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# XML Schema union element * * Complete XML Schema Reference Manual](#) * * * ## Definition and Usage The union element defines a collection of multiple simpleType definitions. ### Element Information * **Parent Element:** simpleType ### Syntax (annotation?,(simpleType*)) (The ? symbol indicates that the element can appear zero or one time within the union element.) | Attribute | Description | | --- | --- | | id | Optional. Specifies a unique ID for the element. | | memberTypes | Optional. Specifies a list of names of built-in data types or simpleType elements defined in the schema. | | any attributes | Optional. Specifies any other attributes with non-schema namespaces. | ### Example 1 This example defines a simple type that combines two simple types: