Understanding SQL Commands: DDL, DML, DCL, TCL, and DQL

1. Data Definition Language (DDL)

DDL commands define and manage the structure of a database, including tables, indexes, and schema. These commands modify the database schema itself.

Common DDL Commands:

• CREATE – Creates a new database object (table, view, index, etc.).

CREATE TABLE Employees (
    ID INT PRIMARY KEY,
    Name VARCHAR(50),
    Age INT,
    Department VARCHAR(50)
);        

• ALTER – Modifies an existing database object.

ALTER TABLE Employees ADD COLUMN Salary DECIMAL(10,2);        

• DROP – Deletes an entire database object.

DROP TABLE Employees;        

• TRUNCATE – Removes all records from a table but retains its structure.

TRUNCATE TABLE Employees;        

2. Data Manipulation Language (DML)

DML commands are used to insert, update and delete data within a database.

Common DML Commands:

• INSERT – Adds new records to a table.

INSERT INTO Employees (ID, Name, Age, Department) 
VALUES (1, 'John Doe', 30, 'IT');        

• UPDATE – Modifies existing records.

UPDATE Employees SET Age = 31 WHERE ID = 1;        

• DELETE – Removes specific records from a table.

DELETE FROM Employees WHERE ID = 1;        

3. Data Control Language (DCL)

DCL commands control access to database objects, ensuring data security and integrity.

Common DCL Commands:

• GRANT – Provides user access rights to a database.

GRANT SELECT, INSERT ON Employees TO user1;        

• REVOKE – Removes access rights from a user.

REVOKE INSERT ON Employees FROM user1;        

4. Transaction Control Language (TCL)

TCL commands manage transactions within a database, ensuring consistency and reliability.

Common TCL Commands:

• COMMIT – Saves all changes made in the current transaction.

COMMIT;        

• ROLLBACK – Reverts changes made in the current transaction.

ROLLBACK;        

• SAVEPOINT – Creates a savepoint within a transaction for partial rollbacks.

SAVEPOINT save1;        

• SET TRANSACTION – Defines transaction properties.

SET TRANSACTION READ ONLY;        

5. Data Query Language (DQL)

DQL is used to retrieve data from the database. It consists mainly of the SELECT statement.

Common DQL Command:

• SELECT – Fetches records from a database based on specified conditions.

SELECT * FROM Employees WHERE Department = 'IT';        

6. Overview

Understanding these SQL command types is essential for database administrators and developers. DDL manages database structure, DML manipulates data, DCL handles permissions, and TCL ensures transactional integrity. Mastering these commands allows for efficient and secure database operations.

By leveraging these SQL commands effectively, organizations can optimize their database performance and maintain data integrity across applications.