单表查询

单表查询的语法及关键字执行的优先级

单表查询语法

SELECT DISTINCT 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

关键字执行的优先级

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from
where
group by
select
distinct
having
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.执行select（去重）

5.将分组的结果进行having过滤

6.将结果按条件排序：order by

7.限制结果的显示条数

简单查询

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,

emp_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 |

| emp_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(emp_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


准备表和记录

建表和数据准备
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#简单查询
    SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id 
    FROM employee;

</span><span style="color: #0000ff;">SELECT</span> <span style="color: #808080;">*</span> <span style="color: #0000ff;">FROM</span><span style="color: #000000;"> employee;

</span><span style="color: #0000ff;">SELECT</span> emp_name,salary <span style="color: #0000ff;">FROM</span><span style="color: #000000;"> employee;



#避免重复DISTINCT

SELECT DISTINCT post FROM employee;


#通过四则运算查询

SELECT emp_name, salary12 FROM employee;

SELECT emp_name, salary12 AS Annual_salary FROM employee;

SELECT emp_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: ',emp_name,'  年薪: ', salary*12)  AS Annual_salary

FROM employee;


CONCAT_WS() 第一个参数为分隔符

SELECT CONCAT_WS(':',emp_name,salary*12)  AS Annual_salary

FROM employee;


结合CASE语句：

SELECT

(

CASE

WHEN emp_name = 'jingliyang' THEN

emp_name

WHEN emp_name = 'alex' THEN

CONCAT(emp_name,'_BIGSB')

ELSE

concat(emp_name, 'SB')

END

) as new_name

FROM

employee;

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小练习：

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:',emp_name,'>    ','<薪资:',salary,'>') from employee;
select distinct depart_id from employee;
select emp_name,salary*12 annual_salary from employee;
答案

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 'e%'
    通配符可以是%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

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#1:单条件查询
    SELECT emp_name FROM employee
        WHERE post='sale';
        
#2:多条件查询
    SELECT emp_name,salary FROM employee
        WHERE post='teacher' AND salary>10000;

#3:关键字BETWEEN AND

    SELECT emp_name,salary FROM employee

WHERE salary BETWEEN 10000 AND 20000;


SELECT emp_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 emp_name,post_comment FROM employee

WHERE post_comment IS NULL;


SELECT emp_name,post_comment FROM employee 
    WHERE post_comment IS NOT NULL;
    
SELECT emp_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 emp_name,salary FROM employee

WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;


SELECT emp_name,salary FROM employee 
    WHERE salary IN (</span>3000,3500,4000,9000<span style="color: #000000;">) ;

SELECT emp_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 emp_name LIKE 'eg%';


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



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小练习：

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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9000-10000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
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select emp_name,age from employee where post = 'teacher';
select emp_name,age from employee where post='teacher' and age > 30; 
select emp_name,age,salary from employee where post='teacher' and salary between 9000 and 10000;
select * from employee where post_comment is not null;
select emp_name,age,salary from employee where post='teacher' and salary in (10000,9000,30000);
select emp_name,age,salary from employee where post='teacher' and salary not in (10000,9000,30000);
select emp_name,salary*12 from employee where post='teacher' and emp_name like 'jin%';
答案

group by 

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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(emp_name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名

    SELECT post,GROUP_CONCAT(emp_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

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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

聚合函数

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#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;
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小练习：

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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
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#题1：分组
mysql> select post,group_concat(emp_name) from employee group by post;
+-----------------------------------------+---------------------------------------------------------+
| post                                    | group_concat(emp_name)                                      |
+-----------------------------------------+---------------------------------------------------------+
| operation                               | 张野,程咬金,程咬银,程咬铜,程咬铁                        |
| sale                                    | 歪歪,丫丫,丁丁,星星,格格                                |
| teacher                                 | alex,wupeiqi,yuanhao,liwenzhou,jingliyang,jinxin,成龙   |
| 老男孩驻沙河办事处外交大使              | egon                                                    |
+-----------------------------------------+---------------------------------------------------------+

#题目2：

mysql> select post,count(id) from employee group by post;

+-----------------------------------------+-----------+

| post                                    | count(id) |








operation

sale

teacher		


     5
     5
     7







老男孩驻沙河办事处外交大使              |         1 |






#题目3：

mysql> select sex,count(id) from employee group by sex;

+--------+-----------+

| sex    | count(id) |








male

female		


    10
     8









#题目4：

mysql> select post,avg(salary) from employee group by post;

+-----------------------------------------+---------------+

| post                                    | avg(salary)   |








operation

sale

teacher		
16800.026000

2600.294000

151842.901429		




老男孩驻沙河办事处外交大使              |   7300.330000 |






#题目5

mysql> select post,max(salary) from employee group by post;

+-----------------------------------------+-------------+

| post                                    | max(salary) |








operation

sale

teacher		


20000.00
 4000.33



1000000.31






老男孩驻沙河办事处外交大使              |     7300.33 |






#题目6

mysql> select post,min(salary) from employee group by post;

+-----------------------------------------+-------------+

| post                                    | min(salary) |








operation

sale

teacher		


10000.13
 1000.37
 2100.00







老男孩驻沙河办事处外交大使              |     7300.33 |






#题目七

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

+--------+---------------+

| sex    | avg(salary)   |








male

female		
110920.077000

7250.183750
答案

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
mysql> select @@sql_mode;
+--------------------+
| @@sql_mode         |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
row in set (0.00 sec)

mysql> select * from emp where salary > 100000;

+----+------+------+-----+------------+---------+--------------+------------+--------+-----------+

| id | emp_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 post,group_concat(emp_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(emp_name) from emp group by post having avg(salary) > 10000;

+-----------+-------------------------------------------------------+

| post | group_concat(emp_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)

验证

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
3. 查询各岗位平均薪资大于10000的岗位名、平均工资
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
#题1：
mysql> select post,group_concat(emp_name),count(id) from employee group by post having count(id) < 2;
+-----------------------------------------+--------------------+-----------+
| post                                    | group_concat(emp_name) | count(id) |
+-----------------------------------------+--------------------+-----------+
| 老男孩驻沙河办事处外交大使              | egon               |         1 |
+-----------------------------------------+--------------------+-----------+

#题目2：

mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;

+-----------+---------------+

| post      | avg(salary)   |








operation

teacher		
16800.026000

151842.901429		






#题目3：

mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000 and avg(salary) <20000;

+-----------+--------------+

| post      | avg(salary)  |








operation
16800.026000
答案

ORDER BY 查询排序

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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;

复制代码

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
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
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
#题目1
mysql> select * from employee ORDER BY age asc,hire_date desc;

#题目2

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

+-----------+---------------+

| post      | avg(salary)   |








operation

teacher		
16800.026000

151842.901429		






#题目3

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

+-----------+---------------+

| post      | avg(salary)   |








teacher

operation		
151842.901429

16800.026000
答案

LIMIT 限制查询的记录数

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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
复制代码

小练习：

1. 分页显示，每页5条
mysql> select * from  employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | emp_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 | emp_name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
















6

7		
jingliyang

jinxin		
female

male		
18

18		
2011-02-11

1900-03-01		
teacher

teacher		
NULL

NULL		
9000.00

30000.00		


401
401





     1
     1







8

9

10		
成龙       | male   |  48 | 2010-11-11 | teacher | NULL         | 10000.00 |    401 |         1 |

歪歪       | female |  48 | 2015-03-11 | sale    | NULL         |  3000.13 |    402 |         2 |

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 | emp_name      | sex    | age | hire_date  | post      | post_comment | salary   | office | depart_id |
















11

12

13

14

15		
丁丁      | female |  18 | 2011-03-12 | sale      | NULL         |  1000.37 |    402 |         2 |

星星      | female |  18 | 2016-05-13 | sale      | NULL         |  3000.29 |    402 |         2 |

格格      | female |  28 | 2017-01-27 | sale      | NULL         |  4000.33 |    402 |         2 |

张野      | male   |  28 | 2016-03-11 | operation | NULL         | 10000.13 |    403 |         3 |

程咬金    | male   |  18 | 1997-03-12 | operation | NULL         | 20000.00 |    403 |         3 |		






rows in set (0.00 sec)

答案

使用正则表达式查询

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SELECT * FROM employee WHERE emp_name REGEXP '^ale';

SELECT * FROM employee WHERE emp_name REGEXP 'on$';


SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';


小结：对字符串匹配的方式

WHERE emp_name = 'egon';

WHERE emp_name LIKE 'yua%';

WHERE emp_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 emp_name regexp '^jin.*[gn]$';
答案

 

posted @ 2019-09-17 11:51  若如初见_you  阅读(242)  评论(0)  编辑  收藏  举报