MySQL functions and related cases and exercises

1.MySQL The main function of

1.1 Aggregate functions

group_concat() Use of functions

CREATE DATABASE bianbian4;
USE bianbian4;
CREATE TABLE emp (
	emp_id INT PRIMARY KEY auto_increment COMMENT ' Number ',
	emp_name VARCHAR ( 20 ) NOT NULL DEFAULT '' COMMENT ' full name ',
	salary DECIMAL ( 10, 2 ) NOT NULL DEFAULT 0 COMMENT ' Wages ',
	department VARCHAR ( 20 ) NOT NULL DEFAULT '' COMMENT ' department ' 
);
INSERT INTO emp ( emp_name, salary, department )
VALUES
	( ' Zhang Jingjing ', 5000, ' Finance Department ' ),
	( ' Wang Feifei ', 5800, ' Finance Department ' ),
	( ' Zhao Gang ', 6200, ' Finance Department ' ),
	( ' Liu Xiaobei ', 5700, ' The personnel department ' ),
	( ' Wang Dapeng ', 6700, ' The personnel department ' ),
	( ' Zhang Xiaopei ', 5200, ' The personnel department ' ),
	( ' Liu yunyun ', 7500, ' The sales department ' ),
	( ' Liu Yunpeng ', 7200, ' The sales department ' ),
	( ' Liu Yunpeng ', 7800, ' The sales department ' );
	
--  Merge the names of all employees into one line 
SELECT GROUP_CONCAT(emp_name) FROM emp;
--  Specify separator merge 
SELECT GROUP_CONCAT(emp_name SEPARATOR ';') FROM emp;
--  Specify sorting method and separator 
SELECT department,GROUP_CONCAT(emp_name ORDER BY salary desc SEPARATOR ';') FROM emp GROUP BY department;

1.2 Mathematical functions

--  Mathematical functions 
SELECT abs(-10);
SELECT abs(10);
SELECT abs( Expression or field ） FROM  surface ;
-- ceil  Upward evidence collection ;
select ceil(1.1); -- 2
select ceil(1.0); -- 1
--  Rounding down 
select floor(1.1); -- 1
select floor(1.9); -- 1
--  Taking the maximum 
select greatest(1,2); -- 2
--  minimum value 
select least(1,2,3); -- 1

1.3 String function

1.4 Date function

1.5 Control flow function

1.5.1 if Logical judgment statement

When judging the status , have access to IF Function to judge ：

1.5.2 case when function

case when amount to C Linguistic switch() sentence

1.6 Window function

Window function classification ：

Window function syntax structure ：

1.6.1 Ordinal function

Ordinal function ：row_number()、rank()、dense_rank(), Can be used to achieve grouping sorting , And add serial number .

use bianbian4;
create table employee(
dname varchar(20),
eid varchar(20),
ename varchar(20),
hiredate date,
salary double
);
--  The data class contains 
insert into 
employee 
values
(' R & D department ','1001',' Liu bei ','2021-11-01',3000),
(' R & D department ','1002',' Guan yu ','2021-11-02',5000),
(' R & D department ','1003',' Zhang Fei ','2021-11-03',7000),
(' R & D department ','1004',' zhaoyun ','2021-11-04',7000),
(' R & D department ','1005',' d ','2021-11-05',4000),
(' R & D department ','1006',' Huang Zhong ','2021-11-06',4000),

(' The sales department ','1001',' Cao Cao ','2021-11-01',2000),
(' The sales department ','1002',' Xu Chu ','2021-11-02',3000),
(' The sales department ','1003',' Dianwei ','2021-11-03',5000),
(' The sales department ','1004',' Zhang liao ','2021-11-04',6000),
(' The sales department ','1005',' Xu Huang ','2021-11-05',9000),
(' The sales department ','1006',' Cao Hong ','2021-11-06',6000);
--  Sort the employees of each department according to their salary , And give the ranking 
select dname,ename,salary, 
row_number() over (partition by dname order by salary desc) as row_rank,
rank() over (partition by dname order by salary desc) as rk,
dense_rank() over (partition by dname order by salary desc) as dense_rk
from employee;

--  Find out the top three employees in each department ,- Grouping calculation topN
select * 
from
(
select dname,ename,salary, 
dense_rank() over (partition by dname order by salary desc) as dense_rk
from employee
) as t
where t.dense_rk<=3;
--  Sort all employees globally 
select dname,ename,salary, 
dense_rank() over (order by salary desc) as dense_rk
from employee

row_number()： Don't consider the differences , The numbers in the order do not change ,1,2,3
rank()： Consider the same situation , Skip number
dense_rank()： Consider the same situation , Don't jump numbers
Here's the picture ：

1.6.2 Windowed aggregate function

--  Accumulate according to the grouping date 
select 
 dname,
 ename,
 hiredate,
 salary, 
 sum(salary) over (partition by dname order by hiredate ) as c1
from employee

--  without order by  Add all the data in the Group 
select 
 dname,
 ename,
 hiredate,
 salary, 
 sum(salary) over (partition by dname  ) as c1
from employee

Realize interval statistics , With sum As an example , The data is the data created above ：

--  Specify the range to add 
--  Start line ： unbounded preceding  To the current line  current row
select 
 dname,
 ename,
 hiredate,
 salary, 
 sum(salary) over (partition by dname order by hiredate rows between unbounded preceding and current row) as c1
from employee

--  The top three lines of the current line 
select 
 dname,
 ename,
 hiredate,
 salary, 
 sum(salary) over (partition by dname order by hiredate rows between 3 preceding and current row) as c1
from employee

--  Three lines up + Current row + Next line 
select 
 dname,
 ename,
 hiredate,
 salary, 
 sum(salary) over (partition by dname order by hiredate rows between 3 preceding and 1 following) as c1
from employee

--  Current line to the end 
select 
 dname,
 ename,
 hiredate,
 salary, 
 sum(salary) over (partition by dname order by hiredate rows between current row and unbounded following) as c1
from employee

1.6.3 Distribution function

cume_dist() Introduction to ：
purpose ： Less than... In the group 、 Equal to current rank The number of rows for the value / The total number of rows in the group
Application scenarios ： Query the comparison column less than or equal to the current salary

cume_dist() Example ：

use bianbian4;
select
 dname,
 ename,
 salary,
 cume_dist() over(order by salary) as rn1,
 cume_dist() over(partition by dname order by salary) as rn2
from employee;
/*  No addition partition by  Is the total number of lines ： rn1  It indicates the proportion of the whole salary less than or equal to the current number  3/12 = 0.25 (3000) 5/12 = 0.4166666666666667 (4000)  Yes partition by, Look at this group ： rn2 Meaning case ： 1 / 6 = 0.16666666666666666 (3000, In the R & D group ） */

cume_dist() Result ：

percent_rank Introduction to ：

percent_rank Code ：

select
 dname,
 ename,
 salary,
 rank() over(partition by dname order by salary desc) as rn1,
 percent_rank() over(partition by dname order by salary desc) as rn2
from employee;
/* rn2:  first line ：（1-1）/ (6-1) = 0  The second line ：（1-1）/ (6-1) = 0  The third line ：（3-1）/ (6-1) = 0.4 */

percent_rank() result ：

1.6.4 Before and after function LAG and LEAD

select
 dname,
 ename,
 salary,
 hiredate,
 --  Put the... In the previous line , Put it behind the current , If there is no default '2000-01-01'
 lag(hiredate,1,'2000-01-01') over(partition by dname order by hiredate ) as rn1,
 --  Put the values of the last two lines , Put it behind the current , If there is no default null
 lag(hiredate,2) over(partition by dname order by hiredate ) as rn2
from employee;

1.6.5 Head tail function first_value and last_value

first_value() and last_value() Case study ：

select
 dname,
 ename,
 salary,
 hiredate,
 --  The first one so far 
 first_value(salary) over(partition by dname order by hiredate ) as first_,
 --  The last one so far 
 last_value(salary) over(partition by dname order by hiredate ) as last_
from employee;

1.6.6 Other functions nth_value(expr,n)、ntile(n)

nth_value(expr,n) Case study ：

select
 dname,
 ename,
 salary,
 hiredate,
 --  No. so far 1 Salary of the line 
 nth_value(salary,1) over(partition by dname order by hiredate ) as first_salary,
 --  No. so far 2 Salary of the line 
 nth_value(salary,2) over(partition by dname order by hiredate ) as second_salary
from employee;

ntile Case study ：

select
 dname,
 ename,
 salary,
 hiredate,
 --  Divide into 3 Group 
 ntile(3) over(partition by dname order by hiredate ) as nt
from employee;
--  Take out the first group of employees in each department 
select * 
from
(
select
 dname,
 ename,
 salary,
 hiredate,
 --  Divide into 3 Group 
 ntile(3) over(partition by dname order by hiredate ) as nt
from employee
) t
where t.nt =1;

Reference resources

https://www.bilibili.com/video/BV1iF411z7Pu?p=99&spm_id_from=pageDriver