单表查询

分类: IT文章 • 2025-01-11 11:14:01

语法：

一、单表查询的语法
    SELECT 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


二、关键字的执行优先级 *****
from
where
group by
having
select
distinct
order by
limit

1.找到表：from
2.拿着where指定的约束条件，去文件或者表中取出记录
3.将取出的记录进行分组 group by，如果没有group by，则整体作为一组
4.将分组的结果进行having过滤
5.执行select
6.去重
7.将结果按条件排序：order by
8.限制结果的显示条数

例子：

创建公司的员工表，表的字段和数据类型如下：

单表查询

#创建表,设置字段的约束条件
create table employee(
    id int primary key 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
    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    |                |
| salart               | double(15,2)                  | YES  |         | NULL    |                |
| office              | int(11)                           | YES  |         | NULL    |                |
| depart_id        | int(11)                           | YES  |         | NULL    |                |
+--------------+-----------------------+------+-----+---------+----------------+
rows in set (0.08 sec)

#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','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),
('xiaomage','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)
;

创建员工表，并插入记录

View Code

(1) where 约束

where句子中可以使用：

1、比较运算符：> < >= <= <> !=

2、between 80 and 100 ：值在80到100之间

3、in(80,90,100) 值在80 90 100里面其中一个

4、like 'abc' : pattern 可以是% 或者_ % 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

#1 :单条件查询
mysql> select id,emp_name from employee where id > 5;
+----+------------+
| id | emp_name   |
+----+------------+
|  6 | jingliyang |
|  7 | jinxin     |
|  8 | xiaomage   |
|  9 | 歪歪       |
| 10 | 丫丫       |
| 11 | 丁丁       |
| 12 | 星星       |
| 13 | 格格       |
| 14 | 张野       |
| 15 | 程咬金     |
| 16 | 程咬银     |
| 17 | 程咬铜     |
| 18 | 程咬铁     |

#2 多条件查询
mysql> select emp_name from employee where post='teacher' and salary>10000;
+----------+
| emp_name |
+----------+
| alex         |
| jinxin     |
+----------+

#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;

#注意''是空字符串，不是null
 SELECT name,post_comment FROM employee WHERE post_comment='';
 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
mysql>  SELECT name,salary FROM employee WHERE salary=3000 OR salary=3500 OR salary=4000 OR salary=9000 ;
+------------+---------+
| name       | salary  |
+------------+---------+
| yuanhao    | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
rows in set (0.00 sec)

mysql>  SELECT name,salary FROM employee  WHERE salary IN (3000,3500,4000,9000) ;
+------------+---------+
| name       | salary  |
+------------+---------+
| yuanhao    | 3500.00 |
| jingliyang | 9000.00 |
+------------+---------+
mysql>  SELECT name,salary FROM employee  WHERE salary NOT IN (3000,3500,4000,9000) ;
+-----------+------------+
| name      | salary     |
+-----------+------------+
| egon      |    7300.33 |
| alex      | 1000000.31 |
| wupeiqi   |    8300.00 |
| liwenzhou |    2100.00 |
| jinxin    |   30000.00 |
| xiaomage  |   10000.00 |
| 歪歪      |    3000.13 |
| 丫丫      |    2000.35 |
| 丁丁      |    1000.37 |
| 星星      |    3000.29 |
| 格格      |    4000.33 |
| 张野      |   10000.13 |
| 程咬金    |   20000.00 |
| 程咬银    |   19000.00 |
| 程咬铜    |   18000.00 |
| 程咬铁    |   17000.00 |
+-----------+------------+
rows in set (0.00 sec)

#6:关键字LIKE模糊查询
通配符’%’
mysql> SELECT * FROM employee WHERE name LIKE 'jin%';
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
| id | name       | sex    | age | hire_date  | post    | post_comment | salary   | office | depart_id |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
|  6 | jingliyang | female |  18 | 2011-02-11 | teacher | NULL         |  9000.00 |    401 |         1 |
|  7 | jinxin     | male   |  18 | 1900-03-01 | teacher | NULL         | 30000.00 |    401 |         1 |
+----+------------+--------+-----+------------+---------+--------------+----------+--------+-----------+
rows in set (0.00 sec)


通配符'_'

mysql> SELECT  age FROM employee WHERE name LIKE 'ale_';
+-----+
| age |
+-----+
|  78 |
+-----+
row in set (0.00 sec)

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

#对应的sql语句
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%';

where约束

练习

（2）group by 分组

分组是发生在where之后，即分组是基于where之后得到的记录而进行的。

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。

注意：

可以按照任意字段分组，但是分组完毕后，如果想查看组内信息，需要借助于聚合函数。

如果想分组，则必须要设置全局的sql的模式为ONLY_FULL_GROUP_BY
mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

#查看MySQL 5.7默认的sql_mode如下：
mysql> select @@global.sql_mode;
+--------------------+
| @@global.sql_mode  |
+--------------------+
| ONLY_FULL_GROUP_BY |
+--------------------+
1 row in set (0.00 sec)

mysql> exit;#设置成功后，一定要退出，然后重新登录方可生效
Bye

继续验证通过group by分组之后，只能查看当前字段,如果想查看组内信息，需要借助于聚合函数

单表查询

（3）聚合函数

max() 最大值
min() 最小值
avg() 平均值
sum() 求和
count() 总个数

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

单表查询

1. 查询岗位名以及岗位包含的所有员工名字 group_concat()
    select post,group_concat(name) from employee group by post;

2. 查询岗位名以及各岗位内包含的员工个数
    select post,count(id) from employee group by post;

3. 查询公司内男员工和女员工的个数
    select sex,count(sex) from employee group by sex;

4. 查询岗位名以及各岗位的平均薪资
    select post,avg(salary) from employee group by post;

5. 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 sex,avg(salary) from employee group by sex;

（4）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

having是从筛选的字段中再筛选，而where是从数据表中的字段直接进行的筛选。

select id,salary from employee having salary>1000; # right

select id,name from employee having salary>1000; # wrong 前面字段中不包括salary

select id,name from employee where salary > 10000; # right

　　3、where后面跟的是数据表中的字段，而 having后面可以跟别名和聚合函数

select post,avg(salary) from employee where salary > 10000 group by post; # right

select post,avg(salary) as ms from employee where ms>10000 group by post;#wrong where 后面不能跟别名

select post,avg(salary) as ms from employee group by post having ms > 10000;#right

select post,avg(salary) as ms from employee group by post having count(id) > 10;#right

注意：必须使用group by才能使用group_concat()函数。

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

2. 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;

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

练习

单表查询

(1) where 约束

（2）group by 分组

（3）聚合函数

（5）order by查询排序

(5) limit 限制查询的记录数

单表查询

(1) where 约束

（2）group by 分组

（3）聚合函数

（5）order by查询排序

(5) limit 限制查询的记录数

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