Why computed values add flexibility in MySQL - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the total price by multiplying quantity and price_per_unit.

MySQL

SELECT quantity * [1] AS total_price FROM orders;

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

Aprice_per_unit

Btotal

Cprice

Dcost

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column name that does not exist in the table.

Forgetting to multiply quantity by the unit price.

The column price_per_unit holds the unit price. Multiplying it by quantity gives the total price.

2fill in blank

medium

Complete the code to calculate the discount amount by multiplying price and discount_rate.

MySQL

SELECT price * [1] AS discount_amount FROM products;

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

Arate

Bdiscount

Cdiscount_rate

Ddiscount_percent

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column that does not represent the discount rate.

Confusing discount amount with discount rate.

The discount_rate column holds the rate to calculate the discount. Multiplying it by price gives the discount amount.

3fill in blank

hard

Fix the error in the code to compute the total cost including tax.

MySQL

SELECT price * quantity + price * quantity * [1] AS total_cost FROM sales;

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

Atax_rate

Btax

Ctaxamount

Drate

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column that does not represent the tax rate.

Forgetting to multiply tax rate by the total price.

The tax_rate column holds the tax percentage. Multiplying it by the total price (price * quantity) adds the tax amount.

4fill in blank

hard

Fill both blanks to calculate the final price after discount.

MySQL

SELECT price * [1] AS discount_amount, price - [2] AS final_price FROM items;

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

Adiscount_rate

Bdiscount_amount

Cdiscount

Drate

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up discount rate and discount amount.

Using the same column for both blanks.

The first blank uses discount_rate to calculate the discount amount. The second blank uses discount_amount to subtract from price for the final price.

5fill in blank

hard

Fill all three blanks to compute total cost including tax and discount.

MySQL

SELECT price * quantity AS subtotal, subtotal * [1] AS tax_amount, subtotal - subtotal * [2] + [3] AS total_cost FROM orders;

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

Atax_rate

Bdiscount_rate

Ctax_amount

Ddiscount_amount

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing tax and discount columns.

Using discount amount before calculating it.

The first blank is tax_rate to calculate tax amount from subtotal. The second blank is discount_rate to calculate discount from subtotal. The third blank is tax_amount to add tax back after discount.

Practice

1. What is the main benefit of using computed values in a MySQL query?

easy

A. They create new data from existing columns without extra storage

B. They permanently change the original data in the table

C. They slow down the query execution significantly

D. They require additional tables to store results

Solution

1. Step 1: Understand computed values

   Computed values are created on the fly using existing columns without storing extra data.

2. Step 2: Compare options

   Only They create new data from existing columns without extra storage correctly states this benefit; others describe incorrect effects.

3. Final Answer:

   They create new data from existing columns without extra storage -> Option A

4. Quick Check:

   Computed values = new data without storage [OK]

Hint: Computed values don't store data, they calculate on demand [OK]

Common Mistakes:

• Thinking computed values change stored data
• Assuming computed values need extra tables
• Believing computed values always slow queries

2. Which of the following is the correct syntax to create a computed column showing total price as quantity * price in a SELECT statement?

easy

A. SELECT quantity + price AS total_price FROM sales;

B. SELECT quantity * price AS total_price FROM sales;

C. SELECT quantity price AS total_price FROM sales;

D. SELECT total_price = quantity * price FROM sales;

Solution

1. Step 1: Identify correct arithmetic operator

   Multiplication uses the * operator, so quantity * price is correct.

2. Step 2: Check SQL syntax for aliasing

   Using AS total_price correctly names the computed column.

3. Final Answer:

   SELECT quantity * price AS total_price FROM sales; -> Option B

4. Quick Check:

   Use * for multiplication and AS for alias [OK]

Hint: Use * for multiplication and AS to name computed columns [OK]

Common Mistakes:

• Using + instead of * for multiplication
• Missing AS keyword for alias
• Writing invalid syntax like quantity price

3. Given the table employees with columns first_name, last_name, and birth_year, what will this query return?

SELECT first_name, last_name, YEAR(CURDATE()) - birth_year AS age FROM employees;

medium

A. An error because YEAR() cannot be used here

B. A list of employees with their birth years only

C. A list of employees with their full names and current ages

D. A list of employees with their ages but no names

Solution

1. Step 1: Understand the computed column

   The expression YEAR(CURDATE()) - birth_year calculates current age by subtracting birth year from current year.

2. Step 2: Analyze selected columns

   The query selects first_name, last_name, and the computed age, so it returns names and ages.

3. Final Answer:

   A list of employees with their full names and current ages -> Option C

4. Quick Check:

   Computed age = current year - birth year [OK]

Hint: Subtract birth year from current year to get age [OK]

Common Mistakes:

• Thinking YEAR() function causes error
• Ignoring the computed age column
• Assuming only birth years are returned

4. Identify the error in this query that tries to compute full name:

SELECT first_name + ' ' + last_name AS full_name FROM users;

medium

A. Using + to concatenate strings causes an error in MySQL

B. Missing AS keyword for alias

C. Using single quotes around space is invalid

D. The table name users is incorrect

Solution

1. Step 1: Check string concatenation syntax in MySQL

   MySQL uses CONCAT() function, not + operator, to join strings.

2. Step 2: Verify other parts of query

   AS keyword and single quotes are correct; table name assumed valid.

3. Final Answer:

   Using + to concatenate strings causes an error in MySQL -> Option A

4. Quick Check:

   Use CONCAT() for strings, not + [OK]

Hint: Use CONCAT() to join strings in MySQL [OK]

Common Mistakes:

• Using + for string concatenation
• Confusing single quotes with double quotes
• Assuming AS is optional

5. You want to create a query that shows each product's name and its discounted price, which is 10% less than the original price stored in price. Which query correctly uses a computed value to achieve this?

hard

A. SELECT product_name, price / 0.10 AS discounted_price FROM products;

B. SELECT product_name, price * 10 AS discounted_price FROM products;

C. SELECT product_name, price + 0.10 AS discounted_price FROM products;

D. SELECT product_name, price - (price * 0.10) AS discounted_price FROM products;

Solution

1. Step 1: Calculate 10% discount correctly

   10% discount means subtracting 10% of price: price - (price * 0.10).

2. Step 2: Check other options for correctness

   SELECT product_name, price * 10 AS discounted_price FROM products; multiplies price by 10 (wrong), C adds 0.10 (wrong), D divides by 0.10 (wrong).

3. Final Answer:

   SELECT product_name, price - (price * 0.10) AS discounted_price FROM products; -> Option D

4. Quick Check:

   Discounted price = price minus 10% [OK]

Hint: Subtract 10% by price - (price * 0.10) [OK]

Common Mistakes:

• Multiplying price by 10 instead of 0.10
• Adding 0.10 instead of subtracting percentage
• Dividing price by 0.10 incorrectly