Parenthesises can be used to group parts of the expression to circumvent operator precedence. Operators AND, OR, and NOT are preceded and followed by periods in the expression to distinguish these operators from strings and variable names. Numbers and operators work just like the common calculator. Variables, on the other hand, have special rules about how they interact with operations and other types of operands. For example, a variable Age can have a minimum value of 10 and a maximum value of 100.
Relational operators are the most useful operators.
When an expression is applied to a category, the expression is actually applied to each site of the category. If the site has a variable of the type specified in the query, the expression is evaluated with that variable s values. If the result of the evaluation is True, the site is included in the current display. If the result of the evaluation is False, the site is dropped from the current display. If the site does not have a variable of the types listed in the expression, the evaluation results in False and the site is dropped from the current display.