SQL learning notes (01) - basic knowledge of database

One 、 What is database technology

1. Scientific organization and storage of data , Efficient data acquisition and processing
2. SQL Language structured query language （ Additions and deletions ）

Two 、 The basic architecture of the database

1. data （data）
   Definition ： A symbolic representation of an objective thing , Such as graphical symbols , Numbers , Letters and so on . Data is the basic object of database storage .

2. database database (DB)
   Definition ： Long term storage in a computer , Organized , Sharable data set .

3. Database management system （DBMS）

Database Define functions ：DDL Language （ Database definition language ）, Three statements are provided ：create( establish ),alter( modify ）,drop ( Delete ), The object of the operation is the basic architecture in the database ---- surface .
Database Operation function ：DML Language （ Database operation language ）, Realize the operation of adding, deleting, modifying and querying the data stored in the database ( insert , delete , update , select )
Database Protection function ： Protecting data integrity and Security
Database Maintenance function ： Database table structure and data maintenance function

4. Database system （DBS）
   Database system （DBS）= database （DB）+ Database management system （DBMS）+ Application system + Database administrator （DBA）+ user
   Three stages of database system ：

	Mesh database	Hierarchical database	relational database
How data is organized	Mesh model	Tree model	relational model
framework	Graphic structure	A tree structure	surface

3、 ... and 、 Three paradigms of database

1. First normal form

If the relationship R All properties of are simple properties ： Each attribute cannot be further divided , said R Satisfy the first paradigm , In fact, there are no duplicate columns . surface 1 Not satisfied with the first paradigm .

surface 1

surface 2 To meet the requirements of the first paradigm , But the data redundancy is too large , The second paradigm is needed .

surface 2

2. Second normal form

If the relationship R First satisfy the first paradigm , Non primary key fields depend entirely on the primary key , said R Satisfying the second paradigm .
example ：
There is an express order form , If the same person sends multiple couriers at the same time , In this case, the sender's name is duplicated , It will cause data redundancy , At this time, we should split it up . As shown in the table 3

surface 3

The order sheet ：

surface 4

Name and number table ：

surface 5

surface 4 And table 5 For the two split tables , Meet the requirements of the second paradigm .

3. Third normal form

If the relationship R Satisfying the second paradigm , There is no dependency between non key fields , That is, any field cannot be derived from other fields , It requires that the fields are not redundant , said R Meet the third paradigm .

A basic relational data must satisfy the first normal form , A complete relational database should meet the third paradigm .

Four 、SQL The classification of sentences

1.DQL Data query language : select sentence
2.DML Data operation language : insert update delete sentence
3.DDL Data definition language : create alter drop sentence
4.DCL Data control language : to grant authorization grant Right to receive revoke
5.TCL Transaction control language : commit Submit rollback Rollback or undo savepoint Set the savepoint