Data Manipulation and Definition

More Than Just Queries: Using INSERT, UPDATE, and DELETE

SQL isn’t just for reading data — you can also add, change, or remove rows.

INSERT → Add new data

INSERT INTO Customers (ID, Name, City)
VALUES (1, 'Alice', 'London');

UPDATE → Change existing data

UPDATE Customers
SET City = 'Paris'
WHERE Name = 'Alice';

DELETE → Remove data

DELETE FROM Customers
WHERE Name = 'Alice';

Building Your Blueprints: A Guide to CREATE TABLE

A table is like a blueprint for your data. You define columns and their data types.

CREATE TABLE Customers (
    ID INT,
    Name VARCHAR(50),
    City VARCHAR(50)
);

This creates a table with 3 columns: ID, Name, and City.

The Power of ALTER TABLE: Modifying Your Database Structure

What if you want to add a new column later? Use ALTER TABLE.

ALTER TABLE Customers
ADD Phone VARCHAR(20);

Now your Customers table looks like this:

+----+----------+-------------+------------+
| ID | Name     | City        | Phone      |
+----+----------+-------------+------------+

Understanding SQL Data Types: From VARCHAR to INT

Every column has a data type, which defines what kind of data it stores.

• INT → whole numbers (e.g., 1, 2, 100)

• VARCHAR(n) → text up to n characters (e.g., ‘Alice’)

• DATE → dates (e.g., 2025-09-04)

• DECIMAL(10,2) → numbers with decimals (e.g., 199.99)

What Are Primary and Foreign Keys? The Keys to Relational Data

• Primary Key → uniquely identifies each row

• Foreign Key → links one table to another

Example:

Customers Table

+----+----------+
| ID | Name     |
+----+----------+
| 1  | Alice    |
| 2  | Bob      |
+----+----------+

Orders Table

+----+-------------+----------+
| ID | Customer_ID | Product  |
+----+-------------+----------+
| 1  | 1           | Laptop   |
| 2  | 2           | Phone    |
+----+-------------+----------+

Here:

• Customers.ID is the Primary Key.

• Orders.Customer_ID is a Foreign Key that references Customers.ID.

✅ Pro Tip

Always define Primary Keys in your tables. Without them, it’s easy to create duplicate or messy data. Foreign Keys help keep relationships between tables clean and accurate.