What is DDL?

Before you can store a single IPL score, the database needs somewhere to put it — a table with named columns, each with the right type of data. Building and reshaping those containers is the job of DDL. In the previous chapters you only read data with SELECT. In this chapter you become the architect: you design the tables yourself.

Definition

DDL (Data Definition Language) is the group of SQL commands that create, change and remove the structure (schema) of database objects — such as tables, columns and constraints — without working on the individual rows of data stored inside them.

Let's unpack that:

• Structure / schema means the shape of a table: its name, its columns, each column's data type, and the rules (constraints) attached to it. Think of it as the blueprint of a stadium, not the spectators sitting in it.
• Database objects are the things that live in a database: tables mostly, but also views, indexes, schemas and more.
• Not the rows — DDL does not add, edit or delete individual players. That is DML's job (next chapter).

The families of SQL commands

The course groups SQL commands into three main families:

Note: You will also hear about two more families: DCL (Data Control Language — GRANT, REVOKE for permissions) and TCL (Transaction Control Language — COMMIT, ROLLBACK). They are not part of this chapter, but interviewers love asking for the full list.

The three DDL commands

Structure vs data — the key idea

Look at the players table:

• The header row — the column names id, name, team, country, runs and their data types — is the structure. DDL works here.
• The rows below the header — Virat Kohli, Rohit Sharma … — are the data. DML works there.

So "add a jersey_no column" is a DDL task (the structure changes), while "change Dhoni's runs to 400" is a DML task (only a value changes).

Example — DDL in action

CREATE TABLE matches (
    match_id  INT         NOT NULL,
    home_team VARCHAR(10) NOT NULL,
    away_team VARCHAR(10) NOT NULL,
    CONSTRAINT pk_matches PRIMARY KEY (match_id)
);

This statement does not return any rows. It simply creates an empty table called matches. In SQL Server Management Studio (SSMS) you would just see the message "Commands completed successfully."

Important facts about DDL

• DDL is usually permanent. A DROP TABLE cannot be "undone" with Ctrl+Z. (SQL Server does let you roll back DDL inside an explicit transaction, but most databases — like MySQL — auto-commit DDL immediately.)
• DDL changes the database catalog — the system's list of what tables and columns exist. That is why tools like SSMS need a refresh of the Object Explorer to show your new table.
• You need permissions. In real companies, ordinary analysts often have permission to run SELECT but not CREATE/DROP.

Common mistakes

• ❌ Thinking DELETE and DROP are the same. ✅ DELETE (DML) removes rows; DROP (DDL) removes the whole table.
• ❌ Calling INSERT a DDL command. ✅ INSERT adds data → it is DML.
• ❌ Expecting CREATE TABLE to show a result grid. ✅ DDL statements return a message, not rows.

Interview tip

Q: What is the difference between DDL and DML? A: DDL (CREATE, ALTER, DROP) defines or changes the structure of database objects such as tables and columns. DML (INSERT, UPDATE, DELETE) adds, changes or removes the rows of data inside those tables. In short: DDL builds the container, DML fills it.