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Concept
Object–relational impedance mismatch
Summary
Object–relational impedance mismatch creates difficulties going from data in relational data stores (relational database management system [“RDBMS”]) to usage in domain-driven object models. Object-orientation (OO) is the default method for business-centric design in programming languages. The problem lies in neither relational nor OO, but in the conceptual difficulty mapping between the two logic models. Both are logical models implementable differently on database servers, programming languages, design patterns, or other technologies. Issues range from application to enterprise scale, whenever stored relational data is used in domain-driven object models, and vice versa. Object-oriented data stores can trade this problem for other implementation difficulties.
The term object–relational impedance mismatch is from impedance matching in electrical engineering .
OO mathematically is directed graphs, where objects reference each other. Relational is tuples in tables with relational algebra. Tuples are data fields grouped into a "row" with typed fields. Links are reversible (INNER JOIN is symmetric to follow foreign keys backwards), forming undirected graphs.
Object encapsulation hides internals. Object properties only show through implemented interfaces. However, many ORMs expose the properties publicly to work with database columns. Metaprogramming ORMs avoid violating encapsulation.
"Private" versus "public" is need-based in relational. In OO it is absolutely class-based. This relativity versus absolutism of classifications and characteristics clashes.
Objects must implement interfaces to expose internals. Relational uses views to vary perspectives and constraints. It lacks OO concepts like classes, inheritance and polymorphism.
Mapping needs tables to be linked to associations in object-oriented analysis.
Relational prohibits by-reference (e.g. pointers), while OO embraces by-reference. Scalar types differ between them, impeding mapping.
SQL supports strings with maximum lengths (faster than without) and collations.
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