Query subjects merge and Join

[kenzhao]

Scenario:
I have 2 query subjects, member and member master, which are joined as

Member Master . pk (parent)  member. fk (child)

In FM, I am planning to merge member and member master, into a single query subject called member_master, and then join it to member . The merge query subject does not store facts/measures, actually a dimension query subject.

Now I have two questions below.
1) why two  query subjects, member and member master have to be joined first before merging?
2) Can the new merged query subject member_master join query subject member? In our current project, we created a new query subject called member and the new merged query subject join a new member query subject, not existing member. Are these two scenario same? Why?

See the attachment for the detail.

Thanks,

Ken

[blom0344]

Merging dimensional subjects is often a way to reduce complexity in the model, like reducing snowflaking to get 'real' star schema.  Merging is also an option when relationship between subjects is strictly 1:1 <--> 1:1

Merging 2 subjects that have no underlying join relationship would simply generate a cartesian product between both subjects as outcome (all rows from subject1 will be combined with those from subject2) You normally do not want that.

The new merged subject would normally be used as a new dimensional subject to the fact subject, so your question is difficult to interpret.

The whole subject of merging a first and foremost a way to reduce complexity of the model