This chapter tells you about JPQL and how it works with persistence units. In this chapter, examples follow the same package hierarchy, which we used in the previous chapter as follows:. It is used to create queries against entities to store in a relational database. Having SQL like syntax is an advantage because SQL is a simple structured query language and many developers are using it in applications.
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Thus, it offers developers an easy platform to manage SQL tasks. JPQL is used by developers for its wide features and advantages. It helps in creating queries against entities that are stored in a relational database. Moreover, a JPQL query can get and return objects more than field values from tables. This is one of the main reasons where JPQL is considered as object-oriented friendly as well as easier to use. The key difference between JPQL and SQL is that former works with classes and objects of Java whereas the latter works with relational database fields, records, and tables.
These two clauses are compulsory in every query that is used for retrieving information. In some cases, certain optional clauses are also included. It will be in the format as shown below. It is analogous to loop variables present in the programming languages. Each identification variable denotes iteration over database objects.
Here, e is a range variable. That is, it describes iteration over every employee objects in the DB. However, multiple variables, as well as JOIN, are not supported in this.
The SET clause describes one or more number of field update expressions. In addition to that, JPQL is considered a case insensitive. However, in some cases, it is case sensitive. It can be entity class names and persistent fields names etc. JPQL is used to perform several database operations, especially on persistent entities. This document clearly explains each and every aspect of JPQL in an efficient manner. This is a guide to JPQL. Here we discuss how Entity JavaBeans Query Language works, along with some features, methods and different examples.
JPA JPQL Introduction
We use MySQL database. Hibernate is an object-relational mapping tool for the Java programming language. It provides a framework for mapping an object-oriented domain model to a relational database. JPQL is used to make queries against entities stored in a relational database. JPQL is object-oriented. This is the Maven build file. The mysql-connector-java is a MySQL driver, the hibernate-core brings the core Hibernate functionality, and the log4j-slf4j-impl is a simple logging facade to the Log4j2 library.
Spring Data JPA @Query
Select queries can be used to read objects from the database. Select queries can return a single object or data element, a list of objects or data elements, or an object array of multiple objects and data. The SELECT clause can contain object expressions, attribute expressions, functions, sub-selects, constructors and aggregation functions. Aggregation functions can include summary information on a set of objects. These will not be managed objects, and the class must define a constructor that matches the arguments of the constructor and their types. Constructor queries can be used to select partial data or reporting data on objects, and get back a class instance instead of an object array. The FROM clause defines what is being queried.