Databases are everywhere in our life , Whether you look up your name in email , Search on the website or use it at the ATM ATM card , We have been using databases .

        A database is a collection of data stored in an organized way , It's a container for organized data （ Usually a file or group of files ）.

Catalog

  One . surface （table）

Two . Column （column） And data types

3、 ... and . That's ok （row）

Four . Primary key （KEY）

  One . surface （table）

   If the database is regarded as a file cabinet . When we put information in the filing cabinet , We can't put , Otherwise, it will be hard to find in the future , We need to set the classification of documents , Put the corresponding data into a specific file .

  In the database , This file is called a table . Is a structured list of certain types of data . It can be regarded as a set of specific partitions . The nature of the same table should be the same , The data stored in the table searches for the same type of data or list . Like in school , We should build two different tables for teachers and students , Otherwise, it will be difficult to retrieve and access in the future .

   Each table in the same database has a unique table name , However, the same table name can be used in different databases .

  How data is stored in tables , What data to store . How to decompose data . Are described by patterns . Schemas are information about the layout and characteristics of databases and tables .

Two . Column （column） And data types

  Table consists of columns , Columns store some information in the table . A column is a field in a table .

  In fact, columns can be said to be obtained by decomposing data , For a student object , full name , Gender , Student number , Are the corresponding columns . By decomposing data , We can use Specific columns classify and filter data .

  Each list has a corresponding number According to the type , It limits the kind of data stored in this column . If there are numbers in the column , The data type should be numeric .

  What's the use of data types ？

  It can prevent illegal input , For example, enter character values in the numeric field . It also helps to classify data correctly , Optimize the use of disks .

3、 ... and . That's ok （row）

The data in the table is stored by row . If the column is the attribute of the object storing this table , So every The row is the corresponding object , Such as storing students , List names , Student number , Gender , Then every line is a student .

Four . Primary key （KEY）

   A column of （ Or a set of columns ）, Its value uniquely identifies each row in the table . For example, the student number of each student , Everyone's ID card . There is no primary key , It is difficult to update and delete specific lines , Because you can't guarantee not to involve other industries , For example, delete students by name , You can't guarantee whether someone has a duplicate name . Defining a primary key is conducive to future data operation and management .

  Want a column as the primary key , It should meet the following conditions ：

1. The primary key of each row is unique .

2. Each row has a primary key .

3.. The value of primary key cannot be modified .

4 Primary key values cannot be reused （ If it's deleted , Its primary key cannot be assigned to new rows in the future ）

A primary key can be a column , You can also combine multiple columns as primary keys , The combination of all columns must be unique （ But a single column can not be unique ）.