First knowledge database

Command operation ：

Access to database ：mysql -h Host name -P Port number -u user name -p password （ I don't mean to write everything , But to avoid mistakes, it's best to write , It is generally recommended to enter the next line of the password to ensure security ）

view the database ：show databases Database name ;

See the table ：show table Table name ;

Using a database ：use Database name ;

DQL:

The execution order of query statements and the keywords included in a query statement ：

  Connection type classification ： Internal connection （inner join）, External connection , Cross connect

Inner connection classification ： Equivalent connection , Non equivalent connection , Self join

External connection classification ： The left outer join (left outer), Right connection (right outer), Full outer join

The left outer join ,left join The main table on the left ; Right connection ,right join The main table on the right

The difference between internal and external connections ： Inner join is the intersection of two tables ; The external connection is the main table plus the connection part

DDL:

establish ：create modify ：alter Delete ：drop

Library management :

establish ：create database 【if not exists( Determine whether the database already exists )】 Library name ;

modify （ Generally, it is the modification of character set ）：alter database Library name character set Character set format ;

Delete ：drop database 【if exists( Determine whether the database already exists )】 Library name ;

The management of the table ：

establish ：create table 【if not exists】 Table name （ Name The type of the column 【（ length ） constraint （ Optional ）】,（ Name The type of the column 【（ length ） constraint （ Optional ）】,... ,（ Name The type of the column 【（ length ） constraint （ Optional ）】）

modify ：alter table Table name add | drop | modify | change column Name 【 Name Column type constraint 】

Delete ：drop table 【if exists】 Table name ;

Copy ：

Just copy the structure of the table , The contents in the table will not be copied ：create table Table name to copy like The name of the copied table

Copy table structure and data ：create table New table select * from The name of the copied table

Copy only part of the data ：create table New table select ...( Filter conditions and columns to be copied ) from The name of the copied table

Just copy the structure of some fields :create table New table select ... where 0（ Use 0 Express false, No data ）

DML:

Insert ：insert into Table name set Name 1= value 1, Name 2= value 2,........;

modify ：update Table name set Column 1= value 1, Column 2= value 2,.... where filter ;

Delete ： Method 1 ：truncate table Table name ;

Method 2 ：delete from Table name where filter

Common constraints ：

classification （ Six constraints ）：

not null： Non empty constraint , Used to ensure that the field cannot be empty

default The default value is ： Default , Used to ensure that the field has a default value

primary key： Used to ensure that the value of this field is unique , And not empty

unioue： only , Used to ensure that the value of this field is unique , Can be null

check（ Inspection conditions ）： Check constraint （mysql China does not support it. , But no mistake , It just didn't work ）

foreign key （ Column name from table ）references Main table name （ Name of main table ）： The associated column of the main table must be a unique column ( It's usually a primary key or unique , It can also be a foreign key , But there is repetition and it is not recommended )

Three paradigms of database ：

First normal form ： First normal form (1NF)： Attributes are inseparable , That is, each attribute is an indivisible atomic item . It is not allowed that attributes can be further divided

Second normal form ： Second normal form (2NF)： Satisfy the first paradigm ; And there are no partial dependencies , That is, the non primary attribute must be completely dependent on the primary attribute . example ： There are dependencies between primary keys and non primary keys . But there is no dependency between non primary keys

Third normal form ： Third normal form (3NF)： Satisfying the second paradigm ; And there is no transitive dependency , That is, non primary attributes cannot have dependencies with non primary attributes , Non primary attributes must be directly dependent on primary attributes , You can't depend on the primary property indirectly .
Non primary keys must directly depend on primary keys , Non primary keys cannot indirectly depend on primary keys

Isolation level classification ：

read uncommitted： Dirty reading will occur , It can't be read repeatedly , Fantasy reading

read committed： Avoid dirty reading , There will be non repeatable reading , Fantasy reading

repeatable read： Avoid dirty reading , It can't be read repeatedly , Fantasy reading

serializable： Avoid dirty reading , It can't be read repeatedly , Fantasy reading , But the performance is low

The transaction ACID attribute ：

1. Atomicity （Atomicity）： Atomicity means that a transaction is an indivisible unit of work , Either the operations in the transaction occur , Or none at all .

2. Uniformity （Consistency）: Transactions must transform data from one consistency state to another .

3. Isolation, （Isolation）: Transaction isolation means that the execution of one transaction cannot be interfered by other transactions , That is, the operations and data used within a transaction are isolated from other concurrent transactions , Transactions that execute concurrently cannot interfere with each other .

4. persistence （Durability）： Persistence means that once a transaction is committed , Its changes to database data are permanent , Other subsequent operations and database failures should not have any impact on it .