Day 47 how to query a table

Summary of today's content

• How to query a table

  """
  select
  where 
  group by
  having
  distinct
  order by
  limit 
  regexp
  like
  ...
  """
  

• Theory of connected table operation

Today's content is detailed

Preparation of the preliminary schedule

create table emp(
  id int not null unique auto_increment,
  name varchar(20) not null,
  sex enum('male','female') not null default 'male', # Most of them are men 
  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, # One department, one room 
  depart_id int
);

# insert record 
# Three departments ： teaching , sales , operating 
insert into emp(name,sex,age,hire_date,post,salary,office,depart_id) values
('jason','male',18,'20170301',' The first handsome image of Zhangjiang ',7300.33,401,1), # Here is the teaching department 
('tom','male',78,'20150302','teacher',1000000.31,401,1),
('kevin','male',81,'20130305','teacher',8300,401,1),
('tony','male',73,'20140701','teacher',3500,401,1),
('owen','male',28,'20121101','teacher',2100,401,1),
('jack','female',18,'20110211','teacher',9000,401,1),
('jenny','male',18,'19000301','teacher',30000,401,1),
('sank','male',48,'20101111','teacher',10000,401,1),
(' ha-ha ','female',48,'20150311','sale',3000.13,402,2),# Here is the sales department 
(' ha-ha ','female',38,'20101101','sale',2000.35,402,2),
(' West ','female',18,'20110312','sale',1000.37,402,2),
(' Lele ','female',18,'20160513','sale',3000.29,402,2),
(' Lala ','female',28,'20170127','sale',4000.33,402,2),
(' Seng long ','male',28,'20160311','operation',10000.13,403,3), # Here's the operations department 
(' Cheng Yaojin ','male',18,'19970312','operation',20000,403,3),
(' Cheng Yaoyin ','female',18,'20130311','operation',19000,403,3),
(' Cheng Yaotong ','male',18,'20150411','operation',18000,403,3),
(' Cheng yaotie ','female',18,'20140512','operation',17000,403,3);


#  When there are many table fields   The display was disorderly   have access to \G Branch display 
select * from emp\G;

#  Individual students' computers will still appear garbled or blank when inserting Chinese   You can set the character encoding to GBK

The execution order of several important keywords

#  Writing order 
select id,name from emp where id > 3;
#  Execution order 
from
where
select


 Be careful ：
"""
 Although the execution order is inconsistent with the writing order   You're writing sql You may not know how to write a statement 
 You just write in writing order sql
	select *  First use * No. occupation 
	 Then go and complete the back sql sentence 
	 The final will be * The specific field you want after number replacement 
	

"""

where filter

#  effect : It is a filtering operation for the overall data 
# 1. Inquire about id Greater than or equal to 3 Less than or equal to 6 The data of 
select id,name,age from emp where id>=3 and id<=6;
select id,name from emp where id between 3 and 6;   The two are equivalent 

# 2. Query salary is 20000 perhaps 18000 perhaps 17000 The data of 
select * from emp where salary=20000 or salary=18000 or salary=17000;
select * from emp where salary in (20000,18000,17000);

# 3. The query employee name contains letters o The name and salary of the employee 
"""
 Fuzzy query 
	like
		%   Match any number of characters 
		_   Match any single character 
"""
select name,salary from emp where name like '%o%';

# 4. The query employee name is composed of four characters   Name and salary   char_length()   _
select name,salary from emp where name like '____';
select name,salary from emp where char_length(name) = 4;

# 5. Inquire about id Less than 3 perhaps id Greater than 6 The data of 
select * from emp where id not between 3 and 6;

# 6. Query salary is not in 20000,18000,17000 Range data 
select * from emp where salary not in (20000,18000,17000);

# 7. Query the employee name and position name with empty position description    in the light of null No equal sign   use is
select name,post from emp where post_comment = NULL;
select name,post from emp where post_comment is NULL;

group by grouping

#  Group actual application scenarios    There are many grouping application scenarios 
	 The male to female ratio 
     Average departmental salary 
     Department baldness rate 
     Statistics between countries 

# 1	 Group by Department 
select * from emp group by post;
"""
 After grouping   The smallest operational unit should be group   It's no longer a single data in a group 
	 The above commands can be executed normally when you do not set strict mode   What is returned is after grouping   The first data in each group   But this does not meet the grouping specification : Individual data should not be considered after grouping   Instead, it should take the group as the operation unit ( After grouping   There is no way to directly obtain individual data in the group )
	 If strict mode is set   Then the above command will directly report an error  
"""
set global sql_mode = 'strict_trans_tables,only_full_group_by';

 After setting strict mode    grouping   By default, you can only get the grouping basis 
select post from emp group by post;  
 According to what group, you can only get the group   Other fields cannot be obtained directly   Some methods are needed ( Aggregate functions )


"""
 When do you need to group ？？？
	 keyword  
		 Every   Average   The highest   The minimum  
	
	 Aggregate functions 
		max
		min
		sum
		count
		avg
"""
# 1. Get the highest salary for each department 
select post,max(salary) from emp group by post;
select post as ' department ',max(salary) as ' The highest salary ' from emp group by post;
select post ' department ',max(salary) ' The highest salary ' from emp group by post;
# as You can alias fields   You can also omit it directly   But not recommended   Because the meaning of omitted words is not clear   It's easy to get confused 

# 2. Get the minimum wage for each department 
select post,min(salary) from emp group by post;
# 3. Get the average salary for each department 
select post,avg(salary) from emp group by post;
# 4. Get the total salary of each department 
select post,sum(salary) from emp group by post;
# 5. Get the number of people in each department （count  You can count any field in the following table   except null Outside ）
select post,count(id) from emp group by post;  #  Commonly used   It's logical 
select post,count(salary) from emp group by post;
select post,count(age) from emp group by post;
select post,count(post_comment) from emp group by post;  null  no way 

# 6. Query the name of the Department after grouping and the names of all employees in each department  
# group_concat It can not only support you to obtain other field values after grouping   It also supports splicing operation 
select post,group_concat(name) from emp group by post;
select post,group_concat(name,'_DSB') from emp group by post;
select post,group_concat(name,':',salary) from emp group by post;
# concat When not grouped, use  
select concat('NAME:',name),concat('SAL:',salary) from emp;

#  Add  as Syntax doesn't just alias fields   You can also temporarily alias the table 
select emp.id,emp.name from emp;  
select emp.id,emp.name from emp as t1;    Report errors 
select t1.id,t1.name from emp as t1;

#  Check everyone's annual salary   12 pay 
select name,salary*12 from emp;

Group notes

#  keyword where and group by At the same time group by Must be in where Behind 
where Filter the overall data before grouping         
where Filter criteria cannot use aggregate functions 
select id,name,age from emp where max(salary) > 3000;     Report errors     

select max(salary) from emp;  #  Not in groups   Default overall （emp） It's a group 

#  The age of each department is in 30 Average salary of employees over years old 
	1  Let's ask all people older than 30 Year old employees 
    	select * from emp where age>30;
    2  Then group the results 
     	select * from emp where age>30 group by post;
    
    select post,avg(salary) from emp where age>30 group by post;

having Filter criteria after grouping

"""
having The grammatical root of where It's consistent 
 It's just having It is the filtering operation after grouping 
 namely having Aggregate functions can be used directly 
"""
#  The age of each department is in 30 The average salary of employees over years old and keep the average salary greater than 10000 The department in charge of the 
select post,avg(salary) from emp 
		where age>30 
    	group by post
        having avg(salary) > 10000
        ;

distinct duplicate removal

"""
 Be sure to pay attention to   The data must be exactly the same before it can be de duplicated ！！！
 Be sure not to neglect   There are cases of gradual existence   It's impossible to go heavy 
[
{'id':1,'name':'jason','age':18},
{'id':2,'name':'jason','age':18},
{'id':3,'name':'egon','age':18}
]
ORM   Object relation mapping     Don't understand SQL Statement can also be very powerful to operate the database 
 surface 								 class 
 Pieces of data 						 object 
 The value corresponding to the field 						 Object properties 

 You write class again   That means creating tables 
 Generate objects with classes   That means re creating data 
 Object point properties   Is to get the value corresponding to the data field 
 The purpose is to reduce python The pressure of programmers   Just need to be able to python Object oriented knowledge points can be operated MySQL
"""
select distinct id,age from emp;
select distinct age from emp;

order by Sort

select * from emp order by salary;
select * from emp order by salary asc;
select * from emp order by salary desc;
"""
order by The default is ascending   asc  The asc You can omit it 
 You can also change it to descending      desc
"""
select * from emp order by age desc,salary asc;
#  First according to age Descending order    If you come across age identical   Then follow salary Ascending row 

#  The age of each department is in 10 The average salary of employees over years old and keep the average salary greater than 1000 The department in charge of the , And then sort the average wage in descending order 
	select post,avg(salary) from emp 
		where age>10 
    	group by post
        having avg(salary) > 1000
        order by avg(salary) desc
        ;

limit Limit the number of displays

select * from emp;
""" In case of too much data   We usually do paging """
select * from emp limit 3;  #  Show only three pieces of data 

select * from emp limit 0,5;    #  From the first to the fifth 
select * from emp limit 5,5;    #  Start with the sixth and take five more 
 The first parameter is the starting position 
 The second parameter is the number of display entries 

Regular （ Understanding can ）

select * from emp where name regexp '^j.*(n|y)$';

^j :  Express j start 
（n|y)$ :  Express  n or y  ending 
 In the middle 

Multi meter operation

Preparation of the preliminary schedule

# Build table 
create table dep(
id int,
name varchar(20) 
);

create table emp(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') not null default 'male',
age int,
dep_id int
);

# insert data 
insert into dep values
(200,' technology '),
(201,' human resources '),
(202,' sales '),
(203,' operating ');

insert into emp(name,sex,age,dep_id) values
('jason','male',18,200),
('egon','female',48,201),
('kevin','male',18,201),
('nick','male',28,202),
('owen','male',18,203),
('jerry','female',18,204);

Table query

select * from dep,emp;  #  result     The cartesian product 
"""
 Understanding can   It doesn't matter if I don't know 
"""

select * from emp,dep where emp.dep_id = dep.id;

"""
MySQL I know   When you query the data later   I'm sure you'll often use the spelling operation  
 So I specially set up corresponding methods for you 
	inner join   Internal connection 
	left join    Left connection 
	right join   The right connection 
	union		 Full connection 
"""
# inner join   Internal connection      （  Commonly used  ）
select * from emp inner join dep on emp.dep_id = dep.id;
#  Splice only the public data parts of the two tables 

# left join    Left connection 
select * from emp left join dep on emp.dep_id = dep.id;
#  All the data in the left table are displayed   If there is no corresponding item, use NULL

# right join   The right connection 
select * from emp right join dep on emp.dep_id = dep.id;
#  All the data in the right table are displayed   If there is no corresponding item, use NULL

# union		 Full connection    All the data in the left and right tables are displayed 
select * from emp left join dep on emp.dep_id = dep.id
union
select * from emp right join dep on emp.dep_id = dep.id;

Subquery

"""
 Subquery is the way we usually solve problems 
	 Solve the problem step by step 
		 First step 
		 The second step 
		...
 Use the result of one query statement as the condition of another query statement 
"""
#  Query the employee information of technical or human resources department 
	1  First get the Department's id Number 
    2  Then go to the employee table to select the corresponding employees 
    select id from dep where name=' technology ' or name = ' human resources ';
    
    select name from emp where dep_id in (200,201);
    
    
    select * from emp where dep_id in (select id from dep where name=' technology ' or name = ' human resources ');

summary

 Table query results can be used as query criteria for other tables 
 You can also alias it as a virtual table root and associate it with other tables 

"""
 There are two ways to query multiple tables 
	1  Splice the table first and then query 
	2  Subquery   Step by step 
"""