XMM-Newton Science Analysis System


selectlib (selectlib-4.77) [xmmsas_20230412_1735-21.0.0]

Null value function

Numerical columns in tables can optionally have associated null values. The usual convention is that data entries that are set to a column's special null value are to be interpreted as 'not applicable/available' by the application. The operator function

isnull(COLNAME)

selects all rows in which the value of COLNAME is equal to the column's null value. If the column has no associated null value defined the function evaluates to false. Please note:

1. The isnull() function can only be used in the above manner, i.e., with the name of a numerical table column as argument. Attempts to use it in any other way will lead to an error condition.
2. Unlike integer-valued columns there is always an implicit null value for floating point data, viz. the special IEEE value NaN (Not-A-Number). It is not possible to define any other real value to serve as 'null' for real-valued columns.