Logical expression

Atomic conditions can be combined into composite conditions using logical connectives. The most important ones are AND, OR, NOT and the bracket pair (…). If we write a composite condition such as Area < 400,000 AND Land Use = 80, we can use it to select areas for which both atomic conditions hold true. This is the meaning of the AND connective. If we had written Area < 400,000 OR Land Use = 80 instead, the condition would have selected areas for which either condition holds, so effectively those with an area size less than 400,000, but also those with land use class 80. (Included, of course, will be areas for which both conditions hold.)

The NOT connective can be used to negate a condition. For instance, the condition NOT (Land Use = 80) would select all areas with a different land use class than 80. (Clearly, the same selection can be obtained by writing Land Use  <> 80 but this is not the point.) Finally, brackets can be applied to force grouping amongst atomic parts of a composite condition . For instance, the condition (Area < 30,000 AND Land Use = 70) OR (Area < 400,000 AND Land Use = 80) will select areas of class 70 less than 30,000 in size, as well as class 80 areas less than 400,000 in size.