Types of SQL Commands & Roadmap

Every SQL statement you will ever write belongs to a small number of command families. A developer builds the structure, an app changes the data, an analyst reads it, an admin controls access, and transactions keep everything safe. Knowing which family a command belongs to is a classic interview question and helps you understand what each statement can and can't do.

Definition

SQL commands are grouped by their purpose into sub-languages: DDL (define structure), DML (manipulate data), DQL (query data), DCL (control access) and TCL (control transactions).

The three main families (taught in this course)

Think of a stadium: DDL builds the stands and seats, DML puts fans into seats or moves them, and DQL counts how many fans are sitting in each stand.

Two more families (good to know)

Note: Some books put SELECT inside DML (because it works on data). Both views are accepted; this course treats DQL separately because reading data is the most-used part of SQL.

Example — one statement from each main family

-- DDL: build the structure
CREATE TABLE fans (
    fan_id    INT PRIMARY KEY,
    fan_name  VARCHAR(50),
    fav_team  CHAR(3)
);

-- DML: put data in and change it
INSERT INTO fans VALUES (1, 'Aarav', 'RCB'), (2, 'Diya', 'CSK'), (3, 'Kabir', 'MI');
UPDATE fans SET fav_team = 'MI' WHERE fan_id = 2;
DELETE FROM fans WHERE fan_id = 3;

-- DQL: read the data
SELECT *
FROM fans;

How it works: CREATE made an empty table; INSERT added three fans; UPDATE changed Diya's team from CSK to MI; DELETE removed Kabir; finally SELECT showed what is left.

Example — DCL and TCL (SQL Server)

-- DCL: let the analyst read players but not change them
GRANT SELECT ON dbo.players TO analyst_user;
REVOKE SELECT ON dbo.players FROM analyst_user;

-- TCL: both changes happen, or neither does
BEGIN TRANSACTION;
    UPDATE players SET runs = runs + 82 WHERE id = 1;
    UPDATE players SET runs = runs + 45 WHERE id = 2;
COMMIT;      -- or ROLLBACK; to undo both

DROP vs DELETE — a classic confusion

Your course roadmap

The order of learning is structure → data → questions, but we start with questions (SELECT) because it is the most used skill.

After that: real SQL projects on IPL data!

Common mistakes

• ❌ Saying DELETE removes the table. ✅ DELETE removes rows; DROP TABLE removes the table.
• ❌ Thinking ALTER changes data. ✅ ALTER changes structure (e.g. adds a column); UPDATE changes data.

Interview tip

Q: Classify CREATE, INSERT, SELECT, GRANT and COMMIT. A: CREATE → DDL, INSERT → DML, SELECT → DQL (sometimes counted as DML), GRANT → DCL, COMMIT → TCL.