10-[记录操作]--数据的查：单表查询，约束

1、 单表查询的语法

select distinct 字段1,字段2,字段3 from 库.表 
    where 条件
    group by 分组条件
    having 过滤
    order by 排序字段
    limit n;      # 限制条数

2、关键字的执行优先级(重点)

# 重点中的重点：关键字的执行优先级
  
  from                # 1.找到表:from
  where               # 2.拿着where指定的约束条件，去文件/表中取出一条条记录
  group by            # 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
  having              # 4.将分组的结果进行having过滤
  select              # 5.执行select
  distinct            # 6.去重
  order by            # 7.将结果按条件排序：order by
  limit               # 8.限制结果的显示条数

详细见：http://www.cnblogs.com/linhaifeng/articles/7372774.html

 　　

3、创建表，插入数据

#创建表
create table employee(
id int not null unique 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, #一个部门一个屋子
depart_id int
);

#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),
('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

 

4、查询

#简单查询
    SELECT id,name,sex,age,hire_date,post,post_comment,salary,office,depart_id FROM employee;

    SELECT * FROM employee;       #效率低下

    SELECT name,salary FROM employee;

#避免重复DISTINCT
    SELECT DISTINCT post FROM employee;               

#通过四则运算查询             别名
    SELECT name, salary*12 FROM employee;
    SELECT name, salary*12 AS Annual_salary FROM employee;
    SELECT 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: ',name,'  年薪: ', salary*12)  AS Annual_salary FROM employee;

   CONCAT_WS() # 第一个参数为分隔符
   SELECT CONCAT_WS(':',name,salary*12)  AS Annual_salary FROM employee;

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

5、WHERE约束

#1:单条件查询
select id,name,age from employee where id > 7;

#2:多条件查询
select name,post,salary from employee where post='teacher' and salary > 8000;

#3:关键字BETWEEN AND
select name,salary from employee where salary >= 20000 and salary <= 30000;
select name,salary from employee where salary between 20000 and 30000;

select name,salary from employee where salary < 20000 or salary > 30000;
select name,salary from employee where salary not between 20000 and 30000;

#5:关键字IN集合查询
select * from employee where age = 73 or age = 81 or age = 28;
select * from employee where age in (73,81,28);

#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 * from employee where post_comment is Null;    
select * from employee where post_comment is not Null;

#6:关键字LIKE模糊查询
select * from employee where name like "jin%";            #  通配符’%’
select * from employee where name like "jin___";        #     通配符’_’

 

 

 

　　

　　

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

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%';

 6、分组group by

　　（1）为什么要分组

 　　（2）设置严格分组格式

#查看MySQL 5.7默认的sql_mode如下：
mysql> select @@global.sql_mode;
ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION

#！！！注意
ONLY_FULL_GROUP_BY的语义就是确定select target list中的所有列的值都是明确语义，简单的说来，在ONLY_FULL_GROUP_BY模式下，target list中的值要么是来自于聚集函数的结果，要么是来自于group by list中的表达式的值。


#设置sql_mole如下操作(我们可以去掉ONLY_FULL_GROUP_BY模式)：
mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

！！！SQL_MODE设置！！！

#由于没有设置ONLY_FULL_GROUP_BY,于是也可以有结果，默认都是组内的第一条记录，但其实这是没有意义的

mysql> set global sql_mode='ONLY_FULL_GROUP_BY';
Query OK, 0 rows affected (0.00 sec)

#设置成功后，一定要退出，然后重新登录方可生效
mysql> select * from emp group by post; #报错
ERROR 1055 (42000): 'db1.emp.id' isn't in GROUP BY
mysql> select post,count(id) from emp group by post; #只能查看分组依据和使用聚合函数

 

　　（3）GROUP_CONCAT()函数

GROUP BY关键字和GROUP_CONCAT()函数一起使用
    SELECT post,GROUP_CONCAT(name) FROM employee GROUP BY post;#按照岗位分组，并查看组内成员名
    SELECT post,GROUP_CONCAT(name) as emp_members FROM employee GROUP BY post;

 　　

7、聚合函数 max、min、avg、sum、count

　　

# 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
 # 分组之后，只能取分组的字段，以及每个组聚合结果

　　

#没有group by则默认整体算作一组
select max(salary) from employee;

 　　　　

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

#题1：分组
mysql> select post,group_concat(name) from employee group by post;
+-----------------------------------------+---------------------------------------------------------+
| post                                    | group_concat(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                               |         5 |
| sale                                    |         5 |
| teacher                                 |         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   |        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                               |    20000.00 |
| sale                                    |     4000.33 |
| teacher                                 |  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                               |    10000.13 |
| sale                                    |     1000.37 |
| teacher                                 |     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   | 110920.077000 |
| female |   7250.183750 |
+--------+---------------+

8、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

 　　　　

#having
select post,group_concat(name),count(id) from employee group by post;

select post,group_concat(name),count(id) from employee group by post having count(id) < 2;

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

 

# 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(name),count(id) from employee group by post having count(id) < 2;
+-----------------------------------------+--------------------+-----------+
| post                                    | group_concat(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 |  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 |
+-----------+--------------+

• • 　　执行顺序 having ----->select ---->order

# having 执行完才执行 select 然后执行 order
select distinct post,count(id) as emp_count from employee
where salary > 1000
group by post
having emp_count > 1    # select中有这个别名
order by emp_count
;
select distinct post,count(id) as emp_count from employee
where salary > 1000
group by post
having count(id) > 1
order by emp_count
;

 

 

9、查询排序:ORDER BY

按单列排序
　　select * from employee order by age asc; 　　#升序
　　select * from employee order by age desc;　　 #降序


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 asc,id desc;　　 #先按照age升序排，如果age相同则按照id降序排

   

 　　　　

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 |  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 |
+-----------+---------------+

10、限制查询的记录数:LIMIT

示例：
select * from employee limit 3;     #默认初始位置为0 
select * from employee order by salary desc limit 1;

  

# 分页   
select * from employee 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 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
select * from employee limit 10,5;
select * from employee limit 15,5;

　　    

　　    

11、正则表达式

select * from employee where name like 'jin%';
select * from employee where name regexp '^jin';
select * from employee where name regexp '^jin.*(g|n)$';

 

12、单表查询语法顺序与执行顺序

　　

　

# 执行顺序：

def from(db,table):
        f=open(r'%s\%s' %(db,table))
        return f
    
def where(condition,f):
    for line in f:
        if condition:
            yield line

def group(lines):
    pass
    
def having(group_res):
    pass

def distinct(having_res):
    pass

def order(distinct_res):
    pass
    
def limit(order_res)
    pass
    
def select():
    f=from('db1','t1')
    lines=where('id>3',f)
    group_res=group(lines)
    having_res=having(group_res)
    distinct_res=distinct(having_res)
    order_res=order(distinct_res)
    res=limit(order_res)
    print(res)
    return res