HAVING for filtering groups in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter groups having more than 3 orders.

PostgreSQL

SELECT customer_id, COUNT(*) FROM orders GROUP BY customer_id HAVING COUNT(*) [1] 3;

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

A>

B<

C=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE instead of HAVING for aggregated conditions.

Using '<' instead of '>' causing wrong filtering.

The HAVING clause filters groups where the count of orders is greater than 3.

2fill in blank

medium

Complete the code to select product categories with total sales equal to 1000.

PostgreSQL

SELECT category, SUM(sales) FROM products GROUP BY category HAVING SUM(sales) [1] 1000;

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

A<

B=

C>

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE instead of HAVING for aggregate conditions.

Using '>' or '<' when exact match is needed.

The HAVING clause filters groups where the total sales exactly equal 1000.

3fill in blank

hard

Fix the error in the code to filter groups with average rating at least 4.

PostgreSQL

SELECT product_id, AVG(rating) FROM reviews GROUP BY product_id HAVING AVG(rating) [1] 4;

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

A<

B=

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' excludes ratings exactly 4.

Using WHERE instead of HAVING for aggregate filtering.

The HAVING clause filters groups where the average rating is greater than or equal to 4.

4fill in blank

hard

Fill both blanks to select departments with more than 5 employees and total salary less than 50000.

PostgreSQL

SELECT department, COUNT(*) AS emp_count, SUM(salary) AS total_salary FROM employees GROUP BY department HAVING COUNT(*) [1] 5 AND SUM(salary) [2] 50000;

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

A>

B<

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE instead of HAVING for aggregate conditions.

Mixing up comparison operators causing wrong filtering.

The HAVING clause filters departments with employee count greater than 5 and total salary less than 50000.

5fill in blank

hard

Fill all three blanks to select cities with average temperature above 20, count of days above 10, and max temperature below 35.

PostgreSQL

SELECT city, AVG(temperature) AS avg_temp, COUNT(*) AS hot_days, MAX(temperature) AS max_temp FROM weather GROUP BY city HAVING AVG(temperature) [1] 20 AND COUNT(*) [2] 10 AND MAX(temperature) [3] 35;

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

A>

B>=

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE instead of HAVING for aggregate filters.

Using wrong comparison operators causing incorrect results.

The HAVING clause filters cities where average temperature is above 20, count of days is at least 10, and max temperature is below 35.

Practice

1. What is the main purpose of the HAVING clause in a PostgreSQL query?

easy

A. To join two tables

B. To filter rows before grouping

C. To sort the result set

D. To filter groups after aggregation

Solution

1. Step 1: Understand the role of HAVING

   The HAVING clause is used to filter groups created by GROUP BY based on aggregate conditions.

2. Step 2: Differentiate HAVING from WHERE

   WHERE filters rows before grouping, HAVING filters groups after aggregation.

3. Final Answer:

   To filter groups after aggregation -> Option D

4. Quick Check:

   HAVING filters groups = D [OK]

Hint: Remember HAVING filters groups, WHERE filters rows [OK]

Common Mistakes:

• Confusing HAVING with WHERE clause
• Thinking HAVING sorts data
• Using HAVING without GROUP BY

2. Which of the following is the correct syntax to filter groups having a total sales greater than 1000 using HAVING?

easy

A. SELECT store, SUM(sales) FROM sales_data HAVING SUM(sales) > 1000 GROUP BY store;

B. SELECT store, SUM(sales) FROM sales_data GROUP BY store HAVING SUM(sales) > 1000;

C. SELECT store, SUM(sales) FROM sales_data WHERE SUM(sales) > 1000 GROUP BY store;

D. SELECT store, SUM(sales) FROM sales_data GROUP BY store WHERE SUM(sales) > 1000;

Solution

1. Step 1: Identify correct order of clauses

   GROUP BY must come before HAVING in SQL syntax.

2. Step 2: Check aggregate usage in HAVING

   HAVING uses aggregate functions like SUM(sales) to filter groups.

3. Final Answer:

   SELECT store, SUM(sales) FROM sales_data GROUP BY store HAVING SUM(sales) > 1000; -> Option B

4. Quick Check:

   GROUP BY before HAVING = B [OK]

Hint: HAVING always follows GROUP BY clause [OK]

Common Mistakes:

• Placing HAVING before GROUP BY
• Using WHERE with aggregate functions
• Incorrect clause order

3. Given the table orders with columns customer_id and amount, what will this query return?

SELECT customer_id, COUNT(*) FROM orders GROUP BY customer_id HAVING COUNT(*) > 2;

medium

A. All customers with more than 2 orders

B. All customers with exactly 2 orders

C. All customers with less than 2 orders

D. Syntax error due to HAVING clause

Solution

1. Step 1: Understand the GROUP BY and COUNT(*)

   The query groups rows by customer_id and counts orders per customer.

2. Step 2: Apply HAVING filter

   HAVING COUNT(*) > 2 keeps only customers with more than 2 orders.

3. Final Answer:

   All customers with more than 2 orders -> Option A

4. Quick Check:

   HAVING COUNT(*) > 2 filters groups with >2 rows [OK]

Hint: HAVING filters groups by aggregate results [OK]

Common Mistakes:

• Confusing > 2 with = 2
• Thinking HAVING causes syntax error
• Ignoring GROUP BY effect

4. Identify the error in this query:

SELECT department, AVG(salary) FROM employees HAVING AVG(salary) > 50000 GROUP BY department;

medium

A. HAVING clause used before GROUP BY

B. AVG() cannot be used in HAVING

C. Missing WHERE clause before HAVING

D. No error, query is correct

Solution

1. Step 1: Check clause order

   SQL requires GROUP BY before HAVING clause.

2. Step 2: Identify error in query

   HAVING appears before GROUP BY, which is invalid syntax.

3. Final Answer:

   HAVING clause used before GROUP BY -> Option A

4. Quick Check:

   GROUP BY must precede HAVING [OK]

Hint: GROUP BY comes before HAVING always [OK]

Common Mistakes:

• Placing HAVING before GROUP BY
• Assuming AVG() not allowed in HAVING
• Confusing WHERE and HAVING

5. You have a sales table with columns region, salesperson, and amount. Write a query to find regions where the total sales exceed 10,000 but only include salespeople who individually sold more than 3,000. Which query correctly applies HAVING to achieve this?

hard

A. SELECT region, salesperson, SUM(amount) FROM sales GROUP BY region, salesperson HAVING SUM(amount) > 3000 AND SUM(amount) > 10000;

B. SELECT region, SUM(amount) FROM sales GROUP BY region HAVING SUM(amount) > 10000 AND salesperson IN (SELECT salesperson FROM sales GROUP BY salesperson HAVING SUM(amount) > 3000);

C. SELECT region, salesperson, SUM(amount) FROM sales GROUP BY region, salesperson HAVING SUM(amount) > 3000 AND region IN (SELECT region FROM sales GROUP BY region HAVING SUM(amount) > 10000);

D. SELECT region, salesperson, SUM(amount) FROM sales WHERE SUM(amount) > 3000 GROUP BY region, salesperson HAVING SUM(amount) > 10000;

Solution

1. Step 1: Understand filtering conditions

   We want regions with total sales > 10,000 and salespeople with sales > 3,000.

2. Step 2: Analyze correct logic

   Group by region and salesperson, filter salespeople with >3000 sales using HAVING SUM(amount) > 3000, and filter regions with total sales > 10000 using a subquery in HAVING: region IN (SELECT region FROM sales GROUP BY region HAVING SUM(amount) > 10000).

3. Step 3: Check HAVING usage

   This correctly applies HAVING on both salesperson and region aggregates using subquery.

4. Final Answer:

   SELECT region, salesperson, SUM(amount) FROM sales GROUP BY region, salesperson HAVING SUM(amount) > 3000 AND region IN (SELECT region FROM sales GROUP BY region HAVING SUM(amount) > 10000); -> Option C

5. Quick Check:

   Use HAVING with subqueries for multi-level group filters [OK]

Hint: Use HAVING with subqueries for multi-level group filters [OK]

Common Mistakes:

• Using WHERE with aggregate functions
• Incorrect clause order
• Combining unrelated HAVING conditions without subqueries