SQL basics 1

SQL First time to know - database 、SQL And data table

Row and primary key

The columns in the table are used to store certain types of information , For example, customer information table , You need to create multiple columns to save multiple attributes , Used to record data . As for the details of each customer , Will take That's ok Is written to the table .

The primary key value is a unique identifier for each row . To ensure that each row can be assigned its own primary key value , Primary key column No null values are allowed , Duplicate values are not allowed .
For the data in the table , The primary key is the only location and record , Other tables may also need to reference the primary key column of this table . therefore , Primary key Modification and deletion are not allowed , Otherwise, references and queries will become invalid .
Because of this , We do not recommend using business-related columns as primary keys , Such as mobile phone number , It seems to be non repetitive and unique to each user , But the mobile phone number may also be modified . In case something similar happens , It's a big project that breaks bones and muscles .
Usually , We all use numbers or ID A primary key , For example, the primary key of the customer table is the customer number , The order table shows the order ID, The employee table shows the job number .

DB、DBMS、SQL

database (DB)、 Database management software (DBMS) and Structured query language (SQL) These three basic concepts , The relationship among the three

SQL It is used to deal with relational databases . The relational database contains multiple two-dimensional tables , Each table consists of rows and columns

A single table describes the relationship between things and attributes , The connection between multiple tables describes the relationship between many different things .

It is different from the forms we usually set up casually , The tables in the database have the following two characteristics ：

each Column There are specific data type ;
each That's ok Have a unique logo ： Primary key