The only real common data relation shared between OO and relationnal data model is the association. The two other one may be more or less emulated with frustrating results and poor performance as other monks have already pointed out in this thread.
Relational models often include 'IS A' relationships for many of the same reasons OO designs do. Supposing you have a table Vehicle, you might need tables Truck and Motorcycle to hold information specific to only some of the rows in Vehicle. This is commonly modeled as a 1 to 0 or 1 relationship between the generic table and the specific table, but other implementations are possible. My results have never been frustrating, and performance is not generally a factor in this design choice.
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