science

Jakarta Persistence Query Language

Jakarta Persistence Query Language is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Jakarta Persistence Query Language rather than just read about it. In short: The Jakarta Persistence Query Language (JPQL; formerly Java Persistence Query Language) is a platform-independent object-oriented query language defined as part of the Jakarta Persistence (JPA; formerly Java Persistence API) specification. JPQL is used to make queries against entities stored in a relational database.

Key takeaways

Jakarta Persistence Query Language belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Jakarta Persistence Query Language to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Jakarta Persistence Query Language from memory before moving on to harder problems.

Reference excerpt

The Jakarta Persistence Query Language (JPQL; formerly Java Persistence Query Language) is a platform-independent object-oriented query language defined as part of the Jakarta Persistence (JPA; formerly Java Persistence API) specification. JPQL is used to make queries against entities stored in a relational database. It is heavily inspired by SQL, and its queries resemble SQL queries in syntax, but operate against JPA entity objects rather than directly with database tables. In addition to retrieving objects (SELECT queries), JPQL supports set based UPDATE and DELETE queries.

Examples Example JPA Classes, getters and setters omitted for simplicity.

Then a simple query to retrieve the list of all authors, ordered alphabetically, would be:

To retrieve the list of authors that have ever been published by XYZ Press:

JPQL supports named parameters, which begin with the colon (:). We could write a function returning a list of authors with the given last name as follows:

Hibernate Query Language JPQL is based on the Hibernate Query Language (HQL), an earlier non-standard query language included in the Hibernate object-relational mapping library. Hibernate and the HQL were created before the JPA specification. As of Hibernate 3 JPQL is a subset of HQL.

Citations

References Bauer, Christian; King, Gavin; Gregory, Gary (2016), Java Persistence with Hibernate (Second ed.), Manning Publications, ISBN 978-1617290459

See also Object-relational mapping Hibernate (framework) Jakarta Persistence

External links Official website Full Query Language Syntax from The Jakarta EE 8 Tutorial JPA Queries and JPQL - a chapter of the ObjectDB Manual Type safe Hibernate (HQL) query engine - TorpedoQuery

Worked examples

Example 1 — a first encounter with Jakarta Persistence Query Language

Start with the simplest possible case. Write down what Jakarta Persistence Query Language claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Jakarta Persistence Query Language before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Jakarta Persistence Query Language ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Jakarta Persistence Query Language

In research: Jakarta Persistence Query Language appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Jakarta Persistence Query Language in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Jakarta Persistence Query Language is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data modeling languages, Java enterprise platform, Query languages, so understanding it makes those chapters shorter.
In everyday life: Look for Jakarta Persistence Query Language outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Jakarta Persistence Query Language in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Jakarta Persistence Query Language means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Jakarta Persistence Query Language out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Jakarta Persistence Query Language in simple terms?

Why does Jakarta Persistence Query Language matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Jakarta Persistence Query Language?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Jakarta Persistence Query Language.