Aggregate Functions in T-SQL

Aggregate Functions in T-SQL: Calculations vs Aggregations, Common Aggregate Functions, Grouping Aggregations, HAVING.

Updated November 3, 2026

1. Calculations vs Aggregations (Apprentice): A calculation works within one row and returns a value for every row; an aggregation works across many rows and returns one value for all of them.
2. Common Aggregate Functions (Apprentice): COUNT counts rows or values, SUM adds them, AVG averages them, and MIN and MAX find the smallest and largest. All but COUNT(*) skip NULL.
3. Grouping Aggregations (Journeyman): GROUP BY splits the rows into groups and runs the aggregates once per group. Every column in the SELECT that isn't aggregated must be in the GROUP BY.
4. HAVING (Journeyman): WHERE filters rows before they're grouped; HAVING filters the groups afterward, by their aggregates. Know which to use, and the order a query's clauses run in.