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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A NoSQL (originally referring to "non-SQL" or "non-relational") database provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases. Such databases have existed since the late 1960s, but the name "NoSQL" was only coined in the early 21st century, triggered by the needs of Web 2.0 companies. NoSQL databases are increasingly used in big data and real-time web applications.
An XML database is a data persistence software system that allows data to be specified, and sometimes stored, in XML format. This data can be queried, transformed, exported and returned to a calling system. XML databases are a flavor of document-oriented databases which are in turn a category of NoSQL database. There are a number of reasons to directly specify data in XML or other document formats such as JSON.
An object database or object-oriented database is a database management system in which information is represented in the form of objects as used in object-oriented programming. Object databases are different from relational databases which are table-oriented. A third type, object–relational databases, is a hybrid of both approaches. Object databases have been considered since the early 1980s. Object-oriented database management systems (OODBMSs) also called ODBMS (Object Database Management System) combine database capabilities with object-oriented programming language capabilities.
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