A3.3.1 Outline the differences between data language types within SQL.

📚 You can find additional information related to these learning standards in the course companion pages 191 to 195

The Big Idea

Structured Query Language (SQL) is the standard language for managing relational databases. Within SQL, there are subsets of commands designed for different tasks. Two of the most fundamental subsets are:

• Data Definition Language (DDL): used to define and modify the structure of database objects (tables, schemas, indexes, etc.).
• Data Manipulation Language (DML): used to manipulate the data inside those objects (inserting, updating, deleting, querying rows).

Each serves a distinct purpose in the lifecycle of a relational database system.

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1. Data Definition Language (DDL)

Purpose

Defines, alters, or removes schema-level structures.

Common DDL Statements

Command	Description
CREATE	Creates new database objects like tables, views, indexes
ALTER	Modifies an existing object (add/remove columns, constraints)
DROP	Permanently deletes objects from the schema
TRUNCATE	Deletes all data from a table but keeps the structure

Example – Create a table

CREATE TABLE students (
    student_id INT PRIMARY KEY,
    name VARCHAR(100),
    birth_date DATE
);

Example – Add a column

ALTER TABLE students ADD COLUMN email VARCHAR(255);

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2. Data Manipulation Language (DML)

Purpose

Performs actions on the data itself, inside the objects defined using DDL.

Common DML Statements

Command	Description
SELECT	Retrieves data from tables
INSERT	Adds new rows
UPDATE	Modifies existing rows
DELETE	Removes rows

Example – Insert a record

INSERT INTO students (student_id, name, birth_date)
VALUES (101, 'Alice Smith', '2006-11-15');

Example – Retrieve records

SELECT name, birth_date FROM students WHERE student_id = 101;

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Summary Table

Category	DDL (Data Definition Language)	DML (Data Manipulation Language)
Focus	Structure/schema	Data/rows
Examples	CREATE, ALTER, DROP, TRUNCATE	SELECT, INSERT, UPDATE, DELETE
Use Case	Create or modify a table	Add, change, remove, or query data in a table
Persistence	Usually auto-committed and affects structure permanently	May require explicit transaction control (e.g., COMMIT)

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Conclusion

In SQL, DDL defines the "shape" of the database, while DML populates and modifies its contents. Understanding the difference is foundational to effective database development: DDL builds the house, DML moves the furniture. Both are essential in the design, implementation, and operation of relational database systems.