SELECT with expressions and calculations in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the SELECT statement do in SQL?

The SELECT statement retrieves data from a database table and returns it as a result set.

beginner

How can you perform calculations in a SELECT statement?

You can use arithmetic operators like +, -, *, / directly in the SELECT clause to calculate values from columns or constants.

beginner

Example: What does this query do?
SELECT price, quantity, price * quantity AS total_cost FROM sales;

It selects the price and quantity columns from the sales table and calculates a new column total_cost by multiplying price and quantity for each row.

intermediate

Can you use functions in SELECT expressions? Give an example.

Yes, you can use functions like ROUND, ABS, or CONCAT in SELECT expressions. For example:
SELECT ROUND(price * quantity, 2) AS total_rounded FROM sales; rounds the total cost to 2 decimal places.

intermediate

What happens if you divide by zero in a SELECT calculation?

Dividing by zero causes an error in SQL. You should handle it using CASE or NULLIF to avoid errors, for example:
SELECT price / NULLIF(quantity, 0) AS safe_division FROM sales;

Which operator is used to multiply two columns in a SELECT statement?

A /

B +

C -

D *

The * operator multiplies values in SQL expressions.

What will this query return?
SELECT 10 + 5 AS sum_result;

A A list of numbers from 10 to 5

B An error because no table is specified

C A column named sum_result with value 15

D NULL

You can select expressions without a table; it returns the calculated value as a column.

How do you rename a calculated column in the result set?

A Using GROUP BY

B Using AS keyword

C Using WHERE clause

D Using ORDER BY

The AS keyword assigns an alias (name) to the calculated column.

Which function rounds a number to a specific decimal place in SQL?

A ROUND

B ABS

C CONCAT

D LENGTH

ROUND rounds numbers to the specified decimal places.

What is the result of dividing by zero in a SQL SELECT expression?

A An error occurs

B Returns zero

C Returns NULL automatically

D Returns infinity

Dividing by zero causes an error; it must be handled to avoid query failure.

Explain how to use expressions and calculations in a SELECT statement with an example.

Think about multiplying two columns and giving the result a name.

Describe how to safely handle division in SELECT expressions to avoid errors.

Consider how to avoid dividing by zero in your calculation.

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