The problem is this:
var result = from tex in ols.COURSEs
from another in ols.UNITs
It is a cross join. It matches every course with every unit. It doesn't use any FK/PK because no relation (navigation property) is used in this query. To use the relation you have to modify it to:
var result = from tex in ols.COURSEs
from another in tex.SomeNavigationProperty // tex