Recently I have been taking many interviews to increase work force in my companies outsourcing establishment. One question I ask to all interview candidates. What is Aggregate Function? So far I have received two different kind of response. First, I do not know. Second, AVG, SUM, COUNT are aggregate functions. The second response is good enough but not technically correct. None of the candidate have gave me good definition of Aggregate Function.
Definition from BOL is Aggregate functions perform a calculation on a set of values and return a single value.
Following functions are aggregate functions.
AVG, MIN, CHECKSUM_AGG, SUM, COUNT, STDEV, COUNT_BIG, STDEVP, GROUPING, VAR, MAX, VARP
Aggregate functions ignores NULL values except COUNT function. HAVING clause is used, along with GROUP BY, for filtering query using aggregate values.