SQL COMMANDS

 

 What is SQL?

SQL (Structured Query Language) is a standard language used to manage and manipulate relational databases. It is used in database systems like:

  • MySQL

  • PostgreSQL

  • Oracle Database

  • Microsoft SQL Server

SQL allows users to create databases, insert data, update records, retrieve information, and control access.

🔹 Types of SQL Commands

SQL commands are divided into five main categories:

  1. DDL – Data Definition Language

  2. DML – Data Manipulation Language

  3. DQL – Data Query Language

  4. DCL – Data Control Language

  5. TCL – Transaction Control Language

🔹 1. DDL (Data Definition Language)

DDL commands are used to define and manage the structure of database objects like tables.

✅ CREATE

Used to create new databases, tables, views, or other database objects.

✅ DROP

Used to permanently delete database objects such as tables or databases.

✅ ALTER

Used to modify the structure of an existing table (add, delete, or modify columns).

✅ TRUNCATE

Used to remove all records from a table quickly. It does not delete the table structure.

✅ RENAME

Used to change the name of a database object.

🔹 2. DML (Data Manipulation Language)

DML commands are used to manage the data inside tables.

✅ INSERT

Used to add new records into a table.

✅ UPDATE

Used to modify existing records in a table.

✅ DELETE

Used to remove specific records from a table.

🔹 3. DQL (Data Query Language)

DQL is used to retrieve data from the database.

✅ SELECT

Used to fetch data from one or more tables.
It can be combined with conditions, sorting, grouping, and joins.

🔹 4. DCL (Data Control Language)

DCL commands are related to permissions and access control.

✅ GRANT

Used to give users permission to access or modify database objects.

✅ REVOKE

Used to remove permissions from users.

🔹 5. TCL (Transaction Control Language)

TCL commands manage transactions in the database.

✅ COMMIT

Saves all changes made during the current transaction permanently.

✅ ROLLBACK

Undo changes made in the current transaction before they are saved.

✅ SAVEPOINT

Sets a point within a transaction to which you can later roll back.

🔹 Important SQL Clauses (Descriptions)

✅ WHERE

Used to filter records based on specific conditions.

✅ ORDER BY

Used to sort the result in ascending or descending order.

✅ GROUP BY

Used to group rows that have the same values in specified columns.

✅ HAVING

Used to filter grouped data.

✅ DISTINCT

Used to return only unique values.

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