Mysql database (2)

---

1. Table structure

create table student3(
	id int, --  Number 
	name varchar(20), --  full name 
	age int, --  Age 
	sex varchar(5),--  Gender 
	address varchar(100),--  Address 
	math int,--  mathematics 
	english int --  English 
);

insert into student3(id,NAME,age,sex,address,math,english) values(1,' Jack ma, ',55,' male ',' Hangzhou ',66,78),(2,' ma ',45,' Woman ',' Shenzhen ',98,87),(3,' Ma Jingtao ',55,' male ',' Hong Kong ',56,77),(4,' Liu Yan ',20,' Woman ',' hunan ',76,65),(5,' Liu Qing ',20,' male ',' hunan ',86,NULL),(6,' Lau Andy ',57,' male ',' Hong Kong ',99,99),(7,' Mard ',22,' Woman ',' Hong Kong ',99,99),(8,' De Marcia ',18,' male ',' nanjing ',56,65);

2. Query statement

--  Flexible query of all data   Partial column data 
-- select * from  Table name ;
-- select  Field name , Field name  from  Table name ;
select * from student3;
select id,name from student3;

--  Query statements can be conditional 
-- select * from student3 where  Conditions ;
--  Comparison operator  > = >= <= != <>
-- between and
-- in  stay .. Inside 
-- like  Fuzzy query 
-- and  and  or  perhaps  not

--  Query math scores greater than 80 The students of grade 
select * from student3 where math > 80;
--  Query math score less than 80 The students of grade 
select * from student3 where math < 80;
--  Query English score less than 80 The students of grade 
select * from student3 where english <= 80;
--  Query English score greater than 80 The students of grade 
select * from student3 where english >= 80;
-- between and  stay .. Section   contain 65, contain 80, Baotou and Baowei 
--  Query English scores in 65 and 80 What are the students between 
-- and Both conditions must be met 
select * from student3 where english >= 65 and english <=80;
select * from student3 where english between 65 and 80;

-- or  and   One condition is true 
select * from student3 where english >= 80 or english <= 99;

-- not  Not   Don't set up 
select * from student3 where not english >= 80;

-- in  stay .. Inside 
--  English scores in 80 branch ,90 branch  english = 80 or english = 90
select * from student3 where english(80,90,70,77);

-- between and  In the interval   A range  english>=65 and english <=80 65<=english<=80
-- in Equal to the value  english = 80 or english = 90 or english = 70 english=80 english=90
select * from student3;
-- like Fuzzy query   Match any number of characters   Find out 
-- % Match any number of characters  _ Match a character 
--  Inquire about   Information of students surnamed ma   Horse %
select * from student3 where name like ' Horse %';
--  The name contains   Horse   I can find out all the student information of the word 
select * from student3 where name like '% Horse %';
--  Horse ending 
select * from student3 where name like '% Horse ';
--  Inquire about   Information of students surnamed ma   Horse _
select * from student3 where name like ' Horse __';

select * from student3 where english is NULL;

3. Sort

-- order by  Field name  asc  The default is asc  From small to large   Descending order from big to small 
--  Sort in ascending order by age 
select * from student3 order by age;
select * from student3 order by age desc;
--  In ascending order of age   Is the descending order of math scores ok ?
select * from student3 order by age asc,math desc;

4. Aggregate functions

--  One by one   Return the result according to the value of a column 
-- max( Name ) Taking the maximum 
-- min( Name ) Minimum value 
-- avg( Name ) Average. 
-- count( Name ) Count the data in this column 
-- sum( Name ) Sum up 
--  Query the oldest students   A data 
select * from student3;
select max(age) from student3;
select min(age) from student3;
select avg(math) from student3;
-- count( Name ) Not counted as null Of 
select count(id) from student3;
select count(*) from student3;
select count(1) from student3;
select sum(math) from student3;

5. grouping

-- group by Will be used with aggregate functions 
-- group by Will not be used alone , It doesn't make sense to use it alone 
-- group by  grouping 
--  grouping   Divide into two groups of men and women   male   and   Woman   Data only 2 That's ok 
select * from student3 group by sex;
--  Count the number of men and women   Statistics   Take an alias  as  Use spaces 
select count(sex) as ' Statistics ',sex ' Gender ' from student3 group by sex;
--  The average score of male and female English 
select avg(english),sex from student3 group by sex;

--  The age of inquiry is 28 year ( contain ) More than students   Group by sex 
select count(sex) as ' Statistics ',sex from student3 where age>=28 group by sex;

--  The age of inquiry is 28 year ( contain ) More than students   Group by sex , The number of inquiry gender is greater than 2 The data of 
--  This condition can only be done after grouping 
select count(sex) as ' Statistics ',sex from student3 where age>=28 group by sex having count(sex)>2;

-- where and having It's all conditional 
-- where Is to filter data before grouping   You can't add aggregate functions 
-- having Is to filter data after grouping 

6. Pagination

-- limit Limit the number of pieces of data on a page 
-- limit Starting number   from 0 Start   How many pieces of data are displayed in total 
--  Just want to show 3 Data   Show 4,5,6 3,3
select * from student3 limit 1,3;
--  Just write a number 
select * from student3 limit 3;
select * from student3 limit 3,3;

7. Link query

--  Foreign keys foreign key  Primary key  primary key
--  The employee table   Number 、 full name 、  Age   Primary key   The number is unique and will not repeat   The foreign key corresponds to the primary key of another surface   The corresponding information can be found through foreign keys 
--  Departmental table   Number 、 name   Primary key   The number is unique and will not repeat 
create table dept( 
 id int primary key auto_increment,
 name varchar(20)
)charset=utf8;

create table emp (
 id int primary key auto_increment,
 name varchar(10),
 gender char(1), --  Gender 
 salary double, --  Wages 
 join_date date, --  Date of entry 
 dept_id int,
 foreign key (dept_id) references dept(id))charset=utf8; --  Foreign keys , Related department table ( The primary key of the Department table ) )
 
insert into dept values(1,' R & D department '),(2,' Testing department '),(3,' Operation and maintenance department '),(4,' The sales department ');
insert into emp(name,gender,salary,join_date,dept_id) values(' The Monkey King ',' male ',7200,'2013-02-24',1);
insert into emp(name,gender,salary,join_date,dept_id) values(' Pig eight quit ',' male ',3600,'2010-12-02',2);
insert into emp(name,gender,salary,join_date,dept_id) values(' Tang's monk ',' male ',9000,'2008-08-08',2);
insert into emp(name,gender,salary,join_date,dept_id) values(' Bones jing ',' Woman ',5000,'2015-10-07',3);
insert into emp(name,gender,salary,join_date,dept_id) values(' Spider essence ',' Woman ',4500,'2011-03-14',1);
select * from emp;
select * from dept;
--  The requirement is to query the employee information , Employee number , full name , Gender , Wages , Entry date and department name 
-- from What watch   With conditions 
--  Implicit query 
select emp.id,emp.name,gender,salary,join_date,dept.name from emp,dept where emp.dept_id = dept.id;

-- a surface  inner join on b surface  on Conditions 
select emp.id,emp.name,gender,salary,join_date,dept.name from emp inner join dept on emp.dept_id = dept.id;

-- a surface left join b surface  on  Conditions   Based on the left table , The data in the left table are all displayed .
select emp.id,emp.name,gender,salary,join_date,dept.name from emp left join dept on emp.dept_id = dept.id;

-- a surface right join b surface  on  Conditions 
select emp.id,emp.name,gender,salary,join_date,dept.name from emp right join dept on emp.dept_id = dept.id;