What 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.

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.

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SQL Aggregation and GROUP BY Practice Questions

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Collapse many rows into one summary row per group.

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GROUP BY: Names the columns that define each group.
SUM, COUNT, AVG: Describe each group with one number (also MIN and MAX).

61 questionsPage 5 of 5

W12: Event Funnel Counts
Interview-style funnel: event counts per event_type with distinct users. Return event_type, n_events, n_users. Order by...
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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.

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SQL Practice Hub

SQL Aggregation and GROUP BY Practice Questions

Leaderboard

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).

61 questionsPage 5 of 5

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—

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.

Track your streak & earn XP

Full-Length Interview Practice Tests100% Free

10 role-based tests for Data Analysts, SQL Developers & Data Engineers with instant scorecards.

Explore 10 Mock Tests

SQL Aggregation and GROUP BY Practice Questions

W12: Event Funnel Counts

SQL Aggregation and GROUP BY practice: common questions

Full-Length Interview Practice Tests100% Free

SQL Aggregation and GROUP BY Practice Questions

W12: Event Funnel Counts

SQL Aggregation and GROUP BY practice: common questions

Full-Length Interview Practice Tests100% Free

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