Constraints - DSA Guide

🎢 Introduction: The Roller Coaster Guard

Imagine you go to a theme park to ride a giant roller coaster. Before you can enter, there is a ride operator standing at the gate with a wooden ruler:

The Height Rule (CHECK): "You must be at least 48 inches tall to ride!" If you are shorter, the operator stops you.
The Ticket Rule (NOT NULL): "You must have a physical ticket!" If your hand is empty, you cannot enter.
The Seat Rule (UNIQUE): "Only one person can sit in seat number 12!" Two people cannot occupy the same seat.

In a database, Constraints are these exact rules! They stand at the gates of your tables and examine every piece of data trying to enter. If a piece of data breaks the rules, the database operator rejects it!

🛡️ Types of Database Constraints

Constraints keep your data clean, accurate, and reliable. Without them, your database would slowly fill with junk data.

       Incoming Data (INSERT / UPDATE)
                   |
                   v
      +-------------------------+
      |       CONSTRAINTS       |
      |  [NOT NULL]   [UNIQUE]  | <--- Gates checking data properties
      |   [CHECK]    [DEFAULT]  |
      +-------------------------+
                   |
          (Passed checks?)
          /                    YES               NO
        /                        v                   v
[Saved in Table]    [Rejected / Error]

NOT NULL: This column cannot be empty. Every row must have a value here.
UNIQUE: No two rows can have the same value in this column (like an email address or license plate).
DEFAULT: If you don't provide a value, the database fills it in automatically (e.g., setting the signup date to "Today").
CHECK: Makes sure the value satisfies a mathematical or logical condition (e.g., age >= 18 or price > 0).
PRIMARY KEY: A combination of NOT NULL and UNIQUE.
FOREIGN KEY: Ensures the value matches a key in another table.

💻 Code Examples

Let's see how we write SQL constraints and handle validation failures.

SQL Setup

sql

CREATE TABLE coaster_riders (
    rider_id INTEGER PRIMARY KEY,
    name TEXT NOT NULL,                  -- Must have a name
    email TEXT UNIQUE,                   -- Emails must be unique
    height_inches INTEGER CHECK(height_inches >= 48), -- Must be tall enough
    status TEXT DEFAULT 'waiting'        -- Automatic default value
);

Multi-Language Execution

⚠️ Common Mistakes

1. Relying on Frontend Checks Only

Thinking your website's forms (HTML/JS) are enough. Hackers can bypass your browser and send bad data directly. Always set constraints in your database as the final line of defense!

Bad: Relying on client-side JS validators only.
Good: Set database checks on critical numeric values.

2. Constraints That are Too Rigid

Setting a constraint that prevents legitimate user inputs. For example, forcing phone numbers to match a strict check of exactly 10 digits, which blocks international country codes!

🔍 Interview Corner

Q1: What is the difference between a UNIQUE constraint and a PRIMARY KEY?

A PRIMARY KEY uniquely identifies each row and cannot contain NULL values. You can have only one PRIMARY KEY per table.
A UNIQUE constraint also guarantees unique values but can contain NULL values. You can set multiple UNIQUE constraints on different columns.

Q2: What is a CHECK constraint, and when should you use it?

A CHECK constraint is a logical rule that validates values before they are written. It is used to enforce range logic (e.g. balance >= 0, age >= 18, status IN ('pending', 'completed')).

📝 Summary

Constraints are safety guardrails built directly into your database schema.
Common checks include NOT NULL, UNIQUE, DEFAULT, and CHECK.
The database engine automatically rejects transactions that violate these rules.