This article mainly introduces the mysql relational database to everyone. I hope it will be helpful to friends in need!

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Relational database

1 Definition

Relational database,is a database built on a relational model (mathematical model).

As for the relational model, it is a so-called model based on relationships, which includes three aspects, namely:

• Data structure: data Storage form, two-dimensional table (rows and columns);

• Operation instruction set: all SQL statements;

• Integrity constraints: Data constraints within tables (fields and fields) and constraints between tables (foreign keys).

2 Design

• Database: Analyze the data requirements that need to be stored. If it is a type of data (entity), it should be designed as Two-dimensional table;

• Two-dimensional table: consists of header (field name, used to specify the name of the data) and data (actually stored content).

Two-dimensional table example:

##ID EEROR_CODE DESCRIPTION ##1 2 3 Here, if the corresponding field value in the table is empty, the system will still allocate storage space for it, which is why relational databases waste space. !

US0001	This user is not registered
OR0001	Order generation failed
SY0001	Unknown system exception

3 Keyword Description

DB: Database, database;
DBMS: Database Management System, database management system;
DBS: Database System = DBMS DB, database system;
DBA: Database Administrator, database administrator.
Row\record:
• row\record

  , essentially refers to a row (a record) in the table. A row is from a structural perspective, and a record is from a structural perspective. From a data perspective.

Column\Field:
• column\field

  , essentially refers to a column (a field) in the table, the column is from the structural point of view, and the field is from From a data perspective.

4 SQL

SQL: Structured Query Language, structured query language (data is mainly query, 99% are query operations).

SQL is mainly divided into three types:

DDL: Data Definition Language, data definition language, used to maintain the structure of stored data (database, table), represents instructions For
• create

  ,dropandalteretc.

DML: Data Manipulation Language, data manipulation language, used to operate data (content in the table). The representative instructions are
• insert

  ,deleteandupdate, etc., but there is a separate classification within DML, namely DQL (Data Query Language), data query language, the representative command isselect.

DCL: Data Control Language, data control language, mainly responsible for (user) rights management, the representative instructions are
• grant

  andrevoke, etc.

SQL is an operation instruction for a relational database. It is a constraint, but not mandatory, similar to W3C, so this means: internal to different database products (such as Oracle and MySQL) There may be some subtle differences.

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