Selections

Selections define a set of rows selected from a catalogue according to some criteria. For example, if a catalogue contained column x then the selection criterion might be `x $>$ 10.0', that is, the selection would comprise the set of rows for which the field of column x was greater than 10.0. The syntax for specifying expressions is described in Appendix .

The attributes of a selection are listed in Table  and described below. With the exception of the comments attribute, COMM, they are all mandatory and immutable and are set automatically when the selection is created.

Table: Attributes of a selection

Attribute	Name	Data type
Expression	EXPR	_CHAR
Number of rows	NUMSEL	_INTEGER
Comments	COMM	_CHAR
Modification date	DATE	_DOUBLE