1. Pre-requisites
Should read first there types of mapping below:
2. Summary mapping strategies
| Feature | Single-table | Joined-tables | Table-per-class |
|---|---|---|---|
| Table support | One table for all classes in the entity hierarchy: – Mandatory columns may be nullable (exclusive of DiscriminatorColumn). – The table grows when more subclasses are added. | One for the parent class, and each subclass has a separate table to store polymorphic properties.Mapped tables are normalized. | One table for each concrete class in the entity hierarchy. |
| Uses discriminator column? | Yes | Yes | No |
| SQL generated for retrieval of entity hierarchy | Simple SELECT | SELECT clause joining multiple tables. | One SELECT for each subclass or UNION of SELECT. |
| SQL for insert and update | Single INSERT or UPDATE for all entities in the hierarchy | Multiple INSERT,UPDATE: one for the root class and one for each involved subclass. | One INSERT or UPDATE for every subclass. |
| Polymorphic relationship | Good | Good | Poor |
| Polymorphic queries | Good | Good | Poor |
Table 1: Summary of OO hierarchy inheritance mapping strategies (source: EJB3 in Action)
References:
- D. Pan, R. Rahman, R. Cuprak, M. Remijan, “Mapping inheritance”, In EJB3 in Action,pp.292-293, Manning, Second Edition, 2014