I respectfully disagree with umalme.
The basic database architecture of a relational database used for an HRMS and a data warehouse preclude them from being "merged" or in a shared database. The data warehouse simply uses different data structures to more easily facilitate multidimensional analytics and business intelligence.
An HRMS and relational database are optimized in data structure to enhance the efficiency of data transactions.
The 2 types of applications by definition are separate data environments.
Just my opinion.
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Doubledubs
http://systematicHR.com