Summary of basic SQL knowledge and summary of SQL

Summary of basic SQL knowledge and summary of SQL

1. Main Types of SQL statements

(1) DDL: Data Definition Languages, Data Definition Language. Common statement keywords mainly include create, drop, alter, etc;

(2) DML: Data Manipulation Language, Data operation statement. Common statement keywords include insert, delete, update, and select;

(3) DCL: Data Control Language, Data Control Language. Common statement keywords include grant and revoke.

2. Difference between UNION and UNION ALL

(1) union all directly merges result sets without deduplication;

(2) UNION performs a distinct operation on the results after union all to remove the results after repeated records.

3. Functions of the MySQL built-in Database

(1) information_schema: It stores information about some database objects in the system, such as user table information, column information, permission information, character set information, and partition information;

(2) mysql: stores the user permission information of the system;

(3) test: The test database automatically created by the System

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