Must group to Aggregate?

Question

Is it a requirement that you have to group by something in your select if aggregating such as using SUM? That you must have a group by clause?

Answer 1

No it is not a requirement.

Aggregates without a GROUP BY are Scalar aggregates and always return exactly one row.

Example

SELECT SUM(high)
FROM master..spt_values
WHERE 1 = 0

Aggregates with a GROUP BY are Vector aggregates and return zero or one row per group.

Example

SELECT [type], SUM(high)
FROM master..spt_values
WHERE 1 = 0
GROUP BY [type]