The relationships between the database tables are generally more effective. Therefore, the relationships between the classes of the entities are treated as relational tables (JPA concept)

• ManyToOne Entity Relation
• OneToMany Entity Relation
• OneToOne Entity Relation
• ManyToMany Entity Relation

@ManyToOne Relation

Many-To-One relation between entities:

Many-To-one relations between entities: Where an entity is linked to one other (column or group of columns) that contains unique values. The column or set of columns. These connections apply to relational databases with the use of a foreign key between tables.

Let us take an example of the relation between Student and subject entities. In a unidirectional that is from Student to subject, the Many-To-One relation is applied. That indicates each record of student holds one subject id, which should be a primary key in the subject table. Subject id is a Foreign Key.

Creating Entities

• Create a package ‘com.world.EclipseLink.entity’ under ‘src’ set. Make a new java class named subject.java under the specified package.
• Make the other entity in this connection - Department entity class Department.java under ‘com.world.EclipseLink.entity’ package.

Persistence.xml

Persistence.xml file has the structure information of database and registration data of entity classes. Persitence.xml will be built by the eclipse IDE during the creation of a JPA Project. The configuration aspects are user stipulations.

Service Classes

This module includes the classes of services that use the initialisation property to implement the relational component. Create a 'src' package called 'com.world.EclipseLink.service' package. Under the given package is the DAO class ManyToOne.java.

After the accumulation and working of the program, you will be notified in the console panel of Eclipse IDE. Check MySQL workbench for output.

@OneToMany Relation

In this relation, many children in another entity are related to each row of an entity. The important part is that several parents cannot record children's records. Each row in Table A is linked to 0,1 or more rows of Table B in a one-to-many relationship between Table A and Table B.

Let us take an example. If the Student and subject are in a reverse unidirectional way, the connection is Many-To-One relation. Build a JPA project in eclipse IDE JPA_Eclipselink_OTM.

Creating Entities

• Create a package ‘com.world.EclipseLink.entity’ under ‘src’ set. Make a new java class named subject.java under the specified package.
• Make the other entity in this connection - Department entity class Department.java under ‘com.world.EclipseLink.entity’ package.

Persistence.xml

Persistence.xml file has the structure information of database and registration data of entity classes. Persitence.xml will be built by the eclipse IDE during the creation of a JPA Project. The configuration aspects are user stipulations.

Service Classes

This module includes the classes of services that use the initialisation property to implement the relational component. Create a 'src' package called 'com.world.EclipseLink.service' package. Under the given package is the DAO class OneToMany.java.

After the accumulation and working of the program, you will be notified in the console panel of Eclipse IDE. Check MySQL workbench for output.

@OneToOne Relation

One item can belong to only one other item in the One-To-One entity relation. It indicates that each row of an entity refers only to one row of a different entity.

Let us take an example of the relation between Student and subject entities. In a unidirectional that is from Student to subject, the One-To-One relation is applied. That indicates each record of student belongs to only one subject.

Create a project called JPA_Eclipselink_OTO in eclipse IDE.

Creating Entities

• Create a package ‘com.world.EclipseLink.entity’ under ‘src’ set. Make a new java class named subject.java under the specified package.
• Make the other entity in this connection - Department entity class Department.java under ‘com.world.EclipseLink.entity’ package.

Service Classes

This module includes the classes of services that use the initialisation property to implement the relational component. Create a 'src' package called 'com.world.EclipseLink.service' package. Under the given package is the DAO class OneToOne.java.

After the accumulation and working of the program, you will be notified in the console panel of Eclipse IDE. Check MySQL workbench for output.

After the accumulation and working of the program, you will be notified in the console panel of Eclipse IDE. Check MySQL workbench for output.

Many-To-Many Entity Relation

A many-To-Many Entity relation is one or more rows from one entity that are connected with more than one row in another entity.

Let us take an example of the relation between Student and subject entities. In a bidirectional way that is from Student to subject, the Many-To-One relation is applied. That indicates each record of student belongs to only one subject. That means each class record should be addressed by the Subject IDs which should be the primary keys of the Teacher's table, which should be kept in the subject_Class database and vice versa. Here, both foreign key fields are included in the subject_Class table.

Create a project called JPA_Eclipselink_MTM in eclipse IDE.

Creating Entities

• Create a package ‘com.world.EclipseLink.entity’ under ‘src’ set. Make a new java class named classes.java under the specified package.
• Make the other entity in this connection - subject entity class subject.java under ‘com.world.EclipseLink.entity’ package.

Persistence.xml

Persistence.xml file has the structure information of database and registration data of entity classes. Persitence.xml will be built by the eclipse IDE during the creation of a JPA Project. The configuration aspects are user stipulations.

Service Classes

This module includes the classes of services that use the initialisation property to implement the relational component. Create a 'src' package called 'com.world.EclipseLink.service' package. Under the given package is the DAO class ManyToMany.java

After the accumulation and working of the program, you will be notified in the console panel of Eclipse IDE. Check MySQL workbench for output.

JPA Criteria API

The API is a preset API for the creation of entity queries. It is the approach to set up a JPQL query. These queries are type-safe, portable and easy to alter by syntactic changes. Like JPQL, the abstract schema and embedded objects are followed. The API information is mixed with API criteria to model persistent entities for queries of criteria.

The main benefit of the API criterion is that bugs can be spotted upfront while compiling. The performance and efficiency of string-based JPQL queries and JPA-based questions are the same.