GROUP BY and aggregates

What this means

Aggregate functions such as COUNT, SUM, AVG, MIN, and MAX calculate a result across rows. GROUP BY creates one aggregate result per group.

In beginner terms, this topic answers one practical question: "What should I write, and why does React care about it?" Do not try to memorize the syntax first. First understand the idea, then connect the syntax to that idea.

Why it matters

Dashboards, reports, and progress summaries depend on aggregation.

When you build real React screens, this idea helps you decide where data should live, what the user should see, and what should happen after an interaction. That is why this lesson is part of the main path instead of being an optional detail.

Step by step

1. Notice the UI problem this topic solves. 2. Look at the smallest possible example. 3. Change one value and predict what should appear. 4. Run the example and compare the result with your prediction. 5. Use the practice task before moving on.

Small example

SELECT track, COUNT(*) AS learners FROM enrollments GROUP BY track;

Common mistake

Do not copy group by and aggregates syntax without explaining what problem it solves and checking the result.

Practice task

Change one part of the example, predict the result, run it, and explain the result in your own words.

Remember this

Every selected non-aggregate column should belong to the grouping.