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|The DBExpressionValues protocol allows an object to be used in a query-language statement. Its principal method, expressionValue, returns a string that gives an object's representation as it should appear in such a statement.
A second method, isDeferredExpression, returns a boolean that indicates whether the invocation of expressionValue should be deferred until the "last possible moment." This is useful for classes, such as DBExpression, that concatenate values stored in separate objects. As the larger expression is being built, the DBExpression asks each of the value-holding objects whether it is deferred. If the values aren't deferred, it can send an expressionValue message as soon as the value-holding object is added. But if any is deferred, it should delay until all the objects are in place, and then send an expressionValue message to each of them.
|(const char *)expressionValue|
|Returns the value of an expression object as a string that represents the expression in the query language.|
|Returns YES if evaluation of the expression should be deferred (for example, until related expressions are ready).|