SQL stands for Structured Query Language.
SQL is a language for Computer to manage, process and retrive large amount of data stored in relational database.
By using SQL you can execute queries against database, insert new data, update data, can delete and modify data.
SQL queries are classified as
Data Definition Language (DDL)
The standard for group of commands which are used to define structures in a database are called Data Definition Language (DDL). DDL statements such as create, alter and remove are used on database tables, users and indexes. Some of the common DDL statements are CREATE, ALTER and DROP.
Create Command : Command use to create databases and tables.
Alter Command : Command use to alter, modify, change tables.
Drop Command : Command use to drop database and tables.
Data manipulation language (DML)
The Standard for group of commands which are used to manipulate data in database tables, indexes are categorized as Data Manipulation Language (DML). Sometimes read-only command such as select are also comes under DML. Other commands are Insert, Update and Delete.
Select Command : Command use to select some or all rows and columns of data.
Insert Command : Command use to insert new data in a table.
Update Command : Command use to update existing data in a table.
Delete Command : Command use to remove some or all of the existing data in a table.
*** CRUD : CRUD is SQL term which stands for Create, Read, Update and Delete.
Data Control Language (DCL)
The Standard for group of commands which are used to control access on data stored in database basically user controls are categorized as Data control Language. Who can access what in database.
GRANT Command : SQL Command for granting permissions to users so that a specified user can only perform specified tasks..
REVOKE Command : SQL Command for canceling permissions which were previously granted or denied.
RDBMS : Relational Database Management System. In RDBMS collection of data is stored in Databases in the form of objects called tables. A table is a collection of data organized in rows and columns.
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Incorrect table definition there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause