Data Manipulation Language (DML) Commands

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Same pattern + real output shown for ALL DML commands — SELECT, INSERT, UPDATE, DELETE.

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📘 Base Table (Used for ALL examples)

customers

customer_id	first_name	last_name	city
1	Amit	Kumar	Patna
2	Neha	Singh	Delhi
3	Ravi	Verma	Lucknow

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🧾 Summary Table (Questions)

No.	DML Command	Question
1	SELECT	Retrieve customer names
2	INSERT	Add a new customer
3	UPDATE	Update customer city
4	DELETE	Delete a customer

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1️⃣ SELECT

Question: Show first name and last name of all customers.

SELECT first_name, last_name
FROM customers;

✅ Output

first_name	last_name
Amit	Kumar
Neha	Singh
Ravi	Verma

✔ Retrieves data from table
✔ Does not modify data

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2️⃣ INSERT

Question: Add a new customer named Mary Doe from Mumbai.

INSERT INTO customers (first_name, last_name, city)
VALUES ('Mary', 'Doe', 'Mumbai');

✅ Table AFTER INSERT

customer_id	first_name	last_name	city
1	Amit	Kumar	Patna
2	Neha	Singh	Delhi
3	Ravi	Verma	Lucknow
4	Mary	Doe	Mumbai

✔ Adds new record
✔ Existing records remain unchanged

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3️⃣ UPDATE

Question: Update city of customer Ravi to Kanpur.

UPDATE customers
SET city = 'Kanpur'
WHERE first_name = 'Ravi';

✅ Table AFTER UPDATE

customer_id	first_name	last_name	city
1	Amit	Kumar	Patna
2	Neha	Singh	Delhi
3	Ravi	Verma	Kanpur
4	Mary	Doe	Mumbai

✔ Modifies existing record
✔ WHERE condition is important

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4️⃣ DELETE

Question: Delete customer Mary Doe.

DELETE FROM customers
WHERE first_name = 'Mary';

✅ Table AFTER DELETE

customer_id	first_name	last_name	city
1	Amit	Kumar	Patna
2	Neha	Singh	Delhi
3	Ravi	Verma	Kanpur

✔ Removes record permanently
✔ Use WHERE to avoid deleting all rows

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🧠 One-Line Memory Trick (DML)

Command	Meaning
SELECT	Read data
INSERT	Add data
UPDATE	Change data
DELETE	Remove data

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⚠️ Exam Trap (Very Important)

DELETE FROM customers;

❌ Deletes ALL records (no WHERE)

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If you want next:

• DDL commands (CREATE, ALTER, DROP, TRUNCATE)

• TCL commands (COMMIT, ROLLBACK, SAVEPOINT)

• MCQs with table outputs

• Hindi explanation for exams

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