Sometimes you cannot capture the necessary query restrictions using a single condition. For example, suppose we have a database that contains information not just on authors and books, but that also includes other published materials such as magazine articles and screenplays. If we want to not just restrict matches to items published before 1990, but to also ensure that we only match book objects, we have to place two conditions on the book vertex to correctly restrict the matching objects:

Both of the conditions ObjType = book and PubDate < 1990 must be satisfied in order for the corresponding database object to match this vertex. Because both conditions must be satisfied, we can more correctly write this as the logical conjunction of two conditions, as shown in Figure 3.17.

In this example, the two conditions are combined using the logical AND operator. For clarity, query diagrams use the English words AND and OR instead of the symbols ∧ and ∨. In principle, complex conditions in QGraph can be any logical combination of simple conditions. For example, we could to modify the above query to find both books and screenplays published before 1990 by creating the following query. (However, see “Complex conditions in Proximity” for important Proximity limitations on the form of complex conditions.)

QGraph also permits the use of the logical NOT operator in complex conditions. The NOT operator has the expected effect of requiring that the condition’s test must fail for the corresponding database entity to match the query. For example, to find items not published in 1990, we can create the query

This query matches objects whose PubDate attribute does not include the value 1990.

Remember that QGraph must consider all of an attribute’s values for an object or link when evaluating a condition. Therefore, when database items have multiple attribute values, we cannot assume that Attribute <> value is equivalent to NOT(Attribute = value). For example, in the query above, if an object’s PubDate attribute has the the value {1984, 1990}, then the condition PubDate <> 1990 will be satisfied because 1984 <> 1990. But the condition NOT(PubDate = 1990) will not be satisfied because 1990 is included in the value for PubDate.

Similarly, the two conditions PubDate <> 1990 and NOT(PubDate = 1990) do not yield the same results when applied to objects or links having no values for the specified attribute. In the case of an inequality condition, if an item has no PubDate attribute, no value exists to satisfy the inequality, therefore the condition cannot be satisfied. In the case of a negated condition, no value exists to satisfy the PubDate = 1990 equality, therefore the condition is satisfied.