Function application of MySQL

One , Definition of function

1. similar Java Define methods

2. With parameters

3. Must have return value ( Line by line ) , a key ：java Method can have no return value , The database function must have a return value
select max(score) from sc

4. Functions can be nested into sql In the sentence
notes ： Stored procedures cannot

5. Functions can be nested round(avg)

Two , Common functions

（1） Date function

1. Date function 
 
   1) NOW()： Get the current date and time 
	
		select now()
	 
   2) DAY()： Get the day in the date 
	 
		select date(NOW())
	 
   3) MONTH()： Gets the month in the date 
	 
		select MONTH(NOW())
		
   4) YEAR()： Gets the year in the date 
	 
		select YEAR(NOW())
	 
   5)  Date conversion function DATE_FORMAT()/STR_TO_DATE()
   
	 select NOW(),DATE_FORMAT(now(),'%Y year %m month %d Japan ')
	 
   DATE_FORMAT()： Convert date to string 
   STR_TO_DATE()： Convert string to date 
	 Tips ：< Case sensitive >
	 select STR_TO_DATE('2022-09-08','%Y-%m-%d')
 
 

design sketch


Case 2

   Check the age of each student , Only by year 
	 
	 select s.*,year(now())-YEAR(s.sage) from t_student s
	 
	 
    According to the date of birth , Current month day  <  The date of birth is , Age minus one 
	 
	  select s.*,year(now())-YEAR(s.sage)+if(month(NOW())<MONTH(s.sage),-1,0) from t_student s
		
    Check out the students who have birthdays this month 
	 	 
		select *from t_student where month(now()) = month(sage)
		select * from t_student
 
   Check out the students who have their birthday next month （ Current month plus 1）
 
     （1）date_add(NOW(), interval 1 MONTH)
		select *from t_student where month(now())+1 = month(sage)
 
 
      （2）select DATE_ADD(NOW(),interval 1 month)
		select *from t_student where DATE_ADD(NOW(),interval 1 month) = month(sage)
 

3、 ... and , String function

Case study

 
   1) UPPER()/LOWER()： Case to case 
	 
	 select UPPER('afsgafsas')
	 select LOWER('HJHKHKHKHK')
	 
   2) REPLACE()： Search and replace substrings in a string 
	 
 
	 SELECT uuid(),REPLACE((uuid()),'-','');
	 
   3) SUBSTRING()： The last substring starting at a position with a specific length 
	 
	 select SUBSTRING('xiong er is a xiong',1,5)
	 select SUBSTRING('xiong er is a xiong',5)

design sketch

Case study 2

 4) TRIM()： Remove the space before and after 
   5) LENGTH()： Get string length 
	 
	 select LENGTH('xiong er is a xiong'),LENGTH(TRIM( ' xiong er is a xiong '))

design sketch

Four , Mathematical functions

Case study

3. Number function 
   1）FLOOR： Rounding down 
   2）CEIL：  Rounding up 
   3）ROUND： rounding 
     select FLOOR(-3.14), CEIL(-3.14),CEIL(-3.14),CEIL(0.98);

design sketch

5、 ... and , Judgment function

(1).IF(expr,v1,v2)

If expr yes TRUE Then return to v1, Otherwise return to v2 -----( Row column conversion )
demand ： The extra linguistic scores of each student

  Judge ： If it is the current score, go to the current score , Use if not 0 Instead of 

     select s.sid,s.sname,
	 SUM(if(sc.cid='01',sc.score,0))  Chinese language and literature ,
	 SUM(if(sc.cid='02',sc.score,0))  mathematics ,
	 SUM(if(sc.cid='03',sc.score,0))  English 
 from 
 t_student s left JOIN t_score sc on s.sid=sc.sid GROUP BY s.sid,s.sname
	 
	 

design sketch

（2）IFNULL(v1,v2)

If v1 Not for NULL, Then return to v1, Otherwise return to v2

grammar ： CASE expr WHEN v1 THEN r1 [WHEN v2 THEN v2] [ELSE rn] END
If expr Is equal to a vn, Then return to the corresponding position THEN The latter result , If you don't want to wait with all the values , Then return to ELSE hinder rn

    select s.sid,s.sname,
	 SUM(case when sc.cid = '01' THEN sc.score end)  Chinese language and literature ,
	 SUM(case when sc.cid = '02' THEN sc.score end)  mathematics ,
	 SUM(case when sc.cid = '03' THEN sc.score end)  English 
 from 
 t_student s left JOIN t_score sc on s.sid=sc.sid GROUP BY s.sid,s.sname

design sketch

6、 ... and 、 Merge (union)

（1） UNION： Merge all the query results , Then remove the same records

（2） UNION ALL： Merge all the query results , The same records will not be removed

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

 Take only two pieces of data 
select 1,'sa',18
union
select 2,'sa',18
union
select 2,'sa',18
 
 Take all the data 
select 1,'sa',18
union all
select 2,'sa',18
union all
select 2,'sa',18