GROUP BY in SQL : COUNT(*) vs COUNT(column) vs COUNT(DISTINCT) Explained
GROUP BY and aggregate functions look simple — one line, one summary. But three rules quietly trip up almost every SQL interview candidate, and this covers all three, plus the exact query pattern interviewers ask for.
What you'll learn:
How GROUP BY collapses rows into one row per group
Rule 1: why every non-aggregated SELECT column must be in GROUP BY
Rule 2: COUNT(*) vs COUNT(column) vs COUNT(DISTINCT column) — three different answers
Rule 3: why HAVING cannot work without GROUP BY
Part of the SQL Interview Questions series — new question every upload.
Subscribe so you don't miss the next one: primary key vs foreign key vs unique key.
#SQL #SQLInterview #DataEngineering #Groupby #Aggregatefunctions