W12: Event Funnel Counts
Interview-style funnel: event counts per event_type with distinct users. Return event_type, n_events, n_users. Order by...
data-engineerw12interviewaggregation
Advanced
5m10 pts
0—
SQL Practice Hub
SQL Aggregation and GROUP BY Practice Questions
Collapse many rows into one summary row per group.
61 questionsFreeRuns in your browser
- GROUP BY
- Names the columns that define each group.
- SUM, COUNT, AVG
- Describe each group with one number (also MIN and MAX).
- HAVING
- Filters groups after aggregation; WHERE cannot use aggregates.
What you will practice
- Know when a column must appear in GROUP BY and why the database rejects it otherwise
- Use WHERE to filter rows before grouping and HAVING to filter groups afterwards
- Understand how COUNT(*), COUNT(column) and COUNT(DISTINCT column) differ on NULLs and duplicates
- Write conditional aggregation with SUM(CASE WHEN ... THEN 1 ELSE 0 END) to pivot a column into metrics
- Combine aggregates with joins without double-counting the fact table
Read the full overview
Aggregation collapses many rows into one summary row per group: GROUP BY names the grouping columns, and SUM, COUNT, AVG, MIN and MAX describe each group. HAVING then filters the groups themselves, which is why it can reference an aggregate while WHERE cannot. This is the largest topic in the question bank because almost every analyst task, from revenue by month to active users per plan, is an aggregation with the right grouping key.
61 questionsPage 5 of 5
FAQ
SQL Aggregation and GROUP BY practice: common questions
Short answers to what people get stuck on most. Tap a question to expand it.
What is the difference between WHERE and HAVING?
WHERE filters rows before grouping. HAVING filters groups after aggregation.
- WHERE cannot use aggregates like SUM or COUNT; HAVING can.
- WHERE also shrinks the data before GROUP BY runs, so it is cheaper.
- Rule of thumb: put every condition you can in WHERE, and keep HAVING for aggregate conditions.
SQL
SELECT region, SUM(amount) AS total
FROM orders
WHERE status = 'paid' -- row filter, before grouping
GROUP BY region
HAVING SUM(amount) > 1000; -- group filter, after groupingWhat is the difference between COUNT(*), COUNT(column) and COUNT(DISTINCT column)?
They count different things: all rows, non-NULL values, or unique non-NULL values.
- COUNT(*): every row in the group, NULLs included.
- COUNT(column): only rows where that column is not NULL.
- COUNT(DISTINCT column): unique non-NULL values, what you want for how many customers after a join repeats them.
- Mixing them up is a common cause of inflated or understated metrics.
SQL
SELECT COUNT(*) AS order_rows,
COUNT(coupon_code) AS orders_with_coupon,
COUNT(DISTINCT user_id) AS unique_customers
FROM orders;Why do I get an error when I select a column that is not in GROUP BY?
Every selected column must be in GROUP BY or inside an aggregate, because the database cannot pick one value for the whole group.
- Add the column to GROUP BY if you want one row per combination (shown here).
- Wrap it in MIN or MAX if any value in the group will do.
- Use a window function if you need every row next to the aggregate.
- MySQL with ONLY_FULL_GROUP_BY off silently picks a random value, which hides the bug.
SQL
-- error: name is neither grouped nor aggregated
SELECT customer_id, name, SUM(amount) FROM sales GROUP BY customer_id;
-- fix: group by it as well
SELECT customer_id, name, SUM(amount) FROM sales GROUP BY customer_id, name;Related practice topics
Track your streak & earn XP
Sign up free to unlock your progress dashboard, daily streaks, and leaderboard ranking.
Full-Length Interview Practice Tests100% Free
10 role-based tests for Data Analysts, SQL Developers & Data Engineers with instant scorecards.