mean
, sum
,
maximum
etc can only be used on values which are already arrays.
In most cases that means on values in table columns which are declared
as array-valued. FITS and VOTable tables can have columns which contain
array values, but other formats such as CSV cannot.
There is also a set of functions named array
with various
numbers of arguments, which let you assemble an array value from a list
of scalar numbers. This can be used for instance to get the mean of
a set of three magnitudes by using an expression like
"mean(array(jmag, hmag, kmag))
".