MySQL -- Introduction and use of single line functions

MySQL—— One line function

1、MySQL Classification of functions in

stay SQL In language , Including built-in functions and custom functions . Different DBMS Functions are different , Most of the DBMS Will have their own specific functions .

MySQL Built in function and classification

MySQL Provides a wealth of built-in functions , These built-in functions can help users deal with the data in the table more conveniently . Functions are stored in the database like predetermined formulas , Every user can call existing functions to complete some functions .

Function can easily realize the reuse of business logic , also MySQL The database allows users to create their own functions , To adapt to the actual business operation . Using functions correctly will allow developers to write SQL Sentences get twice the result with half the effort .

MySQL Function is used to process the data in the data table accordingly , In order to get the data users want , send MySQL The function of database is more powerful .

MySQL The built-in functions provided can be divided into numerical functions from the perspective of realized functions 、 String function 、 Date and time functions 、 Process control functions 、 System information function and encryption function 、 obtain MyMSQL Information function, etc .

SELECT、INSERT、UPDATE and DELETE Statements and their clauses （ for example WHERE、ORDER BY、HAVING etc. ） You can use MySQL function .

The following describes the scope of use of the above functions ：

• Mathematical functions are mainly used to deal with numbers . Such functions include absolute value functions 、 Sine function 、 Cosine function and function to obtain random number, etc .
• The string function is mainly used to process strings . This includes string concatenation functions 、 String comparison function 、 Functions that change the letters of a string into lowercase or uppercase letters, and functions that obtain substrings .
• The date and time function is mainly used to process date and time . This includes functions to get the current time 、 Function to get the current date 、 Functions that return year and date .
• Process control functions are mainly used in SQL Control condition selection in statement . These include IF sentence 、CASE Statement and WHERE Statement etc. .
• The system information function is mainly used to obtain MySQL Database system information . This includes functions to get the database name 、 Get the function of the current user and the function of getting the database version .
• Encryption function is mainly used to encrypt and decrypt strings . Including string encryption function and string decryption function .
• Other functions mainly include formatting functions and locking functions .

MySQL Function official reference document ：https://dev.mysql.com/doc/refman/5.7/en/

Built in functions are further divided into two categories ： One line function 、 Aggregate functions （ Or grouping function ).

2、 One line function

Single line function definition ：

• Manipulate data objects
• The receiving parameter returns a result
• Convert only one row
• Each line returns a result
• Can be nested
• The parameter can be a field or a value

2.1、 Functions of numerical type

The name of the function	do use
ABS()	Find the absolute value
SQRT()	Find the quadratic root
MOD(x,y)	seek x Divide y The remainder of
PI()	Return the PI
CEIL() and CEILING()	The two functions have the same function , All return the minimum integer no less than the parameter , That is, round up
FLOOR()	Rounding down , The return value is converted to a BIGINT
RAND()	Generate a 0~1 Random number between , When the incoming parameters are the same , The generated random number is the same
ROUND(x,y)	Round the parameters passed
TRUNCARE(x,y)	Return to digital x Truncated to y The result of decimal places
SIGN()	Returns the symbol of the parameter . A positive number returns 1, A negative number returns -1,0 return 0
POW(x,y) and POWER(x,y)	The functions of the two functions are the same , All return to x Of y Power
EXP(x)	return e Of x Power , among e It's a constant ,2.718281828459045
LN(x),LOG(x)	Return to e At the bottom of the X The logarithmic , When X<=0 when , The result returned is NULL
BIN(x)	return x Binary code of
HEX(x)	return x Hexadecimal encoding
OCT(x)	return x Octal encoding
SIN()	Find the sine
ASIN()	Find the inverse sine , And functions SIN They're inverse functions to each other
COS()	Find the cosine
ACOS()	Find the inverse cosine , And functions COS They're inverse functions to each other
TAN()	Find tangent
ATAN()	Find the arctangent , And functions TAN They're inverse functions to each other
COT()	Find the cotangent value

Using examples ：

SELECT ABS(-9)  --  The absolute value  9
select pi(); --  PI  3.141593
select mod(4,3); --  Mod  1
SELECT CEILING(9.8) --  Rounding up  10
SELECT FLOOR(9.8) --  Rounding down  9
SELECT RAND() --  Generate random number 0-1 Between 
SELECT SIGN(7) --  Judge the positive and negative of a number   A positive number is 1
SELECT SIGN(-7)--  A negative number is -1
select round(146.647); --  rounding  146
select round(146.647,2); -- 146.65, Keep two decimal places 
select truncate(146.647,2); --  Return to digital x Truncated to y The result of decimal places  146.64
select truncate(round(146.647,2),1); --  Nested functions use  146.6
select bin(4); --  return x Binary code of  100
select hex(100); --  return x Hexadecimal encoding  64
select oct(100); --  return x Octal encoding  144

2.2、 String function

The name of the function	do use
ASCII(s)	Return string s Of the first character in ASCll Code value
CHAR_LENGTHI(s)	Return string s The number of characters . The functions and CHARACTER_LENGTH(s) identical
LENGTH(s)	Returns the byte length of the string , It's about character sets
CONCAT(s1,s2,…,sn)	Merge string functions , The return result is the string generated by the connection parameter , Parameters can make one or more
CONCAT_WS(x,s1,s2,…,sn)	Same as CONCAT(s1,s2,…,sn) function , But add... Between each string x
INSERT(str,idx,len,replacestr)	Replace string functions , The string str From idx Position start ,len Substrings of characters long are replaced with strings replacestr, String index from 1 At the beginning
LOWER(s) or LCASE(s)	The string s Convert letters in to lowercase
UPPER(s) or UCASE(s)	The string s Convert letters in to uppercase
LEFT(str,n)	Take the string from the left side of the word str, Return... To the left of the string n Dry characters
RIGHT(str,n)	Take the string from the right word str, Returns... To the right of the string n Dry characters
LPAD(str,len,pad)	Use string pad Yes str Fill on the far left , until str The length of is len Characters
RPAD(str,len,pad)	Use string pad Yes str Fill on the far right , until str The length of is len Characters
TRIM(s)	Delete the spaces on the left and right of the string
LTRIM(s)	Remove string s The space on the left
RTRIM(s)	Remove string s The space on the right
REPEAT(str,n)	return str repeat n Results of
SPACE(n)	return n A space
STRCMP(s1,s2)	Compare strings s1,s2 Of ASCIl The size of the code value
REPLACE(str,a,b)	String replacement function , Use string b Replace the universal character string str All the strings that appear in a
SUBSTRING	Intercepting string , Returns a character change of a specified length starting at a specified location
REVERSE	String inversion （ The reverse ） function , Returns the string in reverse order to the original string
ELT(m,s1,s2,s3,…,sn)	Returns the string at the specified location , If m=1, Then return to s1, If m=2, Then return to s2, If m=n, Then return to sn
FIELD(s,s1,s2,s3,…,sn)	Return string s The position of the first occurrence in the string list
FIELD_IN_SET(s1,s2)	Return string s1 In string s2 Where in . among , character string s2 Is a comma separated string
NULLIF(s1,s2)	Compare two strings , If s1 And s2 equal , Then return to NULL, Otherwise return to s1

Using examples ：

select ascii('CBcd') from dual; --  Return string s Of the first character in ASCll Code value  67
SELECT CHAR_LENGTH(' ten thousand , Li Gu Yicheng ');  --  Return string length , 6
SELECT CONCAT(' Miles ',' Gu ',' One ',' cheng ');--  String concatenation  , Wan Li Gu Yicheng 
SELECT CONCAT_WS('a',' Miles ',' Gu ',' One ',' cheng ');--  String concatenation , Same as CONCAT(s1,s2,…,sn) function , But add... Between each string a , Miles a Gu a One a cheng 
SELECT INSERT('helloworld',2,4,'b');-- 2: The starting position of the replaced character  4: Length of replaced character  'b': Replace character  ,hbworld
SELECT LOWER('HELLo'); --  Convert to lowercase  hello
SELECT UPPER('hello'); --  Convert to uppercase  HELLO
SELECT left('hello',3); --  Take the string from the left side of the word str, Return... To the left of the string n Dry characters  , hel
SELECT right('hello',3); --  Take the string from the left side of the word str, Return... To the left of the string n Dry characters , llo
SELECT lpad('hello',10,'a'); --  Use string pad Yes str Fill on the far left , until str The length of is len Characters , aaaaahello
SELECT rpad('hello',10,'a'); --  Use string pad Yes str Fill on the far right , until str The length of is len Characters , helloaaaaa
SELECT trim(' hello '); --  Delete the space around the string , hello
SELECT REPEAT('hello',3); --  return str repeat n Results of , hellohellohello
SELECT STRCMP('hello','world'); --  Compare strings s1,s2 Of ASCIl The size of the code value , -1, Indicates that the following number is large ;1, Indicates that the previous number is large 
SELECT INSTR('hellworld','wo'); --  Returns the index of the first occurrence of the string  ,5
SELECT REPLACE('java Senior engineer ',' senior ',' Ultima ');--  Replace the specified string that appears  java Research Engineer 
SELECT SUBSTR('java Senior engineer ',1,4);--  Intercepts the specified string  1： Where to start the capture  4： Intercept the length of the string , java
SELECT REVERSE(' Brother Zhao asked me to do something ');--  Reverse string ,  I entrusted my brother Zhao 
SELECT FIELD('ab','bc','cd','ab','dg','ab');--  Return string s The position of the first occurrence in the string list ,3
SELECT FIND_IN_SET('ab','bc,cd,ab,dg,ab');--  Return string s1 In string s2 Where in ,3
SELECT NULLIF('ab','bc');--  Compare two strings ,ab

2.3、 Date and time functions

The name of the function	do use
CURDATE and CURRENT_DATE	The two functions work the same , Returns the date value of the current system
CURTIME and CURRENT_TIME	The two functions work the same , Returns the time value of the current system
NOW and SYSDATE	The two functions work the same , Returns the date and time values of the current system
UNIX_TIMESTAMP	obtain UNIX Time stamp function , Return one to UNIX A timestamp based unsigned integer
FROM_UNIXTIME	take UNIX Time stamp converted to time format , And UNIX_TIMESTAMP They're inverse functions to each other
MONTH	Get the month in the specified date
MONTHNAME	Get the English name of the month in the specified date
DAYNAME	Get the English name of the day of the week corresponding to the specified date
DAYOFWEEK	Gets the index location value of the week corresponding to the specified date
WEEK	Get the specified date is the week of the year , Whether the return value range is 0〜52 or 1〜53
DAYOFYEAR	The date of acquisition is the day of the year , The return value range is 1~366
DAYOFMONTH	Get the specified date is the day of the month , The return value range is 1~31
YEAR	Get year , The return value range is 1970〜2069
TIME_TO_SEC	Convert the time parameter to seconds
SEC_TO_TIME	Convert seconds to time , And TIME_TO_SEC They're inverse functions to each other
DATE_ADD and ADDDATE	The two functions have the same function , All add the specified time interval to the date
DATE_SUB and SUBDATE	The two functions have the same function , It's all subtracting the specified time interval from the date
ADDTIME	Time addition operation , Add the specified time... To the original time
SUBTIME	Time subtraction , Subtract the specified time from the original time
DATEDIFF	Get the interval between two dates , Returns the parameter 1 Subtract the parameter 2 Value
DATE_FORMAT	Format the specified date , Returns the value of the specified format according to the parameter
WEEKDAY	Get the corresponding working day index of the specified date in a week

Using examples ：

SELECT CURDATE(),CURRENT_DATE(),CURRENT_DATE(); --  Returns the date value of the current system  2022-02-12,2022-02-12,2022-02-12
SELECT CURTIME(),CURRENT_TIME(),CURRENT_TIME(); --  Returns the time value of the current system  16:02:20,16:02:20,16:02:20
SELECT NOW(),SYSDATE(); --  Returns the date and time values of the current system  2022-02-12 16:03:33,2022-02-12 16:03:33
SELECT UNIX_TIMESTAMP(),UNIX_TIMESTAMP(NOW()),NOW(); --  obtain UNIX Time stamp function  1644653034,1644653034,2022-02-12 16:03:54
SELECT FROM_UNIXTIME(1644653034); --  take  UNIX  Time stamp is converted to normal format  2022-02-12 16:03:54
SELECT MONTH('2017-12-15'); --  Returns the month in the specified date  12
SELECT MONTHNAME('2017-12-15'); --  Return date  date  The full English name of the corresponding month  December
SELECT DAYNAME('2022-02-12'); --  return  date  English name of the corresponding working day  Saturday
SELECT DAYOFWEEK('2022-02-12'); --  Returns the week index corresponding to the date  7 ,1 Means Sunday ,2 It means Monday ,……,7  It means Saturday 

/* WEEK()  Function calculates date  date  It's the week of the year  WEEK The function takes two arguments ： date Is to get the date of the week . mode Is an optional parameter , The logic used to determine the calculation of the number of weeks , If you ignore  mode  Parameters , By default  WEEK  The function will use  default_week_format  The value of the system variable . */
SHOW VARIABLES LIKE 'default_week_format'; --  obtain  default_week_format  The current value of the variable  0
SELECT WEEK('2022-02-12',1); -- 6

SELECT DAYOFYEAR('2022-02-12'); --  Returns the day of the year that the specified date is  43
SELECT DAYOFMONTH('2022-02-12'); --  Returns the day of the specified date in January  12
SELECT YEAR(NOW()); --  Get the year of the current time  2022
SELECT TIME_TO_SEC('15:15:15'); --  Convert time value to second value  54915
SELECT SEC_TO_TIME('54925'); --  Convert the second value to the time format  15:15:25
SELECT DATE_ADD('2018-10-31 23:59:59',INTERVAL 1 SECOND) AS T1, --  Add... To the original time 1  second  2018-11-01 00:00:00
       DATE_ADD('2018-10-31 23:59:59',INTERVAL '1:1' MINUTE_SECOND) AS T2, --  Add... To the original time  1  minute  1  second  2018-11-01 00:01:00
       ADDDATE('2018-10-31 23:59:59',INTERVAL 1 SECOND) AS T3; --  Add... To the original time 1  second  2018-11-01 00:00:00
SELECT DATE_SUB('2018-01-02',INTERVAL 31 DAY) AS T1, --  Subtract one month from the original time  2017-12-02
       SUBDATE('2018-01-02',INTERVAL 31 DAY) AS T2, --  Subtract one month from the original time  2017-12-02
       DATE_SUB('2018-01-01 00:01:00',INTERVAL '0 0:1:1' DAY_SECOND) AS T3; --  Subtract... From the original time 1 branch 1 second  2017-12-31 23:59:59
-- DATE_ADD(date,INTERVAL expr type)  and  DATE_SUB(date,INTERVAL expr type)  The function can also specify a negative value when specifying the time period of addition and subtraction , The negative value of addition returns the date and time before the original time , The negative value of subtraction returns the date and time after the original time .
SELECT ADDTIME('2018-10-31 23:59:59','0:1:1'), --  Add time  2018-11-01 00:01:00
       ADDTIME('10:30:59','5:10:37'); -- 15:41:36
SELECT SUBTIME('2018-10-31 23:59:59','0:1:1'), --  Subtract time  2018-10-31 23:58:58
       SUBTIME('10:30:59','5:12:37'); -- 05:18:22
SELECT DATEDIFF('2017-11-30','2017-11-20') AS COL1, --  Return to start time  date1  And end time  date2  Days between , return  date1-date2  After the value of  10
       DATEDIFF('2017-11-30','2017-12-15') AS col2; -- -15
       
/** DATE_FORMAT(date,format)  Functions are based on  format  Display... In the specified format  date  value . DATE_FORMAT()  The function takes two arguments ： date： Is the valid date value to format  format： Is a format string made up of predefined specifiers , Each specifier is preceded by a percentage character (%). */
SELECT DATE_FORMAT('2017-11-15 21:45:00','%W %M %D %Y') AS col1, -- Wednesday November 15th 2017
       DATE_FORMAT('2017-11-15 21:45:00','%h:i% %p %M %D %Y') AS col2; -- 09:i PM November 15th 2017

SELECT WEEKDAY('2022-02-12'); --  Return the working day index corresponding to the date ,0 It means Monday ,1 Tuesday ,……,6 Means Sunday  5

DATE_FORMAT(date,format) Function format The format is shown in the table below ：

specifier	explain
%a	The abbreviated name of a weekday （Sun~Sat)
%b	Abbreviated name of the month （Jan…Dec)
%c	month , Digital form （0~12)
%D	Date of the month with English suffix (0th, 2st, 3nd,…）
%d	Date of the month , Digital form （00~31)
%e	Date of the month , Digital form （0~31)
%f	Microsecond （000000 …999999)
%H	With 2 Digit representation 24 Hours （00~23)
%h, %I	With 2 Digit representation 12 Hours （01~12)
%i	minute , Digital form (00~59)
%j	— Days of the year （001~366)
%k	With 24 Hours （0~23) Express
%l	With 12 Hours （1~12) Express
%M	The name of the month （January~December)
%m	month , Digital form （00~12)
%p	In the morning （AM) Or afternoon （PM)
%r	Time ,12 hourly （ Hours (hh): minute (mm) : Number of seconds (ss) After add AM or PM)
%S, %s	With 2 The number of digits represents seconds （00~59)
%T	Time ,24 hourly （ Hours (hh): minute (mm): Number of seconds (ss))
%U	Zhou （00~53), Sunday is the first day of the week
%u	Zhou （00~53), Monday is the first day of the week
%V	Zhou （01~53), Sunday is the first day of the week , and ％X Use at the same time
%v	Zhou （01~53), Monday is the first day of the week , and %x Use at the same time
%W	Week logo （ Sunday 、 Monday 、 Tuesday … Saturday ）
%w	— Every day of the week （0= Sunday …6= Saturday ）
%X	The year of the week , Sunday is the first day of the week , Digital form ,4 digit , and ％V Use at the same time
%x	The year of the week , Monday is the first day of the week , Digital form ,4 digit , and %v Use at the same time
%Y	4 The number of digits represents the year
%y	2 The number of digits represents the year
%%	% A literal character

2.4、 Process control functions

Process processing functions can be based on different conditions , Perform different processes , Can be in sQL Statement to implement different conditional choices .MySQL The process processing functions in mainly include lF()、IFNULL() and CASE() function .

The name of the function	effect
lF(value,value1,value2)	If value The value of is TRUE, return value1, Otherwise return to value2
IFNULL(value1, value2)	If value1 Not for NULL, return value1, Otherwise return to value2
CASE WHEN Conditions 1 THEN result 1 WHEN Conditions 2 THEN result 2…[ELSE resultn] END	amount to Java Of if…else if…else…
CASE expr WHEN Constant values 1 THEN value 1 WHEN Constant values 1 THEN value 1…[ELSE value n] END	amount to Java Of switch…case…

Practical examples ：

/* Use  IF(expr,v1,v2)  Functions are based on  expr  The expression result returns the corresponding value :  stay  c3  in , First use  STRCMP(s1,s2)  Function to compare the size of two strings , character string  'abc'  and  'ab'  The return value of the comparison result is  1,  That's the expression  expr  The return result of is not equal to  0  And it's not equal to  NULL, The return value is  v1, String character  'yes' 1,×,yes */
SELECT IF(1<2,1,0) c1,IF(1>5,'√','×') c2,IF(STRCMP('abc','ab'),'yes','no') c3; 
SELECT first_name ,salary , IF(salary > 6000,' High salaries ',' commonly ') ' wages ' FROM employees

--  Use  IFNULL(v1,v2)  Functions are based on  v1  The value of returns the corresponding value , If  v1  Not for  NULL, be  IFNULL  The function returns  v1;  Otherwise return to  v2  Result .
SELECT IFNULL(5,8),IFNULL(NULL,'OK'),IFNULL(SQRT(-8),'FALSE'),SQRT(-8); -- 5,OK,FALSE,null
SELECT first_name ,salary,(salary + salary * IFNULL(commission_pct,0)) * 12 ' Annual salary '

/*CASE  Statement to check that the value of the expression matches a set of unique values .  take  < expression >  The value of each  WHEN  Compare values in clause , for example  < value 1>,< value 2>  etc. . If  < expression >  and  < value n>  The values are equal , Then execute the corresponding  WHEN  Command in branch  < operation >.  If  WHEN  In Clause  < value n>  With no  < expression >  The value of matches , be  ELSE  The command in clause will be executed .ELSE  Clauses are optional .  If omitted  ELSE  Clause , And no match found ,MySQL  Error will be raised . */
SELECT CASE WEEKDAY(NOW()) 
WHEN 0 THEN ' Monday ' 
WHEN 1 THEN ' Tuesday ' 
WHEN 2 THEN ' Wednesday ' 
WHEN 3 THEN ' Thursday ' 
WHEN 4 THEN ' Friday ' 
WHEN 5 THEN ' Saturday '
ELSE ' Sunday ' END AS COLUMN1,
NOW(),WEEKDAY(NOW()),DAYNAME(NOW());

/* MySQL  Separate calculation  WHEN  Every condition in clause , Until you find a value of  TRUE  Conditions , And then execute  THEN  Corresponding in clause  < command >.  If there is no condition for  TRUE, execute  ELSE  In Clause  < command >. If you don't specify  ELSE  Clause , And there is no condition for  TRUE,MySQL  An error message will be sent . */
SELECT CASE 
WHEN WEEKDAY(NOW())=0 THEN ' Monday ' 
WHEN WEEKDAY(NOW())=1 THEN ' Tuesday '  
WHEN WEEKDAY(NOW())=2 THEN ' Wednesday ' 
WHEN WEEKDAY(NOW())=3 THEN ' Thursday ' 
WHEN WEEKDAY(NOW())=4 THEN ' Friday ' 
WHEN WEEKDAY(NOW())=5 THEN ' Saturday ' 
WHEN WEEKDAY(NOW())=6 THEN ' Sunday ' END AS COLUMN1,
NOW(),WEEKDAY(NOW()),DAYNAME(NOW());