Database design best practices in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why use normalization in database design?

Which of the following best explains the main purpose of normalization in database design?

A To increase the number of tables for faster queries

B To store all data in a single table for simplicity

C To reduce data redundancy and improve data integrity

D To avoid using primary keys in tables

💡 Hint

Think about how normalization helps keep data consistent and organized.

Normalization organizes data to reduce duplication and ensures data integrity by structuring tables properly.

❓ query_result

intermediate

Identify the correct foreign key relationship

Given two tables, Orders and Customers, which SQL statement correctly creates a foreign key from Orders to Customers?

SQL

CREATE TABLE Customers (
  CustomerID INT PRIMARY KEY,
  Name VARCHAR(100)
);

CREATE TABLE Orders (
  OrderID INT PRIMARY KEY,
  CustomerID INT,
  OrderDate DATE
);

A ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID);

B ALTER TABLE Customers ADD FOREIGN KEY (CustomerID) REFERENCES Orders(CustomerID);

C ALTER TABLE Orders ADD FOREIGN KEY (OrderID) REFERENCES Customers(CustomerID);

D ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Orders(OrderID);

💡 Hint

The foreign key should link the child table to the parent table's primary key.

The foreign key in Orders references the primary key CustomerID in Customers to link orders to customers.

📝 Syntax

advanced

Find the syntax error in table creation

Which option contains a syntax error when creating a table with a primary key and a unique constraint?

SQL

CREATE TABLE Employees (
  EmployeeID INT PRIMARY KEY,
  Email VARCHAR(255) UNIQUE
);

A CREATE TABLE Employees (EmployeeID INT PRIMARY KEY, Email VARCHAR(255) UNIQUE);

B CREATE TABLE Employees (EmployeeID INT PRIMARY KEY, Email VARCHAR(255) UNIQUE NOT NULL);

C CREATE TABLE Employees (EmployeeID INT PRIMARY KEY, Email VARCHAR(255), UNIQUE (Email));

D CREATE TABLE Employees (EmployeeID INT PRIMARY KEY, Email VARCHAR(255) UNIQUE CONSTRAINT);

💡 Hint

Check the placement and usage of the UNIQUE keyword and CONSTRAINT keyword.

Option D incorrectly uses UNIQUE CONSTRAINT together without proper syntax, causing a syntax error.

❓ optimization

advanced

Choose the best indexing strategy

You have a large table with millions of rows and frequent queries filtering by the column last_name. Which indexing strategy improves query speed without wasting much space?

A Create a B-tree index on the last_name column

B Create a full-text index on the last_name column

C Create a unique index on the last_name column

D Create no index and rely on sequential scans

💡 Hint

Think about the type of queries and the best index type for filtering by a column.

B-tree indexes are efficient for filtering and sorting on columns like last_name, improving query speed.

🔧 Debug

expert

Identify the cause of data inconsistency

A database has two tables: Products and Orders. Sometimes, orders reference product IDs that do not exist in Products. What is the most likely cause?

A The Orders table has a primary key on product ID

B The foreign key constraint between Orders and Products is missing or disabled

C The Products table uses a composite primary key

D The primary key in Products is not unique

💡 Hint

Think about what enforces valid references between tables.

Without a foreign key constraint, the database does not enforce that product IDs in Orders must exist in Products, causing inconsistency.

Practice

1. Which of the following is a key reason to use primary keys in database tables?

easy

A. To store large amounts of text data

B. To create duplicate records easily

C. To speed up the database server hardware

D. To uniquely identify each record in the table

Solution

1. Step 1: Understand the role of primary keys

   Primary keys uniquely identify each row in a table, preventing duplicates.

2. Step 2: Compare options with primary key purpose

   Only To uniquely identify each record in the table describes uniqueness, which is the main purpose of primary keys.

3. Final Answer:

   To uniquely identify each record in the table -> Option D

4. Quick Check:

   Primary key = unique record ID [OK]

Hint: Primary keys always ensure unique records [OK]

Common Mistakes:

• Thinking primary keys store large text
• Confusing primary keys with performance hardware
• Believing primary keys allow duplicates

2. Which SQL statement correctly creates a table with a primary key on the id column?

easy

A. CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50));

B. CREATE TABLE users (id INT, name VARCHAR(50) PRIMARY KEY);

C. CREATE TABLE users (id INT, name VARCHAR(50), PRIMARY id);

D. CREATE TABLE users (id INT UNIQUE, name VARCHAR(50));

Solution

1. Step 1: Review correct primary key syntax

   The primary key is declared as id INT PRIMARY KEY inside the column definition.

2. Step 2: Check each option's syntax

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); uses correct syntax; A has invalid syntax; B places PRIMARY KEY on wrong column; D uses UNIQUE but not PRIMARY KEY.

3. Final Answer:

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); -> Option A

4. Quick Check:

   Primary key syntax = column_name TYPE PRIMARY KEY [OK]

Hint: Primary key goes right after the column type [OK]

Common Mistakes:

• Placing PRIMARY KEY on wrong column
• Using UNIQUE instead of PRIMARY KEY
• Incorrect syntax for primary key declaration

3. Given these two tables:
CREATE TABLE authors (author_id INT PRIMARY KEY, name VARCHAR(100));
CREATE TABLE books (book_id INT PRIMARY KEY, title VARCHAR(100), author_id INT, FOREIGN KEY (author_id) REFERENCES authors(author_id));
What will happen if you try to insert a book with author_id = 5, but no author with author_id = 5 exists?

medium

A. The insert will succeed and create a new author automatically

B. The insert will fail due to foreign key constraint violation

C. The insert will succeed but author_id will be set to NULL

D. The insert will succeed but cause duplicate author records

Solution

1. Step 1: Understand foreign key constraints

   Foreign keys require the referenced value to exist in the parent table to maintain data integrity.

2. Step 2: Apply constraint to the insert operation

   Inserting a book with author_id = 5 fails if no author with id 5 exists, causing a constraint violation.

3. Final Answer:

   The insert will fail due to foreign key constraint violation -> Option B

4. Quick Check:

   Foreign key enforces existing parent record [OK]

Hint: Foreign keys block inserts without matching parent [OK]

Common Mistakes:

• Assuming foreign keys auto-create missing parents
• Thinking foreign keys allow NULL by default
• Ignoring constraint errors on insert

4. You have a table orders with columns order_id (primary key), customer_id, and order_date. You want to add a foreign key constraint on customer_id referencing customers(customer_id). Which of these statements has an error?

medium

A. ALTER TABLE orders ADD FOREIGN KEY (customer_id) REFERENCES customers(customer_id);

B. ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(customer_id);

C. ALTER TABLE orders ADD FOREIGN KEY customer_id REFERENCES customers(customer_id);

D. ALTER TABLE orders ADD CONSTRAINT fk_customer FOREIGN KEY (customer_id) REFERENCES customers(customer_id) ON DELETE CASCADE;

Solution

1. Step 1: Review correct syntax for adding foreign keys

   The correct syntax requires parentheses around the column name after FOREIGN KEY.

2. Step 2: Identify the incorrect statement

   ALTER TABLE orders ADD FOREIGN KEY customer_id REFERENCES customers(customer_id); misses parentheses around customer_id, causing syntax error.

3. Final Answer:

   ALTER TABLE orders ADD FOREIGN KEY customer_id REFERENCES customers(customer_id); -> Option C

4. Quick Check:

   Foreign key column must be in parentheses [OK]

Hint: Always use parentheses around foreign key columns [OK]

Common Mistakes:

• Omitting parentheses around column names
• Forgetting to name the constraint (optional but recommended)
• Misplacing ON DELETE options

5. You are designing a database for a library. You want to track books, authors, and the categories each book belongs to. Which design best follows database design best practices?

hard

A. Separate tables for books, authors, and categories with primary keys and foreign keys linking them

B. Separate tables for books and categories only, storing author names as text in books

C. One table for books and authors combined, and a separate table for categories

D. One table with columns: book_id, book_title, author_name, category_name

Solution

1. Step 1: Analyze normalization and relationships

   Best practice is to separate entities into tables and link them with keys to avoid duplication.

2. Step 2: Evaluate each option's design

   Separate tables for books, authors, and categories with primary keys and foreign keys linking them uses separate tables with keys, which supports clear organization and data consistency.

3. Final Answer:

   Separate tables for books, authors, and categories with primary keys and foreign keys linking them -> Option A

4. Quick Check:

   Separate tables + keys = best design [OK]

Hint: Use separate tables and keys for related data [OK]

Common Mistakes:

• Putting all data in one table causing duplicates
• Mixing unrelated data in same table
• Storing related entity names as plain text