Database in everyday apps (social media, banking) in Intro to Computing - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does a social media app use a database to show your friend list?

Imagine you open a social media app and see your list of friends. How does the app use a database to get this list?

A The app asks the database to find all friends linked to your user ID and sends back their names.

B The app stores all friends on your phone and never asks the database.

C The app sends your password to the database to get the friend list.

D The app randomly picks names from the database without checking your user ID.

💡 Hint

Think about how the app knows which friends belong to you.

The app queries the database using your user ID to find friends linked to you. This is how it gets the correct friend list.

❓ trace

intermediate

Trace the steps when you transfer money in a banking app

Follow the steps below and choose what happens right after the app checks your balance:

1. You enter amount to send.
2. App checks your balance.
3. ???

A The app sends a message to your friend without checking balances.

B The app asks you to enter your password again before checking balance.

C The app updates your balance and the receiver's balance in the database.

D The app deletes your account from the database.

💡 Hint

Think about what must happen after confirming you have enough money.

After confirming your balance is enough, the app updates both your and the receiver's balances in the database to reflect the transfer.

❓ Comparison

advanced

Compare how social media and banking apps use databases differently

Which statement best describes a key difference in how social media and banking apps use databases?

A Social media databases focus on fast access to many user connections, while banking databases focus on secure, accurate transaction records.

B Banking databases store only pictures, while social media databases store only numbers.

C Social media apps do not use databases, but banking apps do.

D Both apps use databases only to store passwords.

💡 Hint

Think about what each app needs most from its database.

Social media apps need to quickly access many user connections, while banking apps require secure and accurate transaction records.

❓ identification

advanced

Identify the database operation when you 'like' a post on social media

What database operation happens when you click 'like' on a post?

A The database sends an email to all users.

B The database deletes the post permanently.

C The database creates a new user account.

D The database updates the post's like count by adding your user ID to the list of likes.

💡 Hint

Think about what changes in the database when you like something.

Clicking 'like' updates the post's record in the database to include your user ID in the list of likes, increasing the like count.

🚀 Application

expert

What happens if a banking app database loses connection during a money transfer?

During a money transfer, the app loses connection to the database after deducting money from your account but before adding it to the receiver's account. What is the likely result?

A The transfer completes successfully without any issues.

B Your money is deducted but the receiver does not get it, causing an inconsistent state.

C The app automatically refunds the money without database access.

D The receiver gets double the money.

💡 Hint

Think about what happens if only part of a transaction is saved.

If the database connection is lost mid-transfer, only part of the transaction may be saved, causing money to be deducted from your account but not added to the receiver's, leading to errors.

Practice

1. What is the main purpose of a database in apps like social media or banking?

easy

A. To send emails to users automatically

B. To store and organize information so it can be easily found and used

C. To control the app's colors and fonts

D. To create graphics and animations for the app

Solution

1. Step 1: Understand what apps need

   Apps like social media and banking need to keep lots of information safe and easy to find.

2. Step 2: Role of a database

   A database stores and organizes this information in tables, like a digital filing cabinet.

3. Final Answer:

   To store and organize information so it can be easily found and used -> Option B

4. Quick Check:

   Database = store and organize info [OK]

Hint: Databases hold data, not design or emails [OK]

Common Mistakes:

• Confusing database with app design features
• Thinking database sends emails
• Mixing database with app styling

2. Which of the following is the correct way to describe a table in a database?

easy

A. A program that runs the app on your phone

B. A folder on your computer where files are saved

C. A place where data is stored in rows and columns, like a spreadsheet

D. A list of colors used in the app design

Solution

1. Step 1: Recall what a table is

   A table in a database organizes data in rows and columns, similar to a spreadsheet.

2. Step 2: Compare options

   Only 'A place where data is stored in rows and columns, like a spreadsheet' correctly describes this; others describe unrelated things.

3. Final Answer:

   A place where data is stored in rows and columns, like a spreadsheet -> Option C

4. Quick Check:

   Table = rows and columns [OK]

Hint: Think spreadsheet when you hear 'table' in databases [OK]

Common Mistakes:

• Confusing tables with app programs
• Mixing tables with computer folders
• Thinking tables are design elements

3. Consider a social media app database table named Users with columns UserID, Name, and Age. If the table has these rows:

UserID | Name   | Age
1      | Alice  | 25
2      | Bob    | 30
3      | Carol  | 22

What will be the result of a query that finds all users older than 23?

medium

A. Alice and Bob

B. Bob and Carol

C. Alice, Bob, and Carol

D. Only Carol

Solution

1. Step 1: Identify users older than 23

   Check each user's age: Alice (25) > 23, Bob (30) > 23, Carol (22) ≤ 23.

2. Step 2: List matching users

   Alice and Bob meet the condition; Carol does not.

3. Final Answer:

   Alice and Bob -> Option A

4. Quick Check:

   Age > 23 = Alice, Bob [OK]

Hint: Filter by age > 23 carefully [OK]

Common Mistakes:

• Including Carol who is 22
• Missing Bob who is 30
• Selecting all users without filtering

4. A banking app database has a table Accounts with columns AccountID, Balance. The following SQL query is written:

SELECT AccountID, Balance FROM Accounts WHERE Balance > 1000

But the app returns an error. What is the most likely mistake?

medium

A. The column Balance does not exist or is misspelled

B. The SQL query is missing a semicolon at the end

C. The table name Accounts is misspelled

D. The query should use SELECT * FROM Accounts instead

Solution

1. Step 1: Check query syntax

   The query syntax is correct and semicolon is optional in many systems.

2. Step 2: Verify column names

   If the app errors, likely the column Balance is misspelled or missing in the table.

3. Final Answer:

   The column Balance does not exist or is misspelled -> Option A

4. Quick Check:

   Column name error causes query failure [OK]

Hint: Check column names carefully for typos [OK]

Common Mistakes:

• Assuming missing semicolon causes error
• Thinking table name is wrong without checking
• Believing SELECT * fixes column errors

5. A social media app wants to show a list of friends for a user. The database has two tables:

Users(UserID, Name)
Friends(UserID1, UserID2)

If UserID1 and UserID2 represent friend pairs, which SQL query correctly finds all friends of user with UserID = 5?

hard

A. SELECT Name FROM Users WHERE UserID IN (SELECT UserID2 FROM Friends WHERE UserID1 = 5)

B. SELECT Name FROM Users WHERE UserID = 5

C. SELECT Name FROM Users WHERE UserID IN (SELECT UserID1 FROM Friends WHERE UserID2 = 5)

D. SELECT Name FROM Users WHERE UserID IN (SELECT UserID1 FROM Friends WHERE UserID2 = 5) OR UserID IN (SELECT UserID2 FROM Friends WHERE UserID1 = 5)

Solution

1. Step 1: Understand friend pairs

   Friends table stores pairs (UserID1, UserID2) meaning both are friends.

2. Step 2: Find all friends of UserID 5

   Friends can appear as UserID1 or UserID2, so query must check both sides.

3. Step 3: Analyze options

   SELECT Name FROM Users WHERE UserID IN (SELECT UserID1 FROM Friends WHERE UserID2 = 5) OR UserID IN (SELECT UserID2 FROM Friends WHERE UserID1 = 5) checks both UserID1 and UserID2 for 5, correctly finding all friends.

4. Final Answer:

   SELECT Name FROM Users WHERE UserID IN (SELECT UserID1 FROM Friends WHERE UserID2 = 5) OR UserID IN (SELECT UserID2 FROM Friends WHERE UserID1 = 5) -> Option D

5. Quick Check:

   Check both friend columns for user 5 [OK]

Hint: Friends can be in either column, check both [OK]

Common Mistakes:

• Checking only one side of friend pairs
• Selecting user 5 instead of their friends
• Using wrong columns in subqueries