How to Implement Referential Integrity in Subtypes?

None of that is necessary, especially not doubling up the tables.

The second option is also fraught with issues - e.g. For Sensors (and assuming that SensorNo is a surrogate key), because you don't have a base table, the SensorNo surrogate is either not unique between the sub class tables, unless you use a klugey mechanism to issue keys across all the sub-class tables (or use a guid).

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