The aggregator fails if any group has more than one element.
Example:
table T = with
[| as A, as B |]
[| 0, "a" |]
[| 1, "b" |]
[| 1, "c" |]
table G[gdim] = by T.B
where T.B != "c"
show table "" a1b4 with
gdim
single(T.A)
group by gdim
The aggregator single succeeds on any empty group, and returns the default value for the given data type.