WHERE vs HAVING mental model in SQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to filter rows where the age is greater than 30.

SQL

SELECT * FROM employees WHERE age [1] 30;

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

A<=

B<

C=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will select younger employees.

Using '=' will only select employees exactly 30 years old.

The WHERE clause filters rows before grouping. Here, we want employees older than 30, so we use age > 30.

2fill in blank

medium

Complete the code to show departments having more than 5 employees.

SQL

SELECT department, COUNT(*) FROM employees GROUP BY department HAVING COUNT(*) [1] 5;

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 to filter aggregated results.

Using '<=' will select departments with 5 or fewer employees.

The HAVING clause filters groups after aggregation. We want departments with more than 5 employees, so we use COUNT(*) > 5.

3fill in blank

hard

Fix the error in the query to filter employees with salary above 50000 after grouping by department.

SQL

SELECT department, AVG(salary) FROM employees GROUP BY department HAVING AVG(salary) [1] 50000;

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

AWHERE

B>

CHAVING

DGROUP BY

Attempts:

3 left

💡 Hint

Common Mistakes

Using WHERE to filter aggregated results causes syntax errors.

Placing HAVING before GROUP BY is incorrect.

The WHERE clause cannot filter aggregated results. Use HAVING to filter groups after aggregation.

4fill in blank

hard

Fill both blanks to filter products with price greater than 100 and groups having total quantity sold more than 50.

SQL

SELECT product_id, SUM(quantity) FROM sales WHERE price [1] 100 GROUP BY product_id HAVING SUM(quantity) [2] 50;

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

A>

B<

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' changes the filter logic.

Using WHERE instead of HAVING for aggregated filters.

WHERE filters rows before grouping (price > 100). HAVING filters groups after aggregation (SUM(quantity) > 50).

5fill in blank

hard

Fill all three blanks to select customers with age over 25, group by city, and only show cities with more than 10 customers.

SQL

SELECT city, COUNT(*) FROM customers WHERE age [1] 25 GROUP BY [2] HAVING COUNT(*) [3] 10;

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

A>

Bcity

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using age in GROUP BY instead of city.

Using WHERE to filter aggregated counts.

Use WHERE age > 25 to filter customers before grouping. Group by city. Use HAVING COUNT(*) > 10 to filter cities with more than 10 customers.

Practice

1. Which clause should you use to filter rows before grouping in a SQL query?

easy

A. HAVING

B. WHERE

C. GROUP BY

D. ORDER BY

Solution

1. Step 1: Understand filtering before grouping

   The WHERE clause filters individual rows before any grouping happens in the query.

2. Step 2: Compare WHERE and HAVING roles

   HAVING filters groups after grouping, so it cannot filter rows before grouping.

3. Final Answer:

   WHERE -> Option B

4. Quick Check:

   Filter rows before grouping = WHERE [OK]

Hint: Use WHERE for rows, HAVING for groups after grouping [OK]

Common Mistakes:

• Using HAVING to filter rows before grouping
• Confusing GROUP BY as a filter
• Using ORDER BY to filter data

2. Which of the following SQL queries correctly filters groups having a total sales greater than 1000?

easy

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

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

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

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

Solution

1. Step 1: Identify correct HAVING usage

   HAVING is used to filter groups based on aggregate functions like SUM.

2. Step 2: Check query syntax and order

   SELECT store, SUM(sales) FROM sales_data GROUP BY store HAVING SUM(sales) > 1000; correctly uses GROUP BY first, then HAVING with SUM(sales) > 1000.

3. Final Answer:

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

4. Quick Check:

   Filter groups by aggregate = HAVING [OK]

Hint: HAVING filters aggregates after GROUP BY [OK]

Common Mistakes:

• Using WHERE with aggregate functions
• Placing HAVING before GROUP BY
• Filtering rows instead of groups

3. Given the table orders(order_id, customer_id, amount), what will this query return?

SELECT customer_id, COUNT(*) AS order_count FROM orders WHERE amount > 50 GROUP BY customer_id HAVING order_count > 2;

medium

A. Customers with total amount over 50

B. All customers with orders over 50 regardless of count

C. Syntax error because alias can't be used in HAVING

D. Customers with more than 2 orders where each order amount is over 50

Solution

1. Step 1: Understand alias usage in HAVING

   Many SQL databases allow using column aliases like order_count directly in HAVING clause, but standard SQL does not. However, most practical systems support it.

2. Step 2: Identify correct HAVING syntax

   Using alias in HAVING is often allowed; thus, the query returns customers with more than 2 orders where each order amount is over 50.

3. Final Answer:

   Customers with more than 2 orders where each order amount is over 50 -> Option D

4. Quick Check:

   HAVING filters groups; alias usage depends on SQL dialect [OK]

Hint: Use full aggregate in HAVING or alias depending on SQL dialect [OK]

Common Mistakes:

• Using alias in HAVING clause (may be allowed in some SQL dialects)
• Confusing WHERE and HAVING filters
• Assuming HAVING filters rows

4. Identify the error in this SQL query:

SELECT department, AVG(salary) FROM employees HAVING AVG(salary) > 50000 WHERE department LIKE 'Sales%' GROUP BY department;

medium

A. WHERE clause used after HAVING

B. HAVING clause used before GROUP BY

C. Missing alias for AVG(salary)

D. GROUP BY clause missing

Solution

1. Step 1: Check SQL clause order

   The correct order is WHERE, then GROUP BY, then HAVING.

2. Step 2: Identify misplaced WHERE clause

   In the query, WHERE appears after HAVING, which is invalid syntax.

3. Final Answer:

   WHERE clause used after HAVING -> Option A

4. Quick Check:

   WHERE before GROUP BY, HAVING after [OK]

Hint: WHERE before GROUP BY, HAVING after GROUP BY [OK]

Common Mistakes:

• Placing WHERE after HAVING
• Forgetting GROUP BY clause
• Using HAVING without GROUP BY

5. You want to find all customers who placed more than 3 orders with each order amount greater than 100. Which query correctly applies WHERE and HAVING?

hard

A. SELECT customer_id FROM orders WHERE amount > 100 GROUP BY customer_id HAVING COUNT(*) > 3;

B. SELECT customer_id FROM orders GROUP BY customer_id HAVING COUNT(*) > 3 AND amount > 100;

C. SELECT customer_id FROM orders HAVING COUNT(*) > 3 WHERE amount > 100 GROUP BY customer_id;

D. SELECT customer_id FROM orders WHERE COUNT(*) > 3 GROUP BY customer_id HAVING amount > 100;

Solution

1. Step 1: Filter rows with WHERE

   Use WHERE to keep only orders with amount > 100 before grouping.

2. Step 2: Filter groups with HAVING

   Use HAVING to keep customers with more than 3 such orders (COUNT(*) > 3).

3. Final Answer:

   SELECT customer_id FROM orders WHERE amount > 100 GROUP BY customer_id HAVING COUNT(*) > 3; -> Option A

4. Quick Check:

   WHERE filters rows, HAVING filters groups [OK]

Hint: WHERE filters rows, HAVING filters groups after grouping [OK]

Common Mistakes:

• Using HAVING to filter rows
• Placing WHERE after HAVING
• Using aggregate in WHERE clause