SELECT with expressions and calculations in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

SQL

SELECT quantity * [1] AS total_price FROM orders;

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

Acost

Btotal

Cprice_per_unit

Damount

Attempts:

3 left

💡 Hint

Common Mistakes

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

Forgetting to multiply quantity by the price per unit.

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

2fill in blank

medium

Complete the code to calculate the discounted price by subtracting discount from price.

SQL

SELECT price - [1] AS discounted_price FROM products;

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

Atax

Bcost

Cfee

Ddiscount

Attempts:

3 left

💡 Hint

Common Mistakes

Subtracting tax instead of discount.

Using a column that increases price instead of decreasing.

The discount column holds the amount to subtract from the price to get the discounted price.

3fill in blank

hard

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

SQL

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

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

Atax_rate

Bfee

Cdiscount

Dcost

Attempts:

3 left

💡 Hint

Common Mistakes

Using discount instead of tax rate.

Using a column that does not represent a percentage.

The tax_rate column holds the tax percentage to add to the price for the total cost.

4fill in blank

hard

Fill both blanks to calculate the final price after applying discount and adding tax.

SQL

SELECT price - [1] + price * [2] AS final_price FROM invoices;

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

Adiscount

Btax_rate

Cfee

Dcost

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping discount and tax_rate in the expression.

Using fee or cost instead of discount or tax_rate.

First, subtract the discount from the price, then add the tax calculated by multiplying price by tax_rate.

5fill in blank

hard

Fill all three blanks to select the product name, calculate total revenue, and filter for revenue greater than 1000.

SQL

SELECT product_name, quantity * [1] AS total_revenue FROM sales WHERE quantity * [2] [3] 1000;

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

Aprice_per_unit

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using different columns for revenue calculation in SELECT and WHERE.

Using '<' instead of '>' in the WHERE clause.

Multiply quantity by price_per_unit to get total revenue. Filter rows where total revenue is greater than 1000 using the '>' operator.

Practice

1. What does the SQL expression SELECT price * quantity AS total_cost FROM sales; do?

easy

A. Calculates total cost by multiplying price and quantity for each row.

B. Adds price and quantity values together for each row.

C. Divides price by quantity for each row.

D. Filters rows where price equals quantity.

Solution

1. Step 1: Understand the multiplication in SELECT

   The expression multiplies the values in columns price and quantity for each row.

2. Step 2: Recognize the alias usage

   The result of multiplication is renamed as total_cost for clarity in output.

3. Final Answer:

   Calculates total cost by multiplying price and quantity for each row. -> Option A

4. Quick Check:

   price * quantity = total_cost [OK]

Hint: Look for * operator to identify multiplication [OK]

Common Mistakes:

• Confusing multiplication with addition or division
• Ignoring the AS alias
• Thinking it filters rows

2. Which of the following is the correct syntax to calculate the average price and rename it as avg_price?

easy

A. SELECT AVG(price) AS avg_price FROM products;

B. SELECT AVG(price) avg_price FROM products;

C. SELECT AVG(price) TO avg_price FROM products;

D. SELECT AVG(price) RENAME avg_price FROM products;

Solution

1. Step 1: Identify correct alias syntax

   In SQL, the keyword AS is used to rename columns or expressions in SELECT.

2. Step 2: Check each option's syntax

   SELECT AVG(price) AS avg_price FROM products; uses AVG(price) AS avg_price which is the correct syntax; others use invalid keywords or omit AS.

3. Final Answer:

   SELECT AVG(price) AS avg_price FROM products; -> Option A

4. Quick Check:

   Use AS to rename columns [OK]

Hint: Use AS keyword to rename calculated columns [OK]

Common Mistakes:

• Omitting AS keyword
• Using TO or RENAME instead of AS
• Misplacing parentheses

3. Given a table orders with columns price and quantity, what is the output of this query?

SELECT price, quantity, price * quantity AS total FROM orders WHERE price = 10;

medium

A. All rows with total calculated, ignoring price filter.

B. Syntax error due to missing GROUP BY.

C. Rows where price is 10, showing price, quantity, and total (price*quantity).

D. Rows where quantity equals 10.

Solution

1. Step 1: Understand the WHERE clause effect

   The query filters rows to only those where price equals 10.

2. Step 2: Calculate total for filtered rows

   For each filtered row, it shows price, quantity, and total which is price multiplied by quantity.

3. Final Answer:

   Rows where price is 10, showing price, quantity, and total (price*quantity). -> Option C

4. Quick Check:

   WHERE price=10 filters rows; total = price*quantity [OK]

Hint: Check WHERE filters before calculations [OK]

Common Mistakes:

• Ignoring WHERE clause filtering
• Expecting all rows instead of filtered
• Thinking GROUP BY is needed here

4. Identify the error in this SQL query:

SELECT price + quantity AS total FROM sales WHERE total > 100;

medium

A. Missing FROM clause in the query.

B. You cannot use alias 'total' in WHERE clause directly.

C. The + operator is invalid for numeric addition.

D. AS keyword cannot be used for aliasing.

Solution

1. Step 1: Understand alias scope in SQL

   Aliases defined in SELECT cannot be used in WHERE clause because WHERE is evaluated before SELECT.

2. Step 2: Identify correct fix

   Use the full expression (price + quantity) in WHERE or use a subquery/CTE to filter by total.

3. Final Answer:

   You cannot use alias 'total' in WHERE clause directly. -> Option B

4. Quick Check:

   Alias not usable in WHERE [OK]

Hint: Aliases appear after WHERE; can't use them there [OK]

Common Mistakes:

• Using alias in WHERE clause
• Thinking + is invalid operator
• Ignoring query evaluation order

5. You want to calculate the discounted price for products with a 10% discount and show it as discounted_price. Which query correctly applies the discount and rounds the result to 2 decimals?

hard

A. SELECT price - (price * 0.10) AS discounted_price FROM products;

B. SELECT price * 0.90 AS discounted_price FROM products;

C. SELECT ROUND(price * 0.90) AS discounted_price FROM products;

D. SELECT ROUND(price - (price * 0.10), 2) AS discounted_price FROM products;

Solution

1. Step 1: Calculate 10% discount correctly

   Discounted price is price minus 10% of price, which is price - (price * 0.10) or price * 0.90.

2. Step 2: Round the result to 2 decimals

   Use ROUND(expression, 2) to round to two decimal places for currency formatting.

3. Final Answer:

   SELECT ROUND(price - (price * 0.10), 2) AS discounted_price FROM products; -> Option D

4. Quick Check:

   Use ROUND(..., 2) for 2 decimals [OK]

Hint: Use ROUND(expression, 2) to round decimals [OK]

Common Mistakes:

• Not rounding the result
• Rounding without specifying decimals
• Using incorrect discount calculation