Database base (Study & review for self use)

Let's start with a brain map ：

One ： The four operations of database （ Add or delete check change ）

1: Library operation

1: Create database
sentence ：create database Library name （ You can add character set settings ：character set utf8）
2: view the database
sentence ：show databases
3: Delete database
sentence : drop database Library name
4: Change database
sentence : alter database Library name + Operation of the library
5: Using a database
sentence : use Library name

2: Table operations

1: Create table
sentence ：create table Table name （ You can add character set settings ：character set utf8）
2: See the table
sentence ：show table
3: Delete table
sentence : drop table Table name
4: Change table
sentence : alter table Table name + Operation of the watch

3: Data manipulation

1: insert data ( increase )
sentence ：insert into Table name ( Field ) values( value )
2: Delete data
sentence ：delete from Table name where Conditions
3: Modifying data
sentence : update Table name set Field = value where Conditions
4: Query data
sentence : select Field from Table name where Conditions

Two : Table joins

1: Internal connection
key word :inner join
grammar : select Field from surface 1 inner join surface 2 where Conditions

2: The left outer join
key word :left join
grammar : select Field from Main table left join From the table where Conditions

3: Right connection
key word :right join
grammar : select Field from From the table right join Main table where Conditions

4: Subquery
No keyword
grammar :select Field from Parent table where Field Operator (select Field from Sub table where Conditions )

exists Subquery : The return value is of boolean type
grammar :select Field from Parent table where exists (select Field from Sub table where Conditions )
If returns true, Outer query execution ; if false, The outer query does not execute

notes : The return value of subquery cannot be more than 1 individual
Subqueries can be used properly instead of table joins

3、 ... and : Add

1: Fuzzy query

key word :like
Key characters :
[ ]: contain
^: Remove
%: Any character of any length
_: Any character

example :select name from student where name like ‘ Zhang %’;
That is to query the name of the student surnamed Zhang in the student table

2: Remove duplicate data

key word :distinct
example :select distinct name from student ;
Look up the student's last name in the student table , Remove duplicate names

3: Sort

key word :order by
Ascending order :abs( By default, the system also arranges in ascending order )
Descending order :desc

example :select id,name fromstudent order by id asc( Omission );
Look up the student's name and number in the student table , And arrange them in ascending order

select id,name fromstudent order by id desc;
Look up the student's name and number in the student table , And arrange them in descending order

notes : If you want to sort by the phonetic alphabet of names , You need to modify the character set when querying
example :select id,name from student order by convert (name using gbk);
Look up the student's name and number in the student table , And arrange them in ascending order according to the phonetic alphabet of the names

4: Limit

key word :limit Limit the number of pieces of data queried
example : select id,name from student limit 0 ,5;
Query the data of the first five items in the student table

5: Aggregate functions

1:avg() averaging
2:count() Find the total
3:max() For maximum
4:min() For the minimum
5:sum() Sum up
notes : Aggregate functions must be used with grouped queries

6: grouping

key word :group by
Sentence order :
where > group by > having

select  
...
from
...
where 
...
group by
...
having
...
order by
...

7: Specify query

key word :in
Multiple pieces of data can be queried , As long as the conditions are met, you can query and return
example ：select id,name from student where id in(4,5,6);
Query the student table id by 4,5,6, Number and name of the student

8: Logical operators

And :and
or :or
Not :not

~~ There is still a lot of knowledge about databases , This article only wants to use when reviewing , If there is any mistake or incompleteness , Also please indicate , Correction with an open mind , Thank you very much ！