Returns a single value from a column of a database specified by a set of conditions. This function returns an error if no value or more than one value exist.
Jout in inkele wearde werom út in kolom fan in databank, dy' t foldocht oan in tal betingsten. Dizze funksje jout in foutmelding as der gjin wearde, of mear as ien wearde oanwêzich is.