How to implement polymorphic associations in an existing database

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You could use Option 1 but without an additional surrogate Alternate Key.

Instead, extend the existing Primary Key (of each entity), with an EntityType column (say CHAR(1), that would be E for Events, P for Persons, D for Products).

The compound (EntityId, EntityType) will become then the Primary Key of table Entity and the corresponding compounds in the other 3 subtype tables.

(The EntityType is just an auxilary, reference table, with 3 rows):

Polymorphic_Associations

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