When no special steps are taken, if a null value (blank cell) is encountered in evaluating an expression (usually because one of the columns it relies on has a null value in the row in question) then the result of the expression is also null.
It is possible to exercise more control than this, but it
requires a little bit of care,
because the expressions work in terms of primitive values
(numeric or boolean ones) which don't in general have a defined null
value. The name "
in expressions gives you the java
reference, but this cannot be matched against a primitive value
or used as the return value of a primitive expression.
For most purposes, the following two tips should enable you to work with null values:
NULL_" (use upper case) to the column name or $ID. This will yield a boolean value which is true if the column contains a blank, and false otherwise.
NULL" (upper case). To return a null value from a non-numeric expression (e.g. a String column) use the name "
null" (lower case).
Null values are often used in conjunction with the conditional
? :"; the expression
test ? tval : fvalreturns the value
tvalif the boolean expression
testevaluates true, or
testevaluates false. So for instance the following expression:
Vmag == -99 ? NULL : Vmagcan be used to define a new column which has the same value as the
Vmagcolumn for most values, but if
Vmaghas the "magic" value -99 the new column will contain a blank. The opposite trick (substituting a blank value with a magic one) can be done like this:
NULL_Vmag ? -99 : VmagSome more examples are given in Section 10.7.