What is Data Definition Language (DDL)

What is Data Definition Language (DDL)

03 Jul 2024
9 min read

Data Definition Language (DDL)

DDL, which stands for Data Definition Language is a component of SQL (Structured Query Language). It is utilized in managing database systems (DBMS) to define and oversee all the elements, within a database. DDL instructions are employed for establishing, altering, and removing database configurations without affecting the data. Through the application of DDL commands database frameworks can be, Upheld, guaranteeing the arrangement and format of information stored in the database.

In this DBMS Tutorial, We will learn deep down about Data Definition Language which will include Key DDL Commands, Creating Databases, Creating Tables, etc.

DDL Commands

These SQL commands are used for creating a table, deleting a table, altering a table, and truncating the table. All the commands of DDL are auto-committed i.e. it permanently saves all the changes in the database.
In this category, we have four main commands as shown in below figure.
DDL Commands

Basics of SQL Commands


Here, The CREATE command is especially used to create new database objects such as databases, tables, indexes, and views.


The syntax of the CREATE command is as follows:
CREATE TABLE table_name
column_1 datatype,
column_2 datatype,
column_3 datatype,


Here We can create a table for storing Student information as follows.

    stud_id INT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    admission_date DATE


The ALTER command is used to modify existing database objects, such as adding, deleting, or modifying columns in a table.


The syntax of the ALTER command is as follows:
ALTER TABLE table_name
ADD column_name datatype;




The DROP command is used to delete existing database objects permanently


The syntax of the DROP command is as follows:
DROP TABLE table_name;


DROP TABLE students;


The TRUNCATE command removes all rows from a table without deleting the table itself. It resets any auto-incremented columns.


TRUNCATE TABLE table_name;



Best Practices using DDL

Creating Database Objects

1. Creating Databases

Creating a new database involves defining the database name and establishing its initial structure.

2. Creating Tables

Creating tables involves defining the table's columns, data types, and constraints.
CREATE TABLE departments (
    department_id INT PRIMARY KEY,
    department_name VARCHAR(100)

3. Creating Indexes

Indexes are created to enhance the performance of data retrieval operations.
CREATE INDEX idx_employee_name ON employees (last_name);

Modifying Database

1. Objects Altering Tables

Altering tables allows for adding, dropping, or modifying columns and constraints.

2. Renaming Database

Objects Renaming database objects involves changing the name of existing objects.
ALTER TABLE employees RENAME TO staff;

Deleting Database

1. Dropping Tables and Databases

Dropping tables and databases remove the objects and their data permanently.
DROP DATABASE company_db;
DROP TABLE departments;

2. Truncating Tables

Truncating tables removes all data from a table quickly without affecting the table structure.
TRUNCATE TABLE departments;

Constraints in DDL

Types of Constraints are rules enforced on data columns to maintain data integrity.


  • PRIMARY KEY ensures unique identification for each row.


  • FOREIGN KEY Maintains referential integrity between tables.
Differences between Primary Key and Foreign Key


  • It ensures all values in a column are unique.
Difference between Primary Key and Unique Key


  • NOT NULL ensures a column cannot have NULL values.


  • CHECK constraints ensure that all values in a column satisfy a specific condition.

6. ALTER TABLE Command

  • Adding and Removing Constraints Constraints can be added or removed using the ALTER TABLE command.
    ALTER TABLE employees ADD CONSTRAINT chk_salary CHECK (salary > 0);
    ALTER TABLE employees DROP CONSTRAINT chk_salary;
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Data Definition Language (DDL) is an essential component of SQL. By understanding and utilizing DDL commands effectively, database administrators and developers can create, modify, and delete database schemas, ensuring organized and efficient data management. We can Adopt best practices and troubleshoot common issues that can further enhance the effectiveness of using DDL in DBMS.
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Top 50 SQL Server Interview Questions and Answers


Q1. What is DCL and TCL?

DQL – Data Query Language. 
DML – Data Manipulation Language.
DCL – Data Control Language. TCL – Transaction Control Language.

Q2. What are DML commands?

DML is an abbreviation for Data Manipulation Language. Represents a collection of programming languages explicitly used to make changes to the database, such as: CRUD operations to create, read, update and delete data. Using INSERT, SELECT, UPDATE, and DELETE commands

Q3. What is DDL and its functions?

A Data Definition Language (DDL) refers to a language that is used to modify data and define data structures.
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