COUNT, SUM, AVG, MIN, MAX in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this COUNT query?

Consider a table employees with columns id, department, and salary. The table has 5 rows with these departments: 'HR', 'IT', 'IT', 'HR', 'Finance'. What is the result of this query?

SELECT COUNT(*) FROM employees WHERE department = 'IT';

PostgreSQL

SELECT COUNT(*) FROM employees WHERE department = 'IT';

A 2

B 3

C 5

D 0

💡 Hint

COUNT(*) counts all rows that match the condition.

There are exactly 2 employees in the 'IT' department, so COUNT(*) returns 2.

❓ query_result

intermediate

What is the average salary for the HR department?

Given the employees table with salary values: 50000, 60000, 55000, 70000, 65000 for departments 'HR', 'IT', 'IT', 'HR', 'Finance' respectively, what does this query return?

SELECT AVG(salary) FROM employees WHERE department = 'HR';

PostgreSQL

SELECT AVG(salary) FROM employees WHERE department = 'HR';

A 65000

B 60000

C 55000

D 70000

💡 Hint

AVG calculates the average of the selected salaries.

The HR salaries are 50000 and 70000. Their average is (50000 + 70000) / 2 = 60000.

📝 Syntax

advanced

Which query correctly sums salaries for Finance department?

Choose the query that correctly sums the salaries of employees in the 'Finance' department.

A SELECT SUM(salary) FROM employees WHERE department = 'Finance';

B SELECT SUM salary FROM employees WHERE department = 'Finance';

C SELECT SUM(salary) FROM employees GROUP BY department = 'Finance';

D SELECT SUM(salary) WHERE department = 'Finance' FROM employees;

💡 Hint

SUM needs parentheses and WHERE clause after FROM.

Option A uses correct syntax: SUM(salary) with WHERE clause after FROM. Others have syntax errors.

❓ query_result

advanced

What is the minimum salary in the employees table?

Given the salaries 50000, 60000, 55000, 70000, 65000 in the employees table, what does this query return?

SELECT MIN(salary) FROM employees;

PostgreSQL

SELECT MIN(salary) FROM employees;

A 60000

B 55000

C 50000

D 65000

💡 Hint

MIN returns the smallest value in the column.

The smallest salary is 50000, so MIN(salary) returns 50000.

🧠 Conceptual

expert

Which aggregate function ignores NULL values in its calculation?

In SQL aggregation, which function does NOT count NULL values when computing its result?

A COUNT(*)

B AVG(column_name)

C SUM(column_name)

D COUNT(column_name)

💡 Hint

Think about how COUNT(*) differs from COUNT(column_name).

COUNT(column_name) counts only non-NULL values, ignoring NULLs. COUNT(*) counts all rows including NULLs. SUM and AVG also ignore NULLs in their calculations.

Practice

1. What does the SQL function COUNT(column_name) do in a PostgreSQL query?

easy

A. Finds the average of all values in the specified column.

B. Counts the number of non-null values in the specified column.

C. Calculates the sum of all values in the specified column.

D. Returns the maximum value in the specified column.

Solution

1. Step 1: Understand COUNT function behavior

   The COUNT function counts how many rows have a non-null value in the specified column.

2. Step 2: Differentiate from other aggregate functions

   SUM adds values, AVG finds average, MAX finds maximum, so they do not count rows.

3. Final Answer:

   Counts the number of non-null values in the specified column. -> Option B

4. Quick Check:

   COUNT = number of non-null rows [OK]

Hint: COUNT counts non-null rows, not sums or averages [OK]

Common Mistakes:

• Thinking COUNT sums values
• Assuming COUNT includes nulls
• Confusing COUNT with AVG or MAX

2. Which of the following is the correct syntax to find the average of the price column in a table named products?

easy

A. SELECT AVG price FROM products;

B. SELECT AVG[price] FROM products;

C. SELECT AVG(price) FROM products;

D. SELECT AVERAGE(price) FROM products;

Solution

1. Step 1: Recall correct function syntax

   Aggregate functions like AVG require parentheses around the column name: AVG(column_name).

2. Step 2: Check each option's syntax

   SELECT AVG(price) FROM products; uses correct syntax: AVG(price). Options A, B, and C have syntax errors or invalid function names.

3. Final Answer:

   SELECT AVG(price) FROM products; -> Option C

4. Quick Check:

   AVG(column) uses parentheses [OK]

Hint: Use parentheses around column in aggregate functions [OK]

Common Mistakes:

• Omitting parentheses in AVG
• Using wrong function name like AVERAGE
• Using brackets instead of parentheses

3. Given the table sales with column amount containing values (100, 200, NULL, 300), what is the result of this query?

SELECT COUNT(amount), SUM(amount), AVG(amount), MIN(amount), MAX(amount) FROM sales;

medium

A. (4, 600, 150, 100, 300)

B. (4, 600, 200, 100, 300)

C. (3, 600, 150, 100, 300)

D. (3, 600, 200, 100, 300)

Solution

1. Step 1: Count non-null values in amount

   Values are 100, 200, NULL, 300. COUNT(amount) counts non-null values = 3.

2. Step 2: Calculate SUM, AVG, MIN, MAX ignoring NULL

   SUM = 100+200+300 = 600; AVG = 600/3 = 200; MIN = 100; MAX = 300.

3. Final Answer:

   (3, 600, 200, 100, 300) -> Option D

4. Quick Check:

   COUNT=3, SUM=600, AVG=200 [OK]

Hint: Aggregate functions ignore NULL except COUNT(*) [OK]

Common Mistakes:

• Counting NULL as a value in COUNT(column)
• Dividing AVG by total rows including NULL
• Confusing COUNT(column) with COUNT(*)

4. Identify the error in this query:

SELECT COUNT(*), SUM(price), AVG(price) FROM products WHERE price > 'ten';

medium

A. Comparing price (numeric) with string 'ten' causes error.

B. Using COUNT(*) with WHERE clause is invalid.

C. SUM(price) cannot be used with AVG(price) in same query.

D. Missing GROUP BY clause for aggregate functions.

Solution

1. Step 1: Analyze WHERE clause condition

   The condition price > 'ten' compares a numeric column to a string, which causes a type error in PostgreSQL.

2. Step 2: Check other parts of the query

   COUNT(*) with WHERE is valid; SUM and AVG can be used together; GROUP BY not needed without grouping.

3. Final Answer:

   Comparing price (numeric) with string 'ten' causes error. -> Option A

4. Quick Check:

   Numeric compared to string causes error [OK]

Hint: Compare numeric columns only with numbers, not strings [OK]

Common Mistakes:

• Assuming COUNT(*) cannot be used with WHERE
• Thinking GROUP BY is always required with aggregates
• Mixing data types in WHERE conditions

5. You have a table orders with columns customer_id and total_amount. How do you write a query to find the average order amount for each customer, but only include customers with at least 5 orders?

hard

A. SELECT customer_id, AVG(total_amount) FROM orders GROUP BY customer_id HAVING COUNT(*) >= 5;

B. SELECT customer_id, AVG(total_amount) FROM orders WHERE COUNT(*) >= 5 GROUP BY customer_id;

C. SELECT customer_id, AVG(total_amount) FROM orders GROUP BY customer_id WHERE COUNT(*) >= 5;

D. SELECT customer_id, AVG(total_amount) FROM orders HAVING COUNT(*) >= 5 GROUP BY customer_id;

Solution

1. Step 1: Group orders by customer_id

   Use GROUP BY customer_id to aggregate orders per customer.

2. Step 2: Filter groups with HAVING clause

   HAVING COUNT(*) >= 5 filters customers with at least 5 orders.

3. Step 3: Calculate average order amount per customer

   Use AVG(total_amount) to find average order value per customer.

4. Final Answer:

   SELECT customer_id, AVG(total_amount) FROM orders GROUP BY customer_id HAVING COUNT(*) >= 5; -> Option A

5. Quick Check:

   GROUP BY + HAVING filters groups [OK]

Hint: Use HAVING to filter groups after GROUP BY [OK]

Common Mistakes:

• Using WHERE instead of HAVING for group filters
• Placing HAVING before GROUP BY
• Not grouping by customer_id