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What is a polymorphic query?
A polymorphic query is one were we can query on the Parent class and as a result get instances of Parent, LeftChild and RightChild. If we set up the descriptor to support polymorphic queries then the following query will return instances of all Parent, LeftChild and RightChild objects stored in the database.
Does Hql support polymorphic queries?
Hibernate uses a powerful query language (HQL) that is similar in appearance to SQL. Compared with SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.
JPA Queries – Polymorphic Queries
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What is lazy loading in JPA?
lazy it is interpreted as a hint to the jpa provider that the loading of that field may be delayed until it is accessed for the first time: the property value in case of a @basic annotation, the reference in case of a @manytoone or a @onetoone annotation, or.
What is @MappedSuperclass in Hibernate?
According to the Hibernate docs, MappedSuperclass allows mapping inheritance, where the super class is not treated as an entity, and where polymorphic queries that fetch objects by the base class are not supported.
What is difference between JPQL and HQL?
JPQL is the JPA standard entity query language while HQL extends JPQL and adds some Hibernate-specific features. JPQL and HQL are very expressive and resemble SQL. Unlike Criteria API, JPQL and HQL make it easy to predict the underlying SQL query that’s generated by the JPA provider.
What is difference between HQL and SQL?
SQL is based on a relational database model whereas HQL is a combination of object-oriented programming with relational database concepts. SQL manipulates data stored in tables and modifies its rows and columns. HQL is concerned about objects and its properties.
What is difference between eager and lazy loading?
While lazy loading delays the initialization of a resource, eager loading initializes or loads a resource as soon as the code is executed. Eager loading also involves pre-loading related entities referenced by a resource.
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Understanding JPQL Polymorphic Queries and using TYPE …
All JPQL queries are polymorphic. That means, the FROM clause of a query returns not only instances of the specific entity class(es) to …
Chapter 10. JPA Query
All JPQL queries are polymorphic, which means the from clause of a query includes not only instances of the specific entity class to which it refers, …
14.8. Polymorphic queries
Polymorphic queries. A query like: from Cat as cat. returns instances not only of Cat , but also of subclasses like DomesticCat . Hibernate queries may name …
Hibernate Inheritance Mapping | Baeldung
Entity inheritance means that we can use polymorphic queries for … Since Hibernate is a JPA implementation, it contains all of the above …
What is eager fetching and lazy fetching?
LAZY: It fetches the child entities lazily i.e at the time of fetching parent entity it just fetches proxy(created by cglib or any other utility) of the child entities and when you access any property of child entity then it is actually fetched by hibernate. EAGER: it fetches the child entities along with parent.
What is the difference between lazy and eager loading in Hibernate?
Eager Loading is a design pattern in which data initialization occurs on the spot. Lazy Loading is a design pattern that we use to defer initialization of an object as long as it’s possible.
What is @version in JPA?
Version used to ensure that only one update in a time. JPA provider will check the version, if the expected version already increase then someone else already update the entity so an exception will be thrown.
Polymorphic Association: Mappings of independent classes
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What is Mappedsuperclass in JPA?
JPA – Mapped Superclasses. A quick overview of JPA Mapped Superclasses. A mapped superclass provides persistent entity state and mapping information but is not itself an entity. A mapped superclass, unlike an entity, does not allow querying, persisting, or relationships to the superclass.
What is @EntityListeners?
An entity listener is a stateless class with a no-arg constructor. An entity listener is defined by annotating the entity class with the @EntityListeners annotation: @Entity.
What is native query in JPA?
Native query refers to actual sql queries (referring to actual database objects). These queries are the sql statements which can be directly executed in database using a database client.
What is difference between HQL and Criteria in Hibernate?
With Criteria we are safe with SQL Injection because of its dynamic query generation but in HQL as your queries are either fixed or parametrized, there is no safe from SQL Injection.
Is HQL database independent?
HQL is an object-oriented query language, similar to the native SQL language, and it works with persistent objects. It is a database-independent and case insensitive query language.
Which is faster SQL or HQL?
Traditional SQL code is longer than the HQL code. SQL is usually faster than the non-native HQL, however, by setting the correct cache size of the query plan, HQL can be made to operate as fast as SQL.
Why we use HQL instead of SQL?
Need of HQL
The following are some of the reasons why HQL is preferred over SQL: Provides full support for relational operations. It is possible to represent SQL Queries in the form of objects in HQL which uses classes and properties instead of tables and columns. Return results as objects.
Is Hibernate better than JDBC?
Both Hibernate & JDBC facilitate accessing relational tables with Java code. Hibernate is a more efficient & object-oriented approach for accessing a database. However, it is a bit slower performance-wise in comparison to JDBC.
Can we join two tables without primary key?
Yes we can. No Clause says that for joining of two or more tables there must be a foreign key or primary key constraint.
Bài 6: JpaRepository nâng cao: các kiểu Query và DSL (Domain Specific Language)
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Is Cross join same as inner join?
A cross join matches all rows in one table to all rows in another table. An inner join matches on a field or fields. If you have one table with 10 rows and another with 10 rows then the two joins will behave differently.
How do I join results of multiple tables in Spring JPA repository?
- Make a @OneToOne / @OneToMany relationship from Booking to Book as a start.
- Change your query to do eager fetching on the entity/collection you have mapped as Book.
- Make a POJO and annotate it with JSON annotations the way you want it to be returned by the controller.
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