Aggregation Functions and Group By Questions

Fundamentals of aggregation in Structured Query Language covering aggregate functions such as COUNT, SUM, AVG, MIN, and MAX and how to use them to calculate totals, averages, minima, maxima, and row counts. Includes mastery of the GROUP BY clause to group rows by one or more dimensions such as customer, product, region, or time period, and producing metrics like total revenue by month, average order value by product, or count of transactions by date. Covers the HAVING clause for filtering aggregated groups and explains how it differs from WHERE, which filters rows before aggregation. Also addresses related topics commonly tested in interviews and practical problems: grouping by multiple columns, grouping on expressions and date truncation, using DISTINCT inside aggregates, handling NULL values, ordering and limiting grouped results, using aggregates in subqueries or derived tables, and basic performance considerations when aggregating large datasets. Practice examples include calculating monthly revenue, finding customers with more than a threshold number of orders, and identifying top products by sales.