MySQL functions

Catalog

One 、 Date function

Two 、 String function

3、 ... and 、 Conditional judgment function

Four 、 Merge functions

5、 ... and 、 Aggregate functions

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One 、 Date function

1) NOW()： Get the current date and time
2) DAY()： Get the day in the date
3) MONTH()： Gets the month in the date
4) YEAR()： Gets the year in the date
5) Date conversion function DATE_FORMAT()/STR_TO_DATE()

        ① NOW()： Get the current date and time

select now();

        ②DAY()： Get the day in the date

select day(now());

        ③MONTH()： 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()

DATE_FORMAT()： Convert date to string 
select now(),DATE_FORMAT(now(),'%Y year %m month %d Japan ')
STR_TO_DATE()： Convert string to date 
select STR_TO_DATE('2022-12-12','%Y-%m-%d')

          Case a ： Check the age of each student , Only by year

select s.*,YEAR(now())-year(s.sage) from t_student s

          Case 2 ： 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

 

         Case three ： Check out the students who have birthdays this month （ If not, it will be displayed as NULL）

select * from t_student where MONTH(now()) = MONTH(sage);

          Case four ： Check out the students who have their birthday next month （ The two methods ）：

select * from t_student where MONTH(now())+1 = MONTH(sage);
select * from t_student where date_add(NOW(), interval 1 MONTH) = MONTH(sage);

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Two 、 String function

2. String function  
   1) UPPER()/LOWER()： Case to case
   2) REPLACE()： Search and replace substrings in a string
   3) SUBSTRING()： The last substring starting at a position with a specific length
   4) TRIM()： Remove the space before and after
   5) LENGTH()： Get string length
   6) CONCAT()： Merge string functions , The result is the string generated by the connection parameter , Parameters can be one or more

3. Number function
   1）FLOOR： Rounding down
   2）CEIL： Rounding up
   3）ROUND： rounding

        ① UPPER() / LOWER()： Case to case

select upper(' beautiful girl ')
select lower(' GOOD  MAN')

 

        ②REPLACE()： Search and replace substrings in a string

select 'zhangqiag',REPLACE('zhang-qiag','-',' ');

         ③ SUBSTRING()： The last substring starting at a position with a specific length

select SUBSTRING('zhang qiang na li',2,5)// From the beginning to the end 
select SUBSTRING('zhang qiang na li',5)// From the first few , Until the end 

        ④ TRIM()： Remove the space before and after
        ⑤ LENGTH()： Get string length

select LENGTH('  zhang qiang na li'),LENGTH(trim('  zhang qiang na li   '))

        ⑥CONCAT()： Merge string functions , The return result is the string generated by the connection parameter , Parameters can be one or more

select CONCAT('zhangqiang111','nali222');

        ⑦FLOOR： Rounding down
        ⑧CEIL： Rounding up
        ⑨ROUND： rounding

select FLOOR(-3.14),FLOOR(3.14), CEIL(-3.14), CEIL(3.14),round(-3.14),round(3.14)

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3、 ... and 、 Conditional judgment function

   1）IF(expr,v1,v2)    If expr yes TRUE Then return to v1, Otherwise return to v2

   2）IFNULL(v1,v2)    If v1 Not for NULL, Then return to v1, Otherwise return to v2

   3）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

        ①IF(expr,v1,v2)    If expr yes TRUE Then return to v1, Otherwise return to v2

        ②IFNULL(v1,v2)    If v1 Not for NULL, Then return to v1, Otherwise return to v2

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

        ③case  ...when....

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

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Four 、 Merge functions

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

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

select 1,'zs',18
union
select 2,'ls',22
union
select 2,'ls',22

         

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

 select 1,'zs',18
union all
select 2,'ls',22
union all
select 2,'ls',22

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5、 ... and 、 Aggregate functions

    Aggregate functions (SUM/COUNT/AVG/MAX/MIN)

SUM()： Sum up . Often with GROUP BY Use it together , It can also be used alone
AVG()： averaging . Often with GROUP BY Use it together , It can also be used alone
MAX()： For maximum . Often with GROUP BY Use it together , It can also be used alone
MIN()： For the minimum . Often with GROUP BY Use it together , It can also be used alone
COUNT()： Count the number of records . Often with GROUP BY Use it together , It can also be used alone
I shared it last time, and I'll mention it this time ！