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ISA Operator for Data Queries

  • Článek

Use the ISA operator in the WHERE clause of a data query to request embedded objects in a class hierarchy.

The following example shows the syntax to request embedded objects in a class hierarchy.

SELECT * FROM Class WHERE EmbeddedProp ISA "ParentClass"

The result includes instances of Class having embedded objects that are derived from ParentClass in the EmbeddedProp property. Not every instance of the Class object is derived from ParentClass, but the result returns the embedded objects that are derived from ParentClass.

For example, in the following query, ClassA includes the weakly typed EmbeddedObj property. The ClassA class has ten instances. Five of those instances have embedded objects with a type derived from ClassZ. The other five have embedded objects of other types.

The following example shows the query that returns the five instances, which include the objects that are derived from ClassZ.

SELECT * FROM ClassA WHERE EmbeddedObj ISA "ClassZ"