MySQL functions

Catalog

One 、 Common functions

Two 、 Process control functions

3、 ... and 、 Merge （union）

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One 、 Common functions

1、 Character functions

effect	function	result
Turn lowercase	LOWER('SQL Course')	sql course
Turn capitalization	UPPER('SQL Course')	SQL COURSE
Splicing	CONCAT('Hello', 'World')	HelloWorld
Intercept	SUBSTR('HelloWorld',1,5)	Hello
length	LENGTH('HelloWorld')	10
Character occurrence index value	INSTR('HelloWorld', 'W')	6
Second half of character interception	TRIM('H' FROM 'HelloWorld')	elloWorld
Character substitution	REPLACE('abcd','b','m')	amcd

  Character function cases ：

1.length  Gets the number of bytes of the parameter value 
SELECT LENGTH('john');
SELECT LENGTH(' Little Zhu, little Zhu, little pig ');
SHOW VARIABLES LIKE '%char%'
2.concat  String concatenation 
SELECT CONCAT(last_name,'_',first_name)  full name  t_mysql_FROM employees;
3.upper、lower
SELECT UPPER('john');
SELECT LOWER('joHn');
 Example ： Capitalize your last name , Change the name to lowercase , Then joining together 
SELECT CONCAT(UPPER(last_name),LOWER(first_name))  full name  FROM t_mysql_employees;
4.substr、substring
 Be careful ： Index from 1 Start 
 Intercepts all characters following the specified index 
SELECT SUBSTR('abcccccccccccc',7) out_put;
 Intercepts the character with the specified length from the specified index 
SELECT SUBSTR(' Xiao Zhu Xiao Zhu is chubby ',1,3) out_put;

 Case study ： The first character of a name is capitalized , The other characters are lowercase and then use _ Splicing , Show it 
SELECT CONCAT(UPPER(SUBSTR(last_name,1,1)),'_',LOWER(SUBSTR(last_name,2))) out_put
FROM t_mysql_employees;5.instr  Returns the index of the first occurrence of the substring , If we can't find a way back 0
SELECT INSTR('aaabbbb','aaa') AS out_put;
#6.trim
SELECT LENGTH(TRIM('   Zhu Jiayin   ')) AS out_put;
SELECT TRIM('aa' FROM 'aaaaaaaaa zhu aaaaaaaaaaaa Good news aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa') AS out_put;
7.lpad  Fill the specified length with the specified characters 
SELECT LPAD(' Xiao Zhu ',2,'*') AS out_put;
8.rpad  Fill the specified length with the specified characters 
SELECT RPAD(' Xiao Zhu ',12,'ab') AS out_put;

9.replace  Replace 
SELECT REPLACE(' Xiao Zhu loves to sleep ',' Xiao Zhu ',' Two bears ') AS out_put;

2、 Number function

effect	function	result
rounding	ROUND(45.926, 2)	45.93
truncation	TRUNC(45.926, 2)	45.92
Seeking remainder	MOD(1600, 300)	100

Mathematical function cases ： 

round  rounding 
SELECT ROUND(-1.55);
SELECT ROUND(1.567,2);

#ceil  Rounding up , return >= The smallest integer of this parameter 
SELECT CEIL(-1.02);
#floor  Rounding down , return <= The maximum integer of the parameter 
SELECT FLOOR(-9.99);
#truncate  truncation 
SELECT TRUNCATE(1.69999,1);

 

3、 Date function

effect	function	result
Get current date	now()
Convert the character of date format to the date of specified format	STR_TO_DATE('9-13-1999','%m-%d-%Y')	1999-09-13
Convert date to character	DATE_FORMAT(‘2018/6/6’,‘%Y year %m month %d Japan ’)	2018 year 06 month 06 Japan

  Date function case ：

now  Returns the current system date + Time 
SELECT NOW();
curdate  Returns the current system date , Not including time 
SELECT CURDATE();
curtime  Return current time , No date included 
SELECT CURTIME();

 You can get the specified part , year 、 month 、 Japan 、 Hours 、 minute 、 second 
SELECT YEAR(NOW())  year ;
SELECT YEAR('1998-1-1')  year ;
SELECT YEAR(hiredate)  year  FROM t_mysql_employees;
SELECT MONTH(NOW())  month ;
SELECT MONTHNAME(NOW())  month ;

str_to_date  Converts a character to a date in a specified format 
SELECT STR_TO_DATE('1998-3-2','%Y-%c-%d') AS out_put;


date_format  Convert date to character 
SELECT DATE_FORMAT(NOW(),'%y year %m month %d Japan ') AS out_put;

 

4、 Other functions

SELECT VERSION();
SELECT DATABASE();
SELECT USER();

  Two 、 Process control functions

1.if function ： if else The effect of

SELECT IF(10<5,' Big ',' Small ');
SELECT last_name,commission_pct,IF(commission_pct IS NULL,' No bonus , ha-ha ',' Bonus , Hee hee ')  remarks 
FROM t_mysql_employees;

2.case The use of function one ： switch case The effect of

java in 
switch( Variable or expression ){
case  Constant 1： sentence 1;break;
...
default: sentence n;break;


}

mysql in 

case  The field or expression to determine 
when  Constant 1 then  The value to display 1 Or words 1;
when  Constant 2 then  The value to display 2 Or words 2;
...
else  The value to display n Or words n;
end

3、 ... and 、 Merge （union）

1、union： Merge all the query results together , Then remove the same records

2、union All： Merge all the query results together , The same records will not be removed

Prerequisite ： The number of columns in the result set is the same , The types of columns should also be the same or compatible

Application scenarios ： Project report module , Merge data