Domain types for validation in PostgreSQL - Mini Project: Build & Apply

Using Domain Types for Validation in PostgreSQL

📖 Scenario: You are creating a simple database for a small bookstore. You want to ensure that certain fields like ISBN numbers and book prices follow specific rules to keep your data clean and valid.

🎯 Goal: Build a PostgreSQL database schema that uses domain types to validate ISBN numbers and book prices automatically.

📋 What You'll Learn

Create a domain type called isbn_type that only allows 13-character strings.

Create a domain type called price_type that only allows numeric values greater than 0.

Create a table called books with columns id, title, isbn using isbn_type, and price using price_type.

💡 Why This Matters

🌍 Real World

Domain types help keep data clean by automatically checking values when inserting or updating records, reducing errors in real-world databases like bookstores or libraries.

💼 Career

Understanding domain types is useful for database developers and administrators to enforce business rules at the database level, improving data quality and reliability.

1

Create the isbn_type domain

Create a domain called isbn_type as varchar(13) that checks the length of the string is exactly 13 characters.

PostgreSQL

-- Your code here to create the isbn_type domain

Hint

Use CREATE DOMAIN with a CHECK constraint that uses char_length(VALUE) = 13.

2

Create the price_type domain

Create a domain called price_type as numeric that checks the value is greater than 0.

PostgreSQL

CREATE DOMAIN isbn_type AS varchar(13) CHECK (char_length(VALUE) = 13);
-- Your code here to create the price_type domain

Hint

Use CREATE DOMAIN with a CHECK constraint that ensures VALUE > 0.

3

Create the books table

Create a table called books with columns: id as serial PRIMARY KEY, title as varchar(100), isbn using the isbn_type domain, and price using the price_type domain.

PostgreSQL

CREATE DOMAIN isbn_type AS varchar(13) CHECK (char_length(VALUE) = 13);
CREATE DOMAIN price_type AS numeric CHECK (VALUE > 0);
-- Your code here to create the books table

Hint

Use CREATE TABLE with the specified columns and domain types.

4

Add a constraint to ensure title is not null

Alter the books table to add a NOT NULL constraint on the title column.

PostgreSQL

CREATE DOMAIN isbn_type AS varchar(13) CHECK (char_length(VALUE) = 13);
CREATE DOMAIN price_type AS numeric CHECK (VALUE > 0);
CREATE TABLE books (
    id serial PRIMARY KEY,
    title varchar(100),
    isbn isbn_type,
    price price_type
);
-- Your code here to add NOT NULL constraint on title

Hint

Add NOT NULL after the title varchar(100) column definition.

Practice

1. What is the main purpose of using domain types in PostgreSQL?

easy

A. To speed up query execution by indexing

B. To create custom data types with automatic validation rules

C. To store large binary data efficiently

D. To create temporary tables for session use

Solution

1. Step 1: Understand what domain types are

   Domain types are user-defined data types that include validation rules to ensure data quality.

2. Step 2: Identify the main purpose

   The main purpose is to enforce rules automatically when data is inserted or updated, keeping data clean.

3. Final Answer:

   To create custom data types with automatic validation rules -> Option B

4. Quick Check:

   Domain types = custom types with validation [OK]

Hint: Domains add rules to types, not speed or storage [OK]

Common Mistakes:

• Confusing domains with indexing
• Thinking domains store large files
• Mixing domains with temporary tables

2. Which of the following is the correct syntax to create a domain named positive_int that only allows positive integers?

easy

A. CREATE DOMAIN positive_int AS integer CHECK (VALUE > 0);

B. CREATE DOMAIN positive_int TYPE integer WHERE VALUE > 0;

C. CREATE DOMAIN positive_int AS integer VALIDATE (VALUE > 0);

D. CREATE DOMAIN positive_int AS integer IF VALUE > 0;

Solution

1. Step 1: Recall the correct syntax for domain creation

   The syntax is: CREATE DOMAIN name AS base_type CHECK (condition);

2. Step 2: Match the syntax with options

   CREATE DOMAIN positive_int AS integer CHECK (VALUE > 0); matches the correct syntax with CHECK and VALUE keyword.

3. Final Answer:

   CREATE DOMAIN positive_int AS integer CHECK (VALUE > 0); -> Option A

4. Quick Check:

   CREATE DOMAIN ... AS ... CHECK(...) [OK]

Hint: Use CHECK with VALUE keyword in domain creation [OK]

Common Mistakes:

• Using TYPE instead of AS
• Using WHERE or VALIDATE instead of CHECK
• Missing parentheses around condition

3. Given the domain creation:

CREATE DOMAIN us_zipcode AS varchar(5) CHECK (VALUE ~ '^[0-9]{5}$');

What will happen if you try to insert '1234a' into a table column of type us_zipcode?

medium

A. The insert will succeed because it's a varchar

B. The insert will cause a syntax error

C. The insert will succeed but store NULL instead

D. The insert will fail due to the CHECK constraint violation

Solution

1. Step 1: Understand the domain's CHECK constraint

   The domain requires the value to match exactly 5 digits using a regular expression.

2. Step 2: Check the value '1234a' against the regex

   '1234a' contains a letter, so it does not match the pattern of 5 digits.

3. Final Answer:

   The insert will fail due to the CHECK constraint violation -> Option D

4. Quick Check:

   Regex check fails = insert rejected [OK]

Hint: Regex in CHECK rejects invalid patterns [OK]

Common Mistakes:

• Assuming varchar allows any string without checks
• Thinking invalid data stores as NULL automatically
• Confusing constraint violation with syntax error

4. You have this domain:

CREATE DOMAIN non_empty_text AS text CHECK (LENGTH(VALUE) > 0);

Which of the following INSERT statements will cause an error when inserting into a column of type non_empty_text?

medium

A. INSERT INTO table_name (col) VALUES ('');

B. INSERT INTO table_name (col) VALUES ('world');

C. INSERT INTO table_name (col) VALUES (' ');

D. INSERT INTO table_name (col) VALUES (' hello ');

Solution

1. Step 1: Understand the CHECK condition

   The domain requires text length to be greater than 0, so empty strings fail. Whitespace-only strings pass as they have length > 0.

2. Step 2: Evaluate each insert value

   INSERT INTO table_name (col) VALUES (''); inserts an empty string '', length 0, violating the check.

3. Final Answer:

   INSERT with empty string '' causes error -> Option A

4. Quick Check:

   Empty string fails length > 0 check [OK]

Hint: Empty strings fail length > 0 check [OK]

Common Mistakes:

• Thinking whitespace strings fail (they pass)
• Assuming empty string is allowed
• Confusing syntax errors with constraint errors

5. You want to create a domain rating that stores integers from 1 to 5 inclusive. Which domain definition correctly enforces this range and can be used in multiple tables?

hard

A. CREATE DOMAIN rating AS integer WHERE (VALUE BETWEEN 1 AND 5);

B. CREATE DOMAIN rating AS integer CHECK (VALUE > 1 AND VALUE < 5);

C. CREATE DOMAIN rating AS integer CHECK (VALUE >= 1 AND VALUE <= 5);

D. CREATE DOMAIN rating AS integer CHECK (VALUE IN (1, 2, 3, 4));

Solution

1. Step 1: Understand the required range

   The rating must include 1 and 5, so boundaries are inclusive.

2. Step 2: Analyze each CHECK condition

   The option using WHERE instead of CHECK has invalid syntax.
   CREATE DOMAIN rating AS integer CHECK (VALUE > 1 AND VALUE < 5); excludes 1 and 5.
   CREATE DOMAIN rating AS integer CHECK (VALUE >= 1 AND VALUE <= 5); uses >= 1 AND <= 5, correctly including boundaries.
   CREATE DOMAIN rating AS integer CHECK (VALUE IN (1, 2, 3, 4)); uses IN list which misses 5.

3. Step 3: Choose the best option for clarity and standard usage

   CREATE DOMAIN rating AS integer CHECK (VALUE >= 1 AND VALUE <= 5); is clear, standard, and commonly used for range checks.

4. Final Answer:

   CREATE DOMAIN rating AS integer CHECK (VALUE >= 1 AND VALUE <= 5); -> Option C

5. Quick Check:

   Inclusive range uses >= and <= [OK]

Hint: Use >= and <= for inclusive numeric domain checks [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using WHERE instead of CHECK
• Incomplete IN list misses values