WHERE vs HAVING mental model in SQL - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding WHERE clause filtering

Which statement best describes when the WHERE clause is applied in an SQL query?

A It filters rows before any grouping or aggregation happens.

B It filters groups after aggregation is done.

C It filters columns after the SELECT statement.

D It filters the final result set after ORDER BY.

💡 Hint

Think about when rows are removed in the process: before or after grouping?

The WHERE clause filters individual rows before any grouping or aggregation occurs. It decides which rows enter the grouping stage.

🧠 Conceptual

intermediate

Understanding HAVING clause filtering

Which statement best describes when the HAVING clause is applied in an SQL query?

A It filters individual rows before grouping.

B It filters groups after aggregation is done.

C It filters columns before SELECT.

D It filters rows after ORDER BY.

💡 Hint

Think about filtering groups, not rows.

The HAVING clause filters groups after aggregation. It decides which groups appear in the final result.

❓ query_result

advanced

Output of WHERE vs HAVING filtering

Given the table sales with columns region and amount, what is the output of this query?

SELECT region, SUM(amount) AS total FROM sales WHERE amount > 100 GROUP BY region;

SQL

Table sales:
region | amount
-------|-------
East   | 150
East   | 90
West   | 200
West   | 50
South  | 120
South  | 80

A [{"region": "East", "total": 90}, {"region": "West", "total": 50}, {"region": "South", "total": 80}]

B [{"region": "East", "total": 240}, {"region": "West", "total": 250}, {"region": "South", "total": 200}]

C [{"region": "East", "total": 150}, {"region": "West", "total": 200}, {"region": "South", "total": 120}]

D [{"region": "East", "total": 0}, {"region": "West", "total": 0}, {"region": "South", "total": 0}]

💡 Hint

Remember WHERE filters rows before grouping.

The WHERE clause removes rows where amount ≤ 100 before grouping. So only amounts 150, 200, and 120 remain. The SUM is over these filtered rows.

❓ query_result

advanced

Output of HAVING filtering after aggregation

Given the same sales table, what is the output of this query?

SELECT region, SUM(amount) AS total FROM sales GROUP BY region HAVING SUM(amount) > 200;

SQL

Table sales:
region | amount
-------|-------
East   | 150
East   | 90
West   | 200
West   | 50
South  | 120
South  | 80

A [{"region": "East", "total": 240}, {"region": "West", "total": 250}]

B [{"region": "East", "total": 240}, {"region": "West", "total": 250}, {"region": "South", "total": 200}]

C [{"region": "South", "total": 200}]

D []

💡 Hint

HAVING filters groups after SUM is calculated.

The SUM per region is East=240, West=250, South=200. HAVING keeps groups with total > 200, so South is excluded.

📝 Syntax

expert

Identify the syntax error in combined WHERE and HAVING query

Which option contains a syntax error in this query?

SELECT region, COUNT(*) FROM sales WHERE region = 'East' HAVING COUNT(*) > 1 GROUP BY region;

A The COUNT(*) function cannot be used in HAVING.

B The WHERE clause cannot be used with GROUP BY.

C The GROUP BY clause must come before HAVING, so this query is correct.

D The HAVING clause appears before GROUP BY, causing a syntax error.

💡 Hint

Check the order of clauses in SQL syntax.

In SQL, the GROUP BY clause must come before HAVING. Placing HAVING before GROUP BY causes a syntax error.

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