3 way junction table and redundant value
You need to normalize your schema.  Your Employee_Owns_Asset table is trying to do too many things.  Consider this schema:



Note that your individual assets need to be normalized out into their own table.  This lets you separate the two semantically independent pieces of information:


What are the properties (brand, model, etc.) of each asset?
Who (if anyone!) owns this asset at a particular point in time?


This is a case of a requirement for fourth normal form (4NF), where it is important to isolate independent many-to-many relationships.

Note that if you don't want to track an asset's owner over time, then you could simplify the illustrated model and make ASSET.owning_employee_key a foreign key on the ASSET table and eliminate the EMPLOYEE_OWNS_ASSET table from the model.