Conditions

• Conditions serve to restrict the number of query matches by placing requirements on the attributes of an object or link in the database.

• Conditions are commonly, but not exclusively, used to enforce type restrictions in query matches.

• Attribute value conditions restrict matches to only those database entities that match the attribute values specified in the query.

• Existence conditions test only to see whether the corresponding object or link has an instance of the specified attribute, but do not require that the attribute have any particular value.

• Attribute value conditions can use the comparison operators =, <, >, <=, >=, and <>

• Simple conditions can be combined using disjunctive normal form.

• An attribute value condition is satisfied if any member of the set of attribute values for the corresponding database entity satisfies the condition.

• If an object or link does not have a value for the attribute listed in an attribute value condition, the condition’s expression is false and the condition fails (unless negated by NOT).

• The use of set-valued attributes means that different attribute values for a single object or link may satisfy different condition tests, yielding the result that NOT(attr = value) is not necessarily equivalent to attr <> value.

Case sensitivity

• Proximity ignores case when matching attribute names.

• Proximity obeys case when matching attribute values.

Efficiency considerations

• It is generally more efficient to include more rather than fewer restrictions in the form of conditions.

• Use existence conditions instead of attribute value conditions when they match the same data.

• Taking advantage of some of these efficiency improvements may require perfect information about a database’s schema. In practice, we frequently do not have thorough enough information about a database to be able to use efficiency improvements with confidence.