Aggregation

COUNT Function

Count rows, count values, count unique values — and know the difference

SUM Function

Add up numeric values — with support for conditional totals

AVG Function

Compute the mean — and understand how NULLs change the result

MIN & MAX

Find the smallest and largest values in a column

GROUP BY

Split data into groups and aggregate each one separately

HAVING

Filter groups after aggregation — the WHERE for aggregated results

NULLs in Aggregations

How NULL silently changes COUNT, SUM, AVG, and GROUP BY