MySQL单表查询
MySQL单表查询
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;
三、约束条件
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
- 通配符可以是%或_，
- 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__';
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;
六、过滤(having)
6.1where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)
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; # 降序
SELECT * FROM employee ORDER BY salary DESC; # 升序
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;
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
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$';
SELECT * FROM employee WHERE emp_name REGEXP 'm{2}';
小结：对字符串匹配的方式
WHERE emp_name = 'nick';
WHERE emp_name LIKE 'sea%';
WHERE emp_name REGEXP 'on$';
