Justin-刘清政的博客

python/MySQL入门/11-MySQL单表查询

2020-10-22

一、单表查询的语法及关键字执行的优先级

1.1 单表查询语法

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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

1.2 关键字执行的优先级

  1. from:找到表
  2. where:拿着where指定的约束条件,去文件/表中取出一条条记录
  3. group by: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. select:执行select
  5. distinct:去重
  6. having:将分组的结果进行having过滤
  7. order by:将结果按条件排序:order by
  8. limit:限制结果的显示条数

二、简单查询

2.1 建表和数据准备

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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
('lqz','male',18,'20170301','老男孩驻上海虹桥最帅',7300.33,401,1), # 以下是教学部
('jason','male',78,'20150302','teacher',1000000.31,401,1),
('sean','male',81,'20130305','teacher',8300,401,1),
('tank','male',73,'20140701','teacher',3500,401,1),
('oscar','male',28,'20121101','teacher',2100,401,1),
('mac','female',18,'20110211','teacher',9000,401,1),
('rocky','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
# 简单查询
SELECT id,emp_name,sex,age,hire_date,post,post_comment,salary,office,depart_id
FROM employee;

SELECT * FROM employee;

SELECT emp_name,salary FROM employee;

# 避免重复DISTINCT
SELECT DISTINCT post FROM employee;

# 通过四则运算查询
SELECT emp_name, salary*12 FROM employee;
SELECT emp_name, salary*12 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 = 'mac' THEN
emp_name
WHEN emp_name = 'jason' THEN
CONCAT(emp_name,'_BIGSB')
ELSE
concat(emp_name, 'SB')
END
) as new_name
FROM
employee;

2.2 练习

  1. 查出所有员工的名字,薪资,格式为<名字:lqz> <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
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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;

三、约束条件(where)

where子句中可以使用:

  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 ‘n%’
    • 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 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 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=''; 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 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 (3000,3500,4000,9000) ;

SELECT emp_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 emp_name LIKE 'ni%';

通配符’_’
SELECT * FROM employee
WHERE emp_name LIKE 'ja__';

3.1 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
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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 'mac%';

四、分组(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

注意: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、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:

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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;

5.1 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
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题目1
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 | jason,sean,tank,oscar,mac,rocky,成龙 |
| 老男孩驻上海虹桥最帅 | lqz |
+-----------------------------------------+---------------------------------------------------------+


题目2
mysql> select post,count(id) from employee group by post;
+-----------------------------------------+-----------+
| post | count(id) |
+-----------------------------------------+-----------+
| operation | 5 |
| sale | 5 |
| teacher | 7 |
| 老男孩驻上海虹桥最帅 | 1 |
+-----------------------------------------+-----------+


题目3
mysql> select sex,count(id) from employee group by sex;
+--------+-----------+
| sex | count(id) |
+--------+-----------+
| male | 10 |
| female | 8 |
+--------+-----------+

题目4
mysql> select post,avg(salary) from employee group by post;
+-----------------------------------------+---------------+
| post | avg(salary) |
+-----------------------------------------+---------------+
| operation | 16800.026000 |
| sale | 2600.294000 |
| teacher | 151842.901429 |
| 老男孩驻上海虹桥最帅 | 7300.330000 |
+-----------------------------------------+---------------+

题目5
mysql> select post,max(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | max(salary) |
+-----------------------------------------+-------------+
| operation | 20000.00 |
| sale | 4000.33 |
| teacher | 1000000.31 |
| 老男孩驻上海虹桥最帅 | 7300.33 |
+-----------------------------------------+-------------+

题目6
mysql> select post,min(salary) from employee group by post;
+-----------------------------------------+-------------+
| post | min(salary) |
+-----------------------------------------+-------------+
| operation | 10000.13 |
| sale | 1000.37 |
| teacher | 2100.00 |
| 老男孩驻上海虹桥最帅 | 7300.33 |
+-----------------------------------------+-------------+

题目7
mysql> select sex,avg(salary) from employee group by sex;
+--------+---------------+
| sex | avg(salary) |
+--------+---------------+
| male | 110920.077000 |
| female | 7250.183750 |
+--------+---------------+

六、过滤(having)

6.1 where和having的区别

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

6.1.1 验证

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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 | jason | 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,rocky,mac,oscar,tank,sean,jason |
+-----------+-------------------------------------------------------+
rows in set (0.00 sec)

6.2 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
  2. 查询各岗位平均薪资大于10000的岗位名、平均工资
  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
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题目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) |
+-----------------------------------------+--------------------+-----------+
| 老男孩驻上海虹桥最帅 | lqz | 1 |
+-----------------------------------------+--------------------+-----------+

题目2
mysql> select post,avg(salary) from employee group by post having avg(salary) > 10000;
+-----------+---------------+
| post | avg(salary) |
+-----------+---------------+
| operation | 16800.026000 |
| teacher | 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 |
+-----------+--------------+

七、查询排序(order by)

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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;

7.1 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
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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 | 16800.026000 |
| teacher | 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 | 151842.901429 |
| operation | 16800.026000 |
+-----------+---------------+

八、限制查询的记录数(limit)

示例:

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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,即先出第6条,然后包含这一条在内往后查5条

8.1 练习

  1. 分页显示,每页5条
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mysql> select * from  employee limit 0,5;
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| id | emp_name | sex | age | hire_date | post | post_comment | salary | office | depart_id |
+----+-----------+------+-----+------------+-----------------------------------------+--------------+------------+--------+-----------+
| 1 | lqz | male | 18 | 2017-03-01 | 老男孩驻上海虹桥最帅 | NULL | 7300.33 | 401 | 1 |
| 2 | jason | male | 78 | 2015-03-02 | teacher | | 1000000.31 | 401 | 1 |
| 3 | sean | male | 81 | 2013-03-05 | teacher | NULL | 8300.00 | 401 | 1 |
| 4 | tank | male | 73 | 2014-07-01 | teacher | NULL | 3500.00 | 401 | 1 |
| 5 | oscar | 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 | mac | female | 18 | 2011-02-11 | teacher | NULL | 9000.00 | 401 | 1 |
| 7 | rocky | 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 | emp_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

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

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 = 'lqz';
WHERE emp_name LIKE 'sea%';
WHERE emp_name REGEXP 'on$';

9.1 练习

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