
需要借助聚合函数 max ,min,avg,sum,count
分组的目的是为了以组为单位来处理记录,而不是处理单独的记录
分组依据多条记录有相同字段如部门 ,年龄, 性别

select count(id) from employee;
先根据条件进行分组 然后根据要查询的数量进行查询 查看各部门员工数
select post,count(id) from employee group by post;
这么查没有意义只查这个组的第一条记录
select post,name from employee group by post;
查看teacher部门的员工数
select post,count(id) from employee where post='teacher';

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
select post,max(salary) from employee group by post;
select post,min(salary) from employee group by post;
select post,avg(salary) from employee group by post;
select post,sum(salary) from employee group by post;
查看各部门都有哪些员工
select post,group_concat(name) from employee group by post;
可以加字符串拼接
select post,group_concat('薪资,salary') from employee group by post;

二 ONLY_FULL_GROUP_BY 完整的分组限制

#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 |
+-------------------+
|                   |
+-------------------+
1 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 |
+----+------+--------+-----+------------+----------------------------+--------------+------------+--------+-----------+
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

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

设置

详解 GROUP BY


#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


使用方法:
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


练习:
1. 查询岗位名以及岗位包含的所有员工名字
2. 查询岗位名以及各岗位内包含的员工个数
3. 查询公司内男员工和女员工的个数
4. 查询岗位名以及各岗位的平均薪资
5. 查询岗位名以及各岗位的最高薪资
6. 查询岗位名以及各岗位的最低薪资
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 post,group_concat(name)  from employee group by post;
select post,count(id)  from employee group by post;
select sex,count(id) from employee group by sex;

select sex,avg(salary)from employee group by sex;

详解与练习

Having 过滤

　　having 过滤条件只能放在分组的后面,一般过滤用where足可

　　和where区别是优先级聚合函数

select * from employee where salary > 10000;

#错误：having是在分组之后的，意味着，只能用分组的字段或者聚合函数作为过滤条件
select post from employee
    group by post
    having salary > 10000
    ;

select * from employee
    having salary > 10000;


select * from employee
    having count(id) > 10000;


#取出员工数大于3的部门
select post from employee group by post having count(id) > 3;


#取出id是大于10的,员工数大于3的部门
select post from employee where id > 10 group by post having count(id) > 3;




#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),count(id) from employee group by post having count(id) < 2;


3. 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;

4. 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) > 10000 and avg(salary) < 20000;

查询排序:ORDER BY

排序只有升序和降序两种排列方式可以指定字段进行升降排序

#默认升序
select * from employee order by age asc;
select * from employee order by age;

#降序
select * from employee order by age desc;

#先按照age升序排，如果age相同，则id降序排序
select * from employee order by age asc,id desc;


#查看每个部门的最高工资
select post,max(salary) from employee group by post order by max(salary);
select post,max(salary) as m from employee group by post order by m;


#取出平均工资>10000的部门以及它的平均工资
select post,avg(salary) from employee group by post having avg(salary) > 10000;


#错误，执行having时，select还没有执行呢
select post,avg(salary) as x from employee group by post having x > 10000;





#取出平均工资>10000的部门以及它的平均工资
select post,avg(salary) as x from employee
    group by post
    having avg(salary) > 10000
    order by x desc
    ;

select post,avg(salary) as x from employee
    group by post
    having avg(salary) > 10000
    order by avg(salary) desc
    ;


#错误
select post,avg(salary) as x from employee
    group by post
    having avg(salary) > 10000
    order by post
    ;


select post,avg(salary) as x from employee
    group by post
    having avg(salary) > 10000
    order by x
    ;

限制查询的记录数:LIMIT

主要是用于翻页限制显示

select * from employee limit 3;

#取出工资最高的那个员工信息
select * from employee order by salary desc limit 1;
显示一条记录 最高的两人就显示两条记录

#总共的计数数：30123
#一页只能显示：30


select * from employee limit 0,3; 从0开始 向后查3条
select * from employee limit 3,3;从3开始 向后查3条
select * from employee limit 6,3;从6开始 向后查3条

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$';

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/zgd1234/p/7732382.html