What is DML?

In the DDL chapter you built empty tables. Now it's time to fill them and keep them up to date. A new player is signed (add a row), Dhoni scores more runs (change a value), a player is released (remove a row). All of these are DML jobs.

Definition

DML (Data Manipulation Language) is the group of SQL commands that add, change and remove the rows of data stored inside tables, without changing the tables' structure. Its three commands are INSERT, UPDATE and DELETE.

• Rows of data — DML works on the contents (Virat Kohli, 750 runs), not the columns or data types.
• Without changing structure — after any DML statement the table has exactly the same columns as before.
• INSERT / UPDATE / DELETE — one command for each kind of change.

The families of SQL commands

Note: Some books put SELECT inside DML too (since it "manipulates" data by reading it). This course keeps it separate as DQL, which is also very common.

The three DML commands

Example — the table we will modify

SELECT * FROM players;

Every lesson in this chapter starts from this table, changes it, and then shows the table after the change.

"Rows affected" — how DML reports back

Unlike SELECT, DML statements don't return a result grid. SQL Server shows a message instead:

(1 row affected)

Always read that number! If you expected to update 1 player and SQL Server says (6 rows affected), something is wrong (usually a missing WHERE).

DDL vs DML at a glance

Common mistakes

• ❌ Calling DELETE a DDL command. ✅ It removes rows → DML.
• ❌ Using ALTER to change a player's runs. ✅ ALTER changes structure; use UPDATE for values.
• ❌ Ignoring the "rows affected" message. ✅ Check it every time.

Interview tip

Q: Name the DML commands and say what each does. A: INSERT adds new rows, UPDATE modifies values in existing rows, and DELETE removes rows. (Some definitions also include SELECT and MERGE.) They change data, not table structure.