MySQL (III)

1. A function is a program or code that can be called directly by another program .

2. String function

MySQL There are many built-in string functions , The common ones are as follows :

CONCAT（S1,S2,...Sn) String splicing , take S1, S2,  Sn Stitching into - A string

LOWER(str) The string str All to lowercase

UPPER(str) The string str Capitalize all

LPAD(str,n,pad) padding-left , Use string pad Yes str Fill the left side of the , achieve n String length

RPAD(str,n,pad) Right fill , Use string pad Yes st Fill the right side of the , achieve n String length

TRIM(str) Remove the spaces at the beginning and end of the string

SUBSTRING(str,start,len) Return from string str from start It's from the position len A string of length

3. Numerical function

Common numerical functions are as follows :

CEIL(x) Rounding up

FLOOR(x) Rounding down

MOD(x,y) return x/y The mold

RAND) return 0~1 The random number in

ROUND(x,y) Find parameters x Rounded value of , Retain y Decimal place .

4. Date function

Common date functions are as follows :

CURDATE() Return current date

CURTIME() Return current time

NOW() Returns the current date and time

YEAR(date) Get specified date A year of

MONTH(date) Get specified date Month of

DAY(date) Get specified date Date

DATE_ADD(date,INTERVALLexpr type) Return a date / Time value plus a time interval expr Time value after

DATEDIFF(date1,date2) Return to start time date1 And end time date2 Days between

5, Flow function

Flow function is also a kind of commonly used function , Can be in SQL Statement to implement conditional filtering , So as to improve the efficiency of statements .

IF(value ,t,f) If value by true, Then return to t, Otherwise return to f

IFNULL(value1 ,value2) If value1 Not empty , return value1, Otherwise return to value2

CASE WHEN [val1] THEN [res1] …ELSE [default] END If val1 by true, return res1, . Incense returns default The default value is

CASE [expr] WHEN [val1] THEN [res1] …ELSE [ default] END If expr The value is equal to the val1, return res1, … Otherwise return to default The default value is

6. summary

1. Concept : Constraints are rules that act on fields in a table , Used to limit the data stored in a table .

2. Purpose : Ensure that the data in the database is correct 、 Effectiveness and integrity .

3. classification :

Non empty constraint   Limit the data of this field to null  NOT NULL

Unique constraint   Ensure that all data in this field is unique 、 Not repeated  UNIQUE

Primary key contract   A primary key is the unique identifier of a row of data , It is required to be non empty and unique  PRIMARY KEY

Default constraint   When saving data , If the value of this field is not specified , The default value DEFAULT

Check the invitation card （8.0.16 After the version )  Ensure that the field value meets a certain condition CHECK

Foreign key invitation   Used to establish a connection between the data of two tables , Ensure data consistency and integrity FOREIGN KEY

Be careful ： Constraints act on fields in a table , You can create a table / Add constraints when modifying tables

7.

8.

9. Foreign key constraints

grammar

Add foreign keys

CREATE TABLE Table name (

Field name data type

       …

[CONSTRAINT] [ Name of the foreign key ] FOREIGN KEY( Foreign key field name ) REFERENCES Main table ( Make a list )

);

ALTER TABLE Table name ADD CONSTRAINT Name of the foreign key FOREIGN KEY( Foreign key field name ) REFERENCES Main table ( Make a list );

Delete foreign key

ALTER TABLE Table name DROP FOREIGN KEY Name of the foreign key ;

Delete / Update behavior

NO ACTION When deleting in the parent table / When updating the corresponding record , First, check whether the record has a corresponding foreign key , If there is, it is not allowed to delete / to update .( And RESTRICT Agreement )

RESTRICT When deleting in the parent table / When updating the corresponding record , First, check whether the record has a corresponding foreign key , If there is, it is not allowed to delete / to update .( And NO ACTION- Cause )

CASCADE When deleting in the parent table / When updating the corresponding record , First, check whether the record has a corresponding foreign key , If there is , Then delete / Update the record of the foreign key in the sub table .

SET NULL When deleting the corresponding record in the parent table , First, check whether the record has a corresponding foreign key , If yes, set the external key value in the sub table to null ( This requires that the foreign key be allowed to take nulll).

SET DEFALT When the parent table changes , Sub table will