I have gone through some articles regarding Bigtable and NOSQL. It is very interesting that they avoid JOIN operations.
As a basic example, let's take Employee and Department table and assume the data is spread across multiple tables / servers.
Just want to know, if data is spread across multiple servers, how do we do JOIN or UNION operations?
You would have to do multiple selects, and join the data manually in your application. See this SO post for more information. From that post:
Bigtable datasets can be queried from services like AppEngine using a language called GQL ("gee-kwal") which is a based on a subset of SQL. Conspicuously missing from GQL is any sort of JOIN command. Because of the distributed nature of a Bigtable database, performing a join between two tables would be terribly inefficient. Instead, the programmer has to implement such logic in his application, or design his application so as to not need it.