Category: 10. Working with Constraints

Imagine your database is a school If there are no rules, students can: That would be chaosSo we use constraints to keep data clean, safe, and sensible. Today, we learn two important rules: We’ll learn this using MySQL. What Is a Constraint? A constraint is a rule applied to a column to control what data can go inside it. Think of it as: Traffic rules for your database roads DEFAULT Constraint – The Helpful Friend What Is DEFAULT? The DEFAULT constraint gives a column a predefined valueOnly when you don’t provide one Example If you forget your school uniform, the…

Unique and NOT NULL Constraints Imagine you are making a school register. That’s exactly what NOT NULL and UNIQUE constraints do in a database We’ll learn this using MySQL. What Are Constraints? (Super Simple) Constraints are rules that tell the database: “Hey! Data must behave nicely!” They protect your data from mistakes. NOT NULL Constraint Example: Creating a Table with NOT NULL Allowed Not Allowed Error:Column 'name' cannot be null Adding NOT NULL to Existing Table UNIQUE Constraint Example: Creating a Table with UNIQUE Allowed Not Allowed Error:Duplicate entry 'a@gmail.com' for key 'email' Adding UNIQUE Constraint Later UNIQUE vs NOT…

What Are Keys in a Database? A key is like an ID card for data. We mainly use two keys: Primary Key (The Super ID) Imagine your school roll number: That’s exactly what a Primary Key does! Rules of a Primary Key A Primary Key:✔ Is unique (no duplicates)✔ Is not NULL✔ Identifies one and only one row Example Table: students What’s happening? Wrong Example (Duplicate Primary Key) MySQL says: “Hey! Two students can’t have the same ID!” Foreign Key (The Connector) Imagine: That classroom number is a Foreign Key. A Foreign Key connects one table to another table Example…