Null values

CURSA supports null values in catalogues. Null values are used to represent a field for which no actual value is available. Null values can arise in several ways. They may be present in a catalogue when it is read by CURSA. An example might be a catalogue of multi-colour photometry for a set of stars where measures for some colours were missing for some of the stars. Null values would be used to represent the missing values. Alternatively, they might arise where an expression is being used to compute a new column and evaluation of the expression for the current row results in a `$\div$ by zero' error. No valid value will be available for the expression, so a null value will be substituted.

Throughout CURSA null values have the single, simple meaning that `no value is available for this datum'. It is possible to invent schemes where a set of null values are supported, each with a subtly different gradation of meaning. However, CURSA does not support such schemes.