Aggregations & GROUP BY

Collapsing many rows into one summary row per group — counts, sums, and averages — plus the HAVING clause that filters groups.

Q1.When you `GROUP BY` a column, which other columns can appear in `SELECT` without an aggregate function?

Q2.What does `COUNT(column_name)` do differently from `COUNT(*)`?

Q3.What's a correct use of `GROUP BY` with multiple columns?

Q4.Does `AVG()` include or exclude NULL values in its calculation?