firstval — aggregate function to return the first value from a specified column.
This function returns the first value in the range of values returned by its query predicate.
expr must contain the name of a column in a LiveView table. The returned first value is of the same type as the specified column.
If the query returns no rows, the aggregate returns
null when group-by is not specified, and no rows when a group-by is specified.
This LiveView aggregate function is based on the firstval aggregate function in the StreamBase expression language.