COUNT, SUM, AVG, MIN, MAX in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count all rows in the employees table.

PostgreSQL

SELECT [1](*) FROM employees;

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

AAVG

BSUM

CCOUNT

DMIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUM instead of COUNT will cause an error or wrong result.

Using AVG or MIN does not count rows.

The COUNT function counts the number of rows in a table or group.

2fill in blank

medium

Complete the code to find the total salary from the employees table.

PostgreSQL

SELECT [1](salary) FROM employees;

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

ASUM

BAVG

CMAX

DCOUNT

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT will count rows, not add salaries.

Using AVG will give average salary, not total.

The SUM function adds all values in a column, here it adds all salaries.

3fill in blank

hard

Fix the error in the code to get the average age from the users table.

PostgreSQL

SELECT [1](age) FROM users;

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

ACOUNT

BAVG

CSUM

DMIN

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUM or COUNT will not give the average.

Using MIN gives the smallest value, not average.

The AVG function calculates the average value of a column.

4fill in blank

hard

Fill both blanks to find the minimum and maximum price from the products table.

PostgreSQL

SELECT [1](price) AS min_price, [2](price) AS max_price FROM products;

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

AMIN

BMAX

CSUM

DCOUNT

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping MIN and MAX will give wrong results.

Using SUM or COUNT here is incorrect.

MIN finds the smallest value, and MAX finds the largest value in a column.

5fill in blank

hard

Fill all three blanks to get the count of orders, total amount, and average amount from the orders table.

PostgreSQL

SELECT [1](*) AS total_orders, [2](amount) AS total_amount, [3](amount) AS average_amount FROM orders;

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

ACOUNT

BSUM

CAVG

DMIN

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up SUM and AVG functions.

Using MIN instead of AVG for average.

COUNT(*) counts all orders, SUM(amount) adds all amounts, and AVG(amount) calculates the average amount.

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