Database application

alter table Existing table name rename The new name of the table ： Change database table name

alter table Table name change Name New column names data type ： Change column names

alter table Table name add Name data type ： Add fields

alter table Table name drop Name ： Delete field

Primary key ： Determine the uniqueness of the data

How to add a primary key

create table Table name （

        id int

        ......

        primary key(id)

）;

create table Table name （

        id int primary key,

....

）

alter table Table name add primary key（id）;

The primary key increases automatically

create table Table name （

id int auto_increment;

primary key(id);

）

alter table Table name modify Primary key column name type auto_increment

Foreign keys

create table student1 (

    id int ,

    name varchar(20),

    teacher_id int,

    primary key (id)

);

create table teacher1(

    id int ,

    name varchar(20),

    primary key (id)

);

 alter table Table name add foreign key ( Foreign key column name ) references Table name pointed to ( Primary key column name );、、

conditional

where Judge the condition

and also or perhaps   and The priority ratio or The height of

The statement to determine whether it is empty is is not null;

between......and......

between The number 1 and The number 2;： In numbers 1 And numbers 2 direct Include

in Specify data

Fuzzy query like

% Is to match any character

_ Match a single character   If you want to query % perhaps _ When Need to add \ To escape

order by Name asc（ Ascending ）/desc（ Descending ）

limit  Usually and order by Together with limit Starting value （ barring ）, Number of pieces

Group keywords group by

Common functions are ：

count（*） Total number of articles

max（ Field name ） Maximum

min（ Field name ） minimum value

avg（ Field name ） Average

sum（ Field name ） The sum of the

having Filter   You can do it where It can't be done

having avg>70; You can filter that the average value is greater than 70 Points of

Common functions ：

select version() ; Show the current MySQL Version of the software
select database(); Displays which database you are currently in
select  char_length(' China '); Return the number of characters .
select  length(' China '); Returns the number of bytes occupied by characters ,MySQL in , One UTF8 Coded Chinese characters account for 3 Bytes
select  concat(  'a',  'b',  'c',  'd'); return  'abcd'. String concatenation function
select  concat_ws(  '=',  'a',  'b',  'c'); return  'a=b=c'. String concatenation function , The first is the splice spacer
select   upper('abcd'); return ABCD. Converts all lowercase letters in the parameter to uppercase
select  lower('ABCD'); return abcd. Converts all uppercase letters in the parameter to lowercase
select  substring(  ' System information class ',  1,  3  ); return   System letter . The first 2 Parameters represent from 1 The first few characters , The first 3 Parameters represent the number of intercepted characters
select  trim('  abc  '); return abc. Used to delete all spaces around the parameter
select  curdate(); Return current date
select  curtime(); Return current time
select  now(); Returns the current date time
select  unix_timestamp(); Returns the timestamp corresponding to the current date and time （ Unit second ）
select  unix_timestamp('2018-05-24 20:00:00'); Returns the timestamp corresponding to the date and time specified by the parameter （ Unit second ）
select  from_unixtime(1527163397); Return the time stamp specified by the parameter （ Unit second ） Corresponding date and time
select  datediff(  '2018-05-23',  now()  ); Returns the number of days between the corresponding dates of the two parameters （ Subtract the second parameter from the first parameter ）
select  adddate( now(), -2 ); Returns the number of days before / Date and time after （ The first parameter is date time , The second parameter is the number of days , Adding back is a positive number , Subtracting forward is a negative number ）
select year('2019-02-24'); return 2019 Get the year
select month('2019-02-24')   return 2 Get the month
select day('2019-02-24')   return 24 Acquisition date
select  if(  < Judge the condition >,  < The return value when the condition is true >,  < The return value when the condition is false >  ); amount to Java The binomial operator in < Judge the condition >  ?  < If the condition is true, the return value >  :  < If the condition is false, the return value >.
Such as select  if(1=1,  2,  3); return 2.
select  ifnull(< Expression or field >,  < The expression or field is NULL The return value of >); Usually used to give the possibility of NULL Provide default values in case of .
select ifnull(null,' John doe ') ; null You can write column names here The value of this column will be null Of Show as anonymous
select ifnull(name,' John doe ') from teacher ;