MySQL built-in functions

Catalog

1. Date function

2. String function

3. Mathematical functions ( Less use )

4. Other functions  

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1. Date function

                 

 (1) Function USES

• Get date ：

• Obtain hours, minutes and seconds ：

                 

• Get timestamp ：

                 

• Add a date to the date ：

                 

• Subtract the time from the date ：

                 

• Calculate the number of days between two dates ：

                 

(2) Case study 1: Create a table , Record birthday

                 

(3) Case study 2: Create a message table , And insert data  

                 

① Show all message messages , The release date only shows the date , Don't show the time  

                 

 ② Please check in 2 Posts posted within minutes

                

                

2. String function

                 

(1) Use cases

① obtain msg Tabular name Character set of column  

                 

②  Ask to show exam_result Information in the table , Display format ：“XXX My language is XXX branch , mathematics XXX branch , English XXX branch ”

                         

③ Find the number of bytes occupied by students' names in the student table  

• Be careful ：length Function returns the length of the string , In bytes . If it is a multi byte character, calculate the number of bytes ; If it is a single byte character, it is counted as a byte . such as ： Letter , The array counts as a byte , Chinese means multiple bytes （ Related to character set encoding ）

                 

④  take EMP There are... In all the names in the table M Of is replaced by ' Rice noodles '

                

⑤ Intercept EMP In the table ename The second to third characters of the field  

                 

⑥  Display the names of all employees in lowercase

                 

(2) Other tests

• instr

 

• left 

 

•  strcmp（ Case insensitive ）

 

•  ltrim,rtrim,trim

                 

                

 3. Mathematical functions ( Less use )

                 

① The absolute value

         

② Rounding up ( Round to positive infinity in the number axis )

                 

③ Rounding down ( Round to negative infinity in the number axis )

         

④ Keep two decimal places ( Round the decimal )

         

⑤ Generate random numbers ,rand() What comes out randomly is <0 Number of numbers

                  

                                

4. Other functions  

①user() Query current user

 

                 

②md5(str) To a string md5 Abstract , After the summary, we get a 32 A string

• Generally in mysql in , Save the user password without clear text ,
• Benefits of using summaries ：
• 1.mysql There is no clear text password , Even inside the company DBA( Database administrator ), You can't view the password
• 2. The password is fixed ！（ The database table for storing passwords is easy to design ）
• When I first registered my account , The database encrypts the password into a fixed length string and saves it in the database , After logging in again, you only need to encrypt the password entered by the user again and compare it with the string stored in the database to determine whether the password is correct

                 

③database() Displays the database currently in use

                 

④password() function ,MySQL The database uses this function to encrypt users

                 

⑤ifnull（val1, val2） If val1 by null, return val2, Otherwise return to val1 Value