String function

function	Features
CONCAT(S1,S2,…Sn)	Level S1, S2,...Sn into a string
INSERT(str,x,y,instr)	Replace the string str, starting at position x, a substring of y characters long with instr
LOWER(str)	Convert all characters of string str to lowercase
UPPER(str)	Convert all characters in the string str to uppercase
LEFT(str,x)	Returns the x characters from the left of the string str
RIGHT(str,x)	Returns the x characters on the right side of the string str
LPAD(str,n,pad)	Fill the left side of the string with the string pad until the length is n characters long
RPAD(str,n,pad)	Fill the right side of the string with the string pad until the length is n characters long
LTRIM(str)	Remove the left space of the string
RTRIM(str)	Remove the space on the right side of the string
REPEAT(str,x)	Repeat the string str x times
REPLACE(str,a,b)	Replace all occurrences of a string in string str with string b
STRCMP(s1,s2)	Compare strings s1 and s2
TRIM(str)	Remove all strings at the end and beginning of the string str
SUBSTRING(str,x,y)	Returns a string of length y from position str x

• CONCAT(s1,s2,...sn)Concatenate multiple strings into one string, any string andNULLThe result of the connection isNULL

select CONCAT('ads','133'),CONCAT('abc',NULL);	

• INSERT(str,x,y,instr)Replace the y characters of the string str starting from x with instr

select INSERT('Hello World',7,5,'MySQL');

• LOWER(str)Convert all string str to lowercase

select LOWER('bHUFRhjH');

• UPPER(str)Convert the string str to uppercase

select UPPER('jhssdgvTF');

• LEFT(str,x)withRIGHT(str,x)Returns x characters to the left/right of string str

select LEFT('BEIJIN',3),RIGHT('LOVE',2);

• LPAD(str,n,pad)withRPAD(str,n,pad)Fill the characters on the left/right side of the string str with pad until the length is n characters long

select lpad('2008',20,'beijing'),rpad('beijing',20,'2008');

• LTRIM(str)And ``RTRIM(str)` to remove the spaces on the left/right side of the string str

select  LTRIM('   |b'),RTRIM('h|   ');

• REPEAT(str,n)Repeat the string str n times

select REPEAT('Hello ',4);

• REPLACE(str,a,b)Replace a in str with b

select REPLACE('Hello World','World','MySQL');

• STRCMP(s1,s2)Compare the uppercase of the ASCII code value of the string s1, s2, if s1 is smaller than s2, return -1, if s1 and s2 are equal, return 0, if s1 is greater than s2, return 1

select STRCMP('java','python');

• TRIM(str)Remove str leading and trailing spaces

select TRIM('   |k|   ');

• SUBSTRING(str,x,y)Returns the y characters of the string str starting from x

select  SUBSTRING('Hello',1,2);

Numerical function

function	effect
ABS(x)	Returns the absolute value of x
CEIL(x)	Returns the largest integer value greater than x
FLOOR(x)	Returns the largest integer value less than x
MOD(x,y)	Returns the value of x%y
RAND()	Return a random value from 0 to 1
ROUND(x,y)	Returns the value of x with y decimals after rounding
TRUNCATE(x,y)	Returns the result of the number x truncated to y decimals

• ABS(x)Returns the absolute value of x

select ABS(-8),ABS(8);

• CEIL(x)Returns the largest integer value greater than x

select CEIL(2.6),CEIL(2.1);

• FLOOR(x)Returns the largest integer value less than x

select FLOOR(3.9),FLOOR(3.1);

• MOD(x,y)Returns the value of x%y

select MOD(5,2),5%2;

• RAND()Return a random value from 0 to 1

select RAND(),RAND();

• ROUND(x,y), Returns the value that contains y decimal places after rounding x

select ROUND(2.5674,3); 

Date and time type functions

function	effect
CURDATE()	Return current date
CURTIME()	Return current time
NOW()	Return the current date and time
UNIX_TIMESTAMP(date)	Returns the UNIX timestamp of the date date
FROM_UNIXTIME()	Returns the date value of the UNIX timestamp
WEEK(date)	Return date date is the first few weeks of the year
YEAR(date)	Returns the year of the date date
HOUR(time)	Returns the hour value of time
MIUNTE(time)	Returns the minute value of time
MONTHNAME(date)	Returns the month name of date
DATE_FORMAT(date,fmt)	Returns the date value formatted by the string fmt
DATE_ADD(date,INTERVAL expr type)	Returns a date or time value plus a time interval time value
DATEDIFF(expr,expr2)	Returns the number of days between the start time expr and the end time expr2

• CURDATE()Return the current date value, including only the year, month, and day

select CURDATE();

• CURTIME()Returns the current time including only hours, minutes and seconds

select CURTIME();

• NOW()Return the current date and time

select NOW();

• UNIX_TIMESTAMP(date)Returns the UNIX timestamp of date

select UNIX_TIMESTAMP(NOW());

• FROM_UNIUXTIME(unixtime)Return the date value of the UNIX timestamp, andUNIX_TIMESTAMP(date)in contrast

select FROM_UNIXTIME(1597733300);

• WEEK(date) and YEAR(date)The former returns the given date in the first few weeks of the year, the latter returns the given date in which year

select WEEK(NOW()),YEAR(NOW());

• HOUR(time)withMINUTE(time)The former returns the hour of the given time, the latter returns the minute of the given time

select HOUR(CURTIME()),MINUTE(CURTIME());

• MONTHNAME(date)Returns the name of the month corresponding to date

select MONTHNAME(NOW());

• DATE_FORMAT(date,fmt), Format the date date according to the string fmt, the format characters that can be used are as follows

Format	Format description
%S,%s	Second in two-digit format (00,01,...,59)
%i	Minutes in two-digit format (00,01,...,59)
%H	Two-digit hour, 24 hours (0,1,...,24)
%h,%l	Two-digit hour, 12 hours (0,1,...,12)
%k	Numerical hour, 24 hours (0,1,...,24)
%l	Number of hours, 12 hours (0,1,...,12)
%T	24-hour format (hh:mm:ss)
%r	12-hour format (hh:mm:ssAM or hh:mm:ssPM)
%p	AM or PM
%W	The name of each day of the week (Sunday,Monday,…,Saturday)
%a	Abbreviation of the name of each day of the week (Sun,Mon,…,Sat)
%d	Two digits indicate the number of days in the month (00, 01, ..., 31)
%e	The number indicates the number of days in the month (1,2,...,31)
%D	The English suffix indicates the number of days in the month (1st, 2nd, 3rd...)
%w	Art form represents the number of days of the week (0=Sunday,1=Monday,…,6=Saturday)
%j	Use three digits to indicate the number of days in the year (001, 002,...336)
%U	Week (0,1,52), where Sunday is the first day of the week
%u	Week (0,1,52), where Monday is the first day of the week
%M	Month name, (January,February,…,December)
%b	Abbreviated month (Jan,Feb,...Dec)
%m	Two-digit month (01,02,...,12)
%c	Number of months (1,2,…12)
%Y	4-digit year
%y	Two-digit year
%%	Direct value

select DATE_FORMAT(NOW(),'%Y year%m month%d day%H hour%i minute%S second');

• DATE_ADD(date,INTERVAL expr type)Returns the INTERVAL time period from the given date, where INTERVAL is the time interval type, expr is an expression, type is the interval type, and MySQL provides 13 interval types.

Expression type	description	format
HOUR	hour	hh
MINUTE	minute	mm
SECOND	second	ss
YEAR	year	YY
MONTH	month	MM
DAY	day	DD
YEAR_MONTH	years	YY-MM
DAY_HOUR	Day and time	DD hh
DAY_MINUTE	Day and minute	DD hh:mm
DAY_SECOND	Day and second	DD hh:mm:ss
HOUR_MIUNTE	Hours and minutes	hh:mm
HOUR_SECOND	Hour and second	hh:ss
MINUTE_SECOND	Minutes and seconds	mm:ss

select NOW() CURRENT,DATE_ADD(NOW(),INTERVAL 31 DAY) AFTER_31_DAYS,DATE_ADD(NOW(),INTERVAL '1_2' YEAR_MONTH) AFTER_1YEAR_2MONTH;

• DATEDIFF(date1,date2) calculates the number of days between two dates

select DATEDIFF('20210101',now());

Process function

Process functions are also a very commonly used function. Users can use this type of function to implement conditional selection in a SQL statement, which can improve the efficiency of the statement

function	effect
IF(value,t ,f)	If value is true, return t, otherwise return f
IFNULL(value1,value2)	If value1 is not empty, return value1, otherwise return value2
CASE WHEN [value1] THEN[result1]…ELSE[default] END	If value1 is true, return result1, otherwise return default
CASE[expr] WHEN[value1] THEN[result1] …ELSE[default]END	If expr is equal to value1, return result1, otherwise return default

Create employee salary table

 create table salary (userid int,salary decimal(9,2));

Insert test data

 insert into salary values(1,1000),(2,2000), (3,3000),(4,4000),(5,5000), (1,null);

• IF(value,t,f) If the employee's salary is greater than 3000, it is displayed as "high", otherwise it is displayed as "low"

select IF(salary>3000,'high','low') from salary;	

• IFNULL(value1,value2)Often used to replace NULL

select IFNULL(salary,0) from salary;

• CASE WHEN to achieve a high base salary

select CASE WHEN salary >=2000 then 'higt' ELSE 'low' END  from salary;

Other commonly used functions

Function name	effect
DATABASE()	Returns the current database name
VERSION()	Returns the current database version
USER()	Returns the currently logged in user name
INET_ATON(IP)	Return the numeric representation of the IP address
INET_NTOA(num)	Return the IP address represented by the number
PASSWORD(str)	Returns the encrypted version of the string str
MD5(str)	Returns the MD5 value of the string str

• DATABASE

select DATABASE();

• VERSION()

select VERSION();

• USER()

select USER();

• INET_ATON(ip)

select INET_ATON('192.168.31.51');

• INET_NTOA(num)

select INET_NTOA(3232243507);

• PASSWORD(str)

select PASSWORD('ihgyugjui4564');

• MD5(str)

select MD5('sfcsf4778x');

• INET_ATON(ip)

select INET_ATON('192.168.31.51');

• INET_NTOA(num)

select INET_NTOA(3232243507);