[MySQL database learning]

MySQL database

1、 Connect to database ：

mysql -u root -p
123456

2、 Create database ：

create database Database name （ In letters , Numbers , String of underscores , But don't start with a number ）

3、 view the database ：

show databases

4、 View character set ：

show variables like ‘character%’

5、 View port number ：

show variables like ‘port’

6、 View the data storage path ：

show variables like ‘datadir'

MySQL Database table operation

1、 Create table

create table test(id int(11),name varchar(50));
create table student(sno char(11) primary key,sname varchar(20) not null);

2、 Create table test

use demo
create table test(id int(11),name varchar(50));

3、 Create a table student

create table student(sno char(11) primary key,sname varchar(20) not null);

4、 Create table course

mysql> create table course
    -> (
    -> cno varchar(20) primary key,
    -> course_name varchar(50) not null,
    -> cpno varchar(20),
    -> course_credit decimal(4,1)
    -> );

5、 increase （ Add new fields to the table ）

mysql> alter table student
    -> add ssex char(2) not null
    -> ;

6、 Change （ Set default values for fields in the table ）

mysql> alter table student
    -> alter ssex set default ' male '
    -> ;

7、 Change table name

Law 1

mysql> rename table test to test2;

Law 2

mysql> alter table test2
    -> rename to test;

8、 change student In the table ssex The data type of is enumeration type

（‘male’,'female'）, The default value is ‘male’

mysql> alter table student
    -> modify ssex enum('male','female') not null;

9、 Change the type of the existing field

mysql> alter table student
    -> alter ssex set default 'male';

10、 Add primary key

mysql> alter table test
    -> add primary key(id)
    -> ;

11、 Add foreign keys

mysql> create table sc(sno char(11),
    -> cno varchar(20),
    -> grade decimal(6,2)
    -> );

12、 Combine with multiple keywords

mysql> alter table sc
    -> add foreign key(sno) references student(sno);

mysql> alter table sc
    -> add foreign key(cno) references course(cno);

mysql> alter table sc
    -> add primary key(sno,cno);

DDL( Operations on table structure )：create alter drop

DML（ Table content ）:insert update delete

DQL:select * from Table name #“*” Equivalent to projection

1、 Add data to the table

insert into Table name ( Field 1, Field 2,...) values( value 1, value 2,...)

mysql> insert into test(id,name) values(1,' Zhang San ');
mysql> insert into test(id) values(2);
mysql> insert into test(id,name) values(3,'li');

Shorthand method ： Omit the field section , Values need to correspond to fields one by one

mysql> insert into test values(4,' Wang Wu ');（√）

mysql> insert into test values(' Wang Wu ',4);（×）

Omit fields , And only give part of the value

mysql> insert into test values(5,null);

Method of adding multiple values at a time ：

mysql> insert into test(id) values(6),(7),(8),(9),(10);

mysql Own statements to add records

mysql> insert into test
    -> set id=11,
    -> name=' Wang Meili ';

insert into surface 2 select * from surface 1;

mysql> create table demo like test;
mysql> insert into demo select *from test;

2、 Modification table record

1、 Modify the table id by 2 It's worth it

mysql> update test set name='jerry' where id=2;

2、 modify test Add a age Column , The default value is 20

mysql> alter table test
    -> add age int(3) not null default 20;

3、 Change the two values in the table to use or Connect

mysql> update test set age=20 where id=5 or id=7;

4、 Modify multiple values under the same condition , Simultaneous change name and age

mysql> update test
    -> set name='tom',
    -> age=23
    -> where id=6;

5、 Modify multiple tables , Modify both tables at the same time id by 8 Of name

mysql> update test,demo1
    -> set test.name=' Qin Jianxing ',demo1.name=' Qin Jianxing '
    -> where test.id=8 and test.id=demo1.id;

6、 Delete record

mysql> delete from test where id=10;  # Recycling 

Truncate table Table name /* Delete completely , Deleted data cannot be recovered

Add ：replace Delete a data and send it into the table  /* Relatively low security

mysql> replace into test(id,name,age) values(1,' Wang Wu ',24);

MySQL Indexing and integrity constraints

mysql> create table test2(id int primary key);# Column level constraints 
mysql> create table test3(id int,primary key(id));# Table level constraints 

1、 Create a normal index （ Attribute values can be repeated ）

mysql> create index idx_name on test(name) ;

mysql> desc test;
mysql> show index from test;

1. Create unique index （ Attribute values cannot be repeated , More suitable for candidate code ）

mysql> create unique index idx_age on test(age);

3、 When the table does not exist , Can be created as ：

mysql> create table test4(id int,name varchar(10),
    -> primary key(id),
    -> unique(name)  # Create a unique index while creating a table 
-> );

mysql> insert into test4 values(1,'tom');（√）

mysql> insert into test4 values(2,'tom');（×）

Compare several indexes

1. Only one index can be created for a table ： Primary key （ Indexes ）
2. A table can create multiple ordinary or unique indexes
3. The attribute column value created as an index must be unique ： Primary index and unique index , The values that can be repeated are ordinary indexes

Name the index

mysql> create table test5(id int,name varchar(10),
    -> constraint mypri primary key(id),
    -> constraint myuiq unique(name)
-> );

Delete index （ You can index names more , Delete operation ）：

Delete primary key （ Indexes ）, There is no need to use a name

mysql> alter table test5 drop primary key;

Delete other indexes , Name required

mysql> alter table test5 drop index myuiq;

Or is it

mysql> drop index myuiq on test5;