Why Column constraints (NOT NULL, UNIQUE, CHECK) in PostgreSQL? - Purpose & Use Cases

The Big Idea

What if your database could catch your mistakes before they happen?

The Scenario

Imagine you have a big spreadsheet where you must enter customer data by hand. You want to make sure every customer has a phone number, no two customers share the same email, and ages are always positive numbers.

The Problem

Doing this by eye is slow and mistakes happen often. You might miss empty phone numbers, accidentally type the same email twice, or enter impossible ages. Fixing these errors later wastes time and causes confusion.

The Solution

Column constraints like NOT NULL, UNIQUE, and CHECK automatically enforce these rules for you. The database stops wrong data from entering, so your data stays clean and reliable without extra effort.

Before vs After

✗ Before

INSERT INTO customers (name, email, phone, age) VALUES ('Alice', '[e-mail]', NULL, -5);

✓ After

CREATE TABLE customers (name TEXT, email TEXT UNIQUE, phone TEXT NOT NULL, age INT CHECK (age > 0));

What It Enables

It lets you trust your data and focus on using it, not fixing it.

Real Life Example

A company uses UNIQUE to ensure no two users register with the same username, NOT NULL to require emails for contact, and CHECK to keep ages within a valid range.

Key Takeaways

Constraints prevent bad data from entering your database.

NOT NULL ensures important fields are never empty.

UNIQUE and CHECK keep data accurate and consistent.

Practice

1. Which column constraint in PostgreSQL ensures that a column cannot have empty or missing values?

easy

A. PRIMARY KEY

B. NOT NULL

C. CHECK

D. UNIQUE

Solution

1. Step 1: Understand the NOT NULL constraint

   NOT NULL means the column must always have a value; it cannot be left empty.

2. Step 2: Compare with other constraints

   UNIQUE prevents duplicates but allows NULLs; CHECK enforces custom rules; PRIMARY KEY implies NOT NULL and UNIQUE but is more specific.

3. Final Answer:

   NOT NULL -> Option B

4. Quick Check:

   NOT NULL = no empty values [OK]

Hint: NOT NULL means no empty values allowed in the column [OK]

Common Mistakes:

• Confusing UNIQUE with NOT NULL
• Thinking CHECK prevents NULLs
• Assuming PRIMARY KEY is the only way to disallow NULL

2. Which of the following is the correct syntax to add a UNIQUE constraint to the column email in a table users?

easy

A. ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email);

B. ALTER TABLE users ADD UNIQUE email;

C. ALTER TABLE users ADD UNIQUE (email) CONSTRAINT unique_email;

D. ALTER TABLE users ADD CONSTRAINT email UNIQUE;

Solution

1. Step 1: Recall the syntax for adding UNIQUE constraint

   The correct syntax is: ALTER TABLE table_name ADD CONSTRAINT constraint_name UNIQUE (column_name);

2. Step 2: Match the syntax with options

   ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email); matches the correct syntax exactly, naming the constraint and specifying the column.

3. Final Answer:

   ALTER TABLE users ADD CONSTRAINT unique_email UNIQUE (email); -> Option A

4. Quick Check:

   ADD CONSTRAINT name UNIQUE (column) [OK]

Hint: Use ADD CONSTRAINT name UNIQUE (column) to add UNIQUE [OK]

Common Mistakes:

• Omitting CONSTRAINT keyword
• Placing UNIQUE before CONSTRAINT name incorrectly
• Not specifying column in parentheses

3. Given the table definition:

CREATE TABLE products (
  id SERIAL PRIMARY KEY,
  price NUMERIC CHECK (price > 0),
  sku TEXT UNIQUE
);

What happens if you try to insert price = -5 and sku = 'A123'?

medium

A. Insert fails due to UNIQUE constraint violation on sku

B. Insert succeeds because CHECK allows negative values

C. Insert fails due to CHECK constraint violation on price

D. Insert succeeds because price can be negative

Solution

1. Step 1: Analyze the CHECK constraint on price

   The CHECK constraint requires price > 0, so negative values like -5 violate this rule.

2. Step 2: Consider the UNIQUE constraint on sku

   Since 'A123' is new, UNIQUE is not violated here.

3. Final Answer:

   Insert fails due to CHECK constraint violation on price -> Option C

4. Quick Check:

   CHECK (price > 0) rejects negative price [OK]

Hint: CHECK enforces rules; negative price breaks price > 0 [OK]

Common Mistakes:

• Ignoring CHECK constraint effects
• Confusing UNIQUE violation with CHECK violation
• Assuming negative numbers are allowed

4. You have this table:

CREATE TABLE employees (
  id SERIAL PRIMARY KEY,
  email TEXT UNIQUE NOT NULL,
  age INT CHECK (age >= 18)
);

Which statement will cause an error when inserting data?

medium

A. INSERT INTO employees (email, age) VALUES ('[e-mail]', 25);

B. INSERT INTO employees (email, age) VALUES ('[e-mail]', 20);

C. INSERT INTO employees (email, age) VALUES ('[e-mail]', 18);

D. INSERT INTO employees (email, age) VALUES (NULL, 30);

Solution

1. Step 1: Check NOT NULL constraint on email

   Email cannot be NULL because of NOT NULL constraint.

2. Step 2: Check CHECK constraint on age

   Age must be 18 or older; the ages in A(25), B(20), and D(18) all satisfy this.

3. Final Answer:

   INSERT with NULL email causes error due to NOT NULL constraint -> Option D

4. Quick Check:

   NOT NULL rejects NULL email [OK]

Hint: NOT NULL disallows NULL values, CHECK enforces conditions [OK]

Common Mistakes:

• Focusing on CHECK constraint instead of NOT NULL
• Ignoring NOT NULL on email
• Assuming UNIQUE causes error on first insert

5. You want to create a table orders where the quantity column must be a positive integer and the order_code must be unique and never empty. Which is the best column definition for quantity and order_code?

hard

A. quantity INT CHECK (quantity > 0), order_code TEXT UNIQUE NOT NULL

B. quantity INT NOT NULL, order_code TEXT CHECK (order_code != '')

C. quantity INT CHECK (quantity >= 0), order_code TEXT UNIQUE

D. quantity INT, order_code TEXT NOT NULL UNIQUE CHECK (order_code IS NOT NULL)

Solution

1. Step 1: Ensure quantity is positive integer

   CHECK (quantity > 0) enforces positive values only, better than >= 0 which allows zero.

2. Step 2: Ensure order_code is unique and not empty

   UNIQUE prevents duplicates; NOT NULL disallows empty; this combination ensures order_code is unique and present.

3. Final Answer:

   quantity INT CHECK (quantity > 0), order_code TEXT UNIQUE NOT NULL -> Option A

4. Quick Check:

   CHECK positive + UNIQUE NOT NULL = correct constraints [OK]

Hint: Use CHECK for positive, UNIQUE + NOT NULL for unique non-empty [OK]

Common Mistakes:

• Allowing zero quantity
• Missing NOT NULL on unique column
• Using CHECK without NOT NULL for order_code