Jakarta Query

Object-oriented query languages

A data structure in an object-oriented language is a graph of objects interconnected by unidirectional object references, which may be polymorphic. Some non-relational databases support similar representations. On the other hand, relational databases represent relationships between entities using foreign keys, and therefore SQL has no syntactic construct representing navigation of an association. Similarly, inheritance and polymorphism can be easily represented within the relational model, but are not present as first-class constructs in the SQL language. An object-oriented query language is a dialect of SQL with support for associations and subtype polymorphism.

Historical background

Object-oriented dialects of SQL have existed since at least the early 90s. The Object Query Language (OQL) was an early example, targeting object databases, but was never widely used, since object databases were themselves not widely adopted. Hibernate Query Language (HQL) and the Enterprise JavaBeans Query Language (EJB-QL) were both introduced in 2001 as query languages intended for use with object/relational mapping. HQL was widely adopted by the Java community and was eventually standardized as the Java Persistence Query Language (JPQL) by JSR-220 in 2006. JPQL has been implemented by at least five different products and is in extremely wide use today. On the other hand, since JPQL is defined as part of the Jakarta Persistence specification, it has not been reused outside the context of object/relational mapping in Java. More recently, Jakarta Data 1.0 introduced the Jakarta Data Query Language (JDQL), a strict subset of JPQL intended for use with non-relational databases. It is now inconvenient that JDQL and JPQL are maintained separately by different groups, and there is some risk of divergence between the two specifications.