Summary of commonly used operators and functions in MySQL

Let’s build the data table first.

use test;
create table `employee`(
 emp_no int unsigned,
 emp_name varchar(30),
 emp_sex varchar(3),
 emp_age tinyint unsigned,
 sal double,
 history datetime
);
insert into employee values(1, '张三', '男', 18, 5000, '2012-04-23'),
(2, 'Li Si', 'Male', 27, 4500, '2013-05-23'),
(3, '王五', '男', 23, 4700, '2012-04-21'),
(4, 'Zilong', 'Male', 19, 3800, '2011-03-04'),
(5, 'Li Bai', 'Male', 15, 6200, '2015-09-09'),
(6, 'Liu Bei', 'Male', 28, 2500, '2016-02-11'),
(7, 'Lu Bu', 'Male', 21, 6000, '2010-10-18'),
(8, 'Shangxiang', 'female', 16, 4500, '2011-09-26'),
(9, 'Xiao Qiao', 'Female', 15, null, '2013-07-05'),
(10, '大乔', '女', 16, 5000, '2017-09-01');

Commonly used operators:
1: equal to ( = )

 select * from employee where sal = 3800;
 select * from employee where sal = null; --No null data can be found here

2: Equal to ( <=> )

 select * from employee where sal <=> 3800;
 select * from employee where sal <=> null; --Here you can query the data that is null

3: is judgment (null)

 select * from employee where sal is null;
 select * from employee where sal is not null;

4: You can also use isnull() to judge null value;

 select * from employee where isnull(sal);
 select * from employee where !isnull(sal);

5: between min and max ps: this is a closed interval

select * from employee where sal between 4500 and 5000;

6: Not in the range

select * from employee where sal not between 4500 and 5000; included

7: and and or

 select * from employee where sal not between 4500 and 5000 or sal is null;
 select * from employee where sal = 4500 and emp_sex = '女';

8: less than (<), greater than (>), less than or equal to (<=), greater than or equal to (>=)

select * from employee where sal >= 4500;


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Mathematical functions
1: rand();

 select rand() from dual; --dual is a pseudo table select 1+1 from dual;
 select rand(); --can be abbreviated

2: least(value1, value2, ...) returns the minimum value

 select least(54,76,4,65,76,87,87,56,65,654,45,23,1,76);
 select least(54,76,4,65,76,87,87,56,65,654,45,23,1,76) as min_value; --The column name can have an alias

3: greatest(value1, value2, ...) returns the greatest value

select greatest(54,76,4,65,76,87,87,56,65,654,45,23,1,76);

4: round(M, D); Returns the rounded value of M, D indicates how many decimal places to keep, the default value is 0

 select round(1.69);
 select round(1.69, 1);

5: abs() absolute value

 select 5-10;
 select abs(5-10);

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Summary functions

1: avg();

 select * from employee where sal >= 6000;
 select avg(sal) from employee where sal >= 6000;

2: count()

 select count(*) from employee;
 select count(emp_name) from employee;
 select count(sal) from employee; --print 9. Null values ​​will be ignored here. select count(*) from employee where sal >= 4000;
 select count(*) from employee where sal <= 4000 or sal is null;

3: sum()

select sum(sal) from employee where sal >= 6000;

4: min()

select min(sal) from employee;

5: max()

select max(sal) from employee;

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Date functions

1: Get the current date and time

 select now(), sysdate(), current_timestamp();
 select now(6), sysdate(6), current_timestamp(6);
 ps: now(), current_timestamp(); no difference, indicates the time when sql starts to execute sysdate() indicates the start time of this function

2: Get the current date

select curdate(); --Only year, month and day

3: Get the current time

select curtime(); --only hours, minutes and seconds

4: Date addition operation date_add

 select history, date_add(history, interval '1 12:10' day_minute) from employee; --date_add(history, interval '1 12:10' day_minute)
 select history, date_add(history, interval '1-1' year_month) from employee; --date_add(history, interval '1-1' year_month)
 select history, date_add(history, interval '1' second) from employee; --date_add(history, interval '1' second)

5: Date subtraction operation data_sub

select history, date_sub(history, interval '1-1' year_month) from employee;

6: Calculate date difference

select history, sysdate(), datediff(sysdate(), history) from employee; --以天數來表示

7: Get the specified part of the date (convert the date to the specified format) date_format()

 select history, date_format(history, '%Y year %m month %d number') from employee;
 select history, date_format(history, '%d号') from employee;
 select history, date_format(history, '%Y year %m month %d date %H hour %i minute %s second') from employee;

8: Calculate the day of the week for a date

select history, dayname(history) from employee;

9: Chinese date string conversion date str_to_date()

 insert into employee values(11, '张飞', '男', 22, 3000, '2017年02月01号'); --Error insert into employee values(11, '张飞', '男', 22, 3000, str_to_date('2017年02月01号', '%Y年%m月%d号%H时%i分钟%s秒'));

insert into employee values(12, 'Second brother', 'Male', 22, 3000, str_to_date('2017-02-01 23:02:02', '%Y year %m month %d date %H hour %i minute %s second'));
insert into employee values(12, 'Second brother', 'Male', 22, 3000, str_to_date('2017-02-01 11:02:02', '%Y year %m month %d date %h hour %i minute %s second'));
ps: If it is h, it means 12-hour system, if it is big H, it means 24-hour system;

String functions

1: left(str, len) returns len characters from the left end of the string str

select left('abcdefg', 5);

2: length()

select length('abcdefg');

3: lower(str) returns the lowercase string str

select lower('HELLO');

4: substring() takes a substring, the second parameter is the starting position of the interception, and the third parameter is the length to be intercepted

select substring('helloworld',2,3);

5: concat() string concatenation

select concat(emp_name, '員工') from employee;

6: replace

select replace(emp_name, '李', '老') from employee where emp_name = '李四';