Why table design matters in SQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select all columns from the employees table.

SQL

SELECT [1] FROM employees;

Drag options to blanks, or click blank then click option'

AALL

B*

Ceverything

Dcolumns

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'ALL' instead of '*'

Writing column names when asked for all columns

The asterisk (*) selects all columns from the table.

2fill in blank

medium

Complete the code to create a table named customers with an id column as the primary key.

SQL

CREATE TABLE customers (id [1] PRIMARY KEY);

Drag options to blanks, or click blank then click option'

AINT

BVARCHAR(10)

CTEXT

DDATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using TEXT or VARCHAR for primary key without reason

Choosing DATE type for an id

The INT type is commonly used for primary keys as unique identifiers.

3fill in blank

hard

Fix the error in the code to add a new column named email of type VARCHAR(255) to the users table.

SQL

ALTER TABLE users [1] COLUMN email VARCHAR(255);

Drag options to blanks, or click blank then click option'

ACHANGE

BMODIFY

CADD

DUPDATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using MODIFY or CHANGE which are for altering existing columns

Using UPDATE which is for data changes

To add a new column, use the ADD keyword in ALTER TABLE.

4fill in blank

hard

Fill both blanks to create a table named orders with order_id as primary key and order_date as a date column.

SQL

CREATE TABLE orders (order_id [1] PRIMARY KEY, order_date [2]);

Drag options to blanks, or click blank then click option'

AINT

BVARCHAR(100)

CDATE

DTEXT

Attempts:

3 left

💡 Hint

Common Mistakes

Using VARCHAR for order_date

Using TEXT for order_id

Primary keys are often integers (INT), and dates use the DATE type.

5fill in blank

hard

Fill all three blanks to select customer_name and order_date from orders where order_date is after 2023-01-01.

SQL

SELECT [1], [2] FROM orders WHERE [3] > '2023-01-01';

Drag options to blanks, or click blank then click option'

Acustomer_name

Border_date

Dcustomer_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using customer_id instead of customer_name

Filtering by wrong column in WHERE

You select customer_name and order_date columns, and filter where order_date is greater than the date given.

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