Mysql---- function

Catalog

Preface

One 、 Definition of function

Two 、 Common functions

        1、 Date function

        2、 String function

        3、 Number function

3、 ... and 、 Conditional judgment function

Four 、 Aggregate functions

5、 ... and 、 Merge

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Preface

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One 、 Definition of function

1、 Be similar to Java How to define

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

4、 Functions can be nested in SQL In the sentence  

Be careful ： Stored procedures cannot

5、 Functions can be called nested round(avg())

Two 、 Common functions

        1、 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()

DATE_FORMAT()： Convert date to string
 

 

 

    STR_TO_DATE()： Convert string to date

 

 

 

        2、 String function

   1) UPPER()/LOWER()： Case to case

UPPER()

LOWER()

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

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

Be careful ： Database subscript from 1 Start

   4) TRIM()： Remove the space before and after

   5) LENGTH()： Get string length

   6) CONCAT()： Merge string functions , The return 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

 

 

 

3、 ... and 、 Conditional judgment function

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

                 Before using ：

                After using ：

 

 

   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

     

 

 

Four 、 Aggregate functions

   1) SUM()： Sum up . Often with GROUP BY Use it together , It can also be used alone

   2) AVG()： averaging . Often with GROUP BY Use it together , It can also be used alone

   3) MAX()： For maximum . Often with GROUP BY Use it together , It can also be used alone

   4) MIN()： For the minimum . Often with GROUP BY Use it together , It can also be used alone

   5) COUNT()： Count the number of records . Often with GROUP BY Use it together , It can also be used alone

 

 

5、 ... and 、 Merge

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
        Use scenarios ： In the project statistical report module , Used to merge data