Data Definition Language (DDL) Commands

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Below is DDL commands explained in the SAME PATTERN you liked:

• Base state

• Question

• SQL

• Result / Output (table structure or data)

• ✔ clear takeaways

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📘 Base State (Before DDL Commands)

Assume no table exists initially, then we work step-by-step.

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🧾 Summary Table (DDL Questions)

No.	DDL Command	Question
1	CREATE	Create an employees table
2	ALTER	Add a new column
3	DROP	Remove the table
4	TRUNCATE	Delete all data from table

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1️⃣ CREATE

Question: Create a table named employees with basic details.

CREATE TABLE employees (
  employee_id INT PRIMARY KEY,
  first_name VARCHAR(50),
  last_name VARCHAR(50),
  age INT
);

✅ Result (Table Structure Created)

Column Name	Data Type	Key
employee_id	INT	PRIMARY KEY
first_name	VARCHAR(50)	—
last_name	VARCHAR(50)	—
age	INT	—

✔ Creates a new table
✔ Defines columns and data types
✔ No data inserted yet

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2️⃣ ALTER

Question: Add an email column to the employees table.

ALTER TABLE employees
ADD email VARCHAR(100);

✅ Result (Updated Table Structure)

Column Name	Data Type
employee_id	INT
first_name	VARCHAR(50)
last_name	VARCHAR(50)
age	INT
email	VARCHAR(100)

✔ Modifies existing table
✔ Table data (if any) remains safe

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3️⃣ DROP

Question: Delete the employees table completely.

DROP TABLE employees;

✅ Result

❌ Table employees no longer exists

✔ Structure deleted
✔ Data deleted permanently
⚠️ Cannot be rolled back

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4️⃣ TRUNCATE

Question: Remove all records from customers table but keep the structure.

TRUNCATE TABLE customers;

✅ Result (Before & After)

Before TRUNCATE

customer_id	name
1	Amit
2	Neha

After TRUNCATE

customer_id	name
(no rows)

✔ Deletes all data
✔ Table structure remains
✔ Faster than DELETE

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🔥 TRUNCATE vs DELETE (Exam Favorite)

Feature	TRUNCATE	DELETE
Removes data	Yes (all)	Yes (conditional)
WHERE allowed	❌ No	✅ Yes
Rollback	❌ No	✅ Yes
Table structure	Remains	Remains
Speed	Very fast	Slower

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🧠 One-Line Memory Trick (DDL)

Command	Meaning
CREATE	Build structure
ALTER	Change structure
DROP	Destroy structure
TRUNCATE	Clean data only

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