Defining a Default Member
The default member of an attribute hierarchy is used to evaluate expressions when an attribute hierarchy is not included in a query. The default member is ignored whenever a query includes an attribute hierarchy or user hierarchy that contains the attribute that sources the attribute hierarchy. This is because the member specified in the query is used.
The default member for an attribute hierarchy is set by specifying an attribute member as the DefaultMember property value for the attribute hierarchy. You can set this property on the Dimension Structure tab in Dimension Designer, or in the cube's calculation script on the Calculation tab in Cube Designer in Business Intelligence Development Studio. You can also specify the DefaultMember property for a security role (overriding the default member set on the dimension) on the Dimension Data tab when defining dimension security. To avoid name resolution problems, define the default member in the cube's MDX script in the following situations: if the cube refers to a database dimension more than once, if the dimension in the cube has a different name than the dimension in the database, or if you want to have different default members in different cubes.
The default member on an attribute is used to evaluate expressions when an attribute is not included in a query. The default member for an attribute is specified by the DefaultMember property on the attribute. Whenever a hierarchy from a dimension is included in a query, all the default members from attributes corresponding to levels in the hierarchy are ignored. If no hierarchy of a dimension is included in a query, default members are used for all the attributes in the dimension.
Resolving the Default Member When No Default Member is Specified
If no default member is specified for an attribute hierarchy, and the attribute hierarchy is aggregatable (the IsAggregatable property on the attribute is set to True), the (All) member is the default member. If no default member is specified and the attribute hierarchy is not aggregatable (the IsAggregatable property on the attribute is set to False), a default member is selected from the attribute hierarchy's top level.