Why table design matters in SQL - Challenge Your Understanding

Challenge - 5 Problems

❓ query_result

intermediate

Effect of missing primary key on query results

Consider a table Employees without a primary key. What will be the result of this query?

SELECT EmployeeID, COUNT(*) FROM Employees GROUP BY EmployeeID;

Assuming EmployeeID is not unique in the table.

SQL

CREATE TABLE Employees (EmployeeID INT, Name VARCHAR(100));
INSERT INTO Employees VALUES (1, 'Alice'), (1, 'Alice'), (2, 'Bob');
SELECT EmployeeID, COUNT(*) FROM Employees GROUP BY EmployeeID;

A Empty result set

B EmployeeID 1 with count 1, EmployeeID 2 with count 1

C Error due to missing primary key

D EmployeeID 1 with count 2, EmployeeID 2 with count 1

💡 Hint

Think about how GROUP BY works with duplicate rows.

Without a primary key, duplicate EmployeeID values can exist. GROUP BY counts all rows per EmployeeID.

🧠 Conceptual

intermediate

Why normalization improves data integrity

Which of the following best explains why normalization is important in table design?

A It reduces data duplication and prevents update anomalies

B It makes queries run slower but uses less disk space

C It allows storing multiple data types in one column

D It automatically creates indexes on all columns

💡 Hint

Think about what happens when the same data is stored in many places.

Normalization organizes data to reduce repetition and avoid inconsistencies when updating data.

📝 Syntax

advanced

Identify the correct SQL to add a foreign key

Which SQL statement correctly adds a foreign key constraint to the Orders table referencing Customers?

SQL

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

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

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

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

D ALTER TABLE Orders ADD CONSTRAINT fk_customer FOREIGN KEY (CustomerID) Customers(CustomerID);

💡 Hint

Check the syntax for adding constraints with parentheses.

The correct syntax requires parentheses around the column name and the REFERENCES clause.

❓ optimization

advanced

Choosing indexes for faster queries

You have a large Sales table with columns SaleID, ProductID, and SaleDate. Which index will speed up this query?

SELECT * FROM Sales WHERE ProductID = 101 AND SaleDate = '2024-01-01';

A CREATE INDEX idx_product ON Sales(ProductID);

B CREATE INDEX idx_product_date ON Sales(ProductID, SaleDate);

C CREATE INDEX idx_date ON Sales(SaleDate);

D CREATE INDEX idx_date_product ON Sales(SaleDate, ProductID);

💡 Hint

Think about which columns appear together in the WHERE clause and their order.

A composite index on (ProductID, SaleDate) matches the query filter and speeds up lookups.

🔧 Debug

expert

Diagnose data inconsistency from poor table design

A table Inventory stores product quantities but allows duplicate ProductID rows. What problem can this cause?

A The database will reject inserts with duplicate ProductID

B Queries will always return zero rows

C Total quantity calculations may be incorrect due to duplicates

D The table will automatically merge duplicate rows

💡 Hint

Think about what happens when the same product appears multiple times with quantities.

Duplicates cause sums or counts to be wrong because the same product is counted multiple times.

Practice

1. Why is it important to design tables properly in a database?

easy

A. To keep data organized and easy to find

B. To make the database look colorful

C. To increase the size of the database unnecessarily

D. To confuse users with complex structures

Solution

1. Step 1: Understand the purpose of table design

   Good table design organizes data logically so it is easy to access and manage.

2. Step 2: Identify the benefits of proper design

   Proper design improves speed and reduces errors in data handling.

3. Final Answer:

   To keep data organized and easy to find -> Option A

4. Quick Check:

   Good design = organized data [OK]

Hint: Good design means organized, fast, and error-free data [OK]

Common Mistakes:

• Thinking design is about appearance
• Ignoring data organization
• Assuming bigger tables are better

2. Which of the following is the correct way to define a primary key in a SQL table?

easy

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

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

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

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

Solution

1. Step 1: Review SQL syntax for primary key

   The correct syntax places PRIMARY KEY after the column type.

2. Step 2: Check each option's syntax

   CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(50)); correctly uses 'id INT PRIMARY KEY'. Others misuse keywords or order.

3. Final Answer:

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

4. Quick Check:

   PRIMARY KEY follows column type [OK]

Hint: PRIMARY KEY goes right after the column type [OK]

Common Mistakes:

• Placing PRIMARY before KEY
• Using UNIQUE and PRIMARY together incorrectly
• Misordering keywords

3. Given the table employees(id INT PRIMARY KEY, name VARCHAR(50), department_id INT) and departments(id INT PRIMARY KEY, name VARCHAR(50)), what will this query return?

SELECT e.name, d.name FROM employees e JOIN departments d ON e.department_id = d.id;

medium

A. A list of all employees without department names

B. A syntax error due to missing WHERE clause

C. A list of employee names with their department names

D. A list of department names only

Solution

1. Step 1: Understand the JOIN operation

   The JOIN matches employees with departments where department_id equals department's id.

2. Step 2: Analyze the SELECT columns

   The query selects employee names and their matching department names.

3. Final Answer:

   A list of employee names with their department names -> Option C

4. Quick Check:

   JOIN on keys returns combined rows [OK]

Hint: JOIN matches rows by keys to combine related data [OK]

Common Mistakes:

• Thinking JOIN needs WHERE clause
• Expecting only one table's data
• Confusing JOIN with UNION

4. You have a table orders(order_id INT, customer_id INT, order_date DATE) but forgot to set a primary key. What problem might this cause?

medium

A. Duplicate orders might be stored without warning

B. The database will reject all inserts

C. Queries will run faster automatically

D. You cannot store dates without a primary key

Solution

1. Step 1: Understand the role of primary keys

   Primary keys ensure each row is unique and prevent duplicates.

2. Step 2: Identify consequences of missing primary key

   Without a primary key, duplicate rows can be inserted without errors.

3. Final Answer:

   Duplicate orders might be stored without warning -> Option A

4. Quick Check:

   No primary key = possible duplicates [OK]

Hint: Primary key prevents duplicate rows [OK]

Common Mistakes:

• Thinking inserts are blocked without primary key
• Assuming primary key affects date storage
• Believing queries always run faster without keys

5. You want to design a table to store product reviews with columns: review_id, product_id, user_id, rating, and comment. Which design choice improves efficiency and avoids mistakes?

hard

A. Store all data in one text column to simplify

B. Make all columns PRIMARY KEY to ensure uniqueness

C. Use only product_id as PRIMARY KEY and ignore others

D. Set review_id as PRIMARY KEY and product_id, user_id as FOREIGN KEYS

Solution

1. Step 1: Identify keys for uniqueness and relationships

   review_id uniquely identifies each review, so it should be PRIMARY KEY.

2. Step 2: Use FOREIGN KEYS for related data

   product_id and user_id link to other tables, so they should be FOREIGN KEYS.

3. Step 3: Evaluate other options

   Making all columns PRIMARY KEY is invalid; using one key ignores uniqueness; storing all data in one column loses structure.

4. Final Answer:

   Set review_id as PRIMARY KEY and product_id, user_id as FOREIGN KEYS -> Option D

5. Quick Check:

   Primary and foreign keys keep data correct and linked [OK]

Hint: Use primary key for unique ID, foreign keys for links [OK]

Common Mistakes:

• Making multiple columns primary keys incorrectly
• Ignoring foreign key relationships
• Storing all data in one column