MySQL单表查询
目录
MySQL完整教程目录：https://www.cnblogs.com/nickchen121/p/14709373.html
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
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
- from：找到表
- where：拿着where指定的约束条件，去文件/表中取出一条条记录
- 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
- select：执行select
- distinct：去重
- having：将分组的结果进行having过滤
- order by：将结果按条件排序：order by
- limit：限制结果的显示条数
二、简单查询
2.1 建表和数据准备
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
('nick','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
- 查出所有员工的名字，薪资，格式为
<名字:nick> <薪资:3000> - 查出所有的岗位（去掉重复）
- 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;
三、约束条件（where）
where子句中可以使用：
- 比较运算符：> < >= <= <> !=
- between 80 and 100 值在80到100之间
- in(80,90,100) 值是80或90或100
- like 'n%'
通配符可以是%或_，
%表示任意多字符
_表示一个字符
8. 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 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
- 查看岗位是teacher的员工姓名、年龄
- 查看岗位是teacher且年龄大于30岁的员工姓名、年龄
- 查看岗位是teacher且薪资在9000-1000范围内的员工姓名、年龄、薪资
- 查看岗位描述不为NULL的员工信息
- 查看岗位是teacher且薪资是10000或9000或30000的员工姓名、年龄、薪资
- 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
- 查看岗位是teacher且名字是jin开头的员工姓名、年薪
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）
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：
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
- 查询岗位名以及岗位包含的所有员工名字
- 查询岗位名以及各岗位内包含的员工个数
- 查询公司内男员工和女员工的个数
- 查询岗位名以及各岗位的平均薪资
- 查询岗位名以及各岗位的最高薪资
- 查询岗位名以及各岗位的最低薪资
- 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
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 | nick |
+-----------------------------------------+---------------------------------------------------------+
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
- 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。
- 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
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
- 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
- 查询各岗位平均薪资大于10000的岗位名、平均工资
- 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 | nick | 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）
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
- 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
- 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
- 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 | 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：
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
- 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 | nick | 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
SELECT * FROM employee WHERE emp_name REGEXP '^jas';
SELECT * FROM employee WHERE emp_name REGEXP 'on$';
SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';
小结：对字符串匹配的方式
WHERE emp_name = 'nick';
WHERE emp_name LIKE 'sea%';
WHERE emp_name REGEXP 'on$';
9.1 练习
- 查看所有员工中名字是mac开头，n或者g结果的员工信息
select * from employee where emp_name regexp '^mac.*[gn]$';
