{"id":128429,"date":"2024-11-21T10:37:00","date_gmt":"2024-11-21T08:37:00","guid":{"rendered":"https:\/\/www.javacodegeeks.com\/?p=128429"},"modified":"2024-11-18T10:58:56","modified_gmt":"2024-11-18T08:58:56","slug":"query-jpa-single-table-inheritance","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/query-jpa-single-table-inheritance.html","title":{"rendered":"Query JPA Single Table Inheritance"},"content":{"rendered":"

In Java Persistence API (JPA), inheritance mapping provides a way to map Java class hierarchies to database tables. Single Table Inheritance is one of the inheritance strategies where all entities in a class hierarchy are stored in a single database table. Although simple to implement, it requires a discriminator column<\/code> to distinguish between various subclasses. Let us delve into understanding JPA inheritance with single table strategy, exploring how it allows different entity types to be stored in a single database table while distinguishing them using a discriminator column.<\/p>\n

<\/a>1. Single Table Inheritance SubTypes<\/h2>\n

Single Table Inheritance is managed by using the @Inheritance<\/code> annotation with InheritanceType.SINGLE_TABLE<\/code>. In this approach, a single table is created to store all fields of both the superclass and subclasses. A discriminator column is used to identify the type of entity being stored. <\/p>\n

<\/a>2. Code Example<\/h2>\n

2.1 Setting up the Project and adding dependencies<\/h3>\n

We\u2019ll start by creating a Spring Boot project with the help of Spring Initializr<\/a> and add the necessary dependencies. In the pom.xml<\/code> file, add the following dependencies:<\/p>\n

\n<dependency>\n    <groupId>org.springframework.boot<\/groupId>\n    <artifactId>spring-boot-starter-data-jpa<\/artifactId>\n<\/dependency>\n\n<dependency>\n    <groupId>org.springframework.boot<\/groupId>\n    <artifactId>spring-boot-starter-web<\/artifactId>\n<\/dependency>\n<\/pre>\n
2.2 Setting up the Mock data<\/h3>\n
We will be using a PostgreSQL database running on Docker. Once the PostgreSQL database is up and running on Docker use the below SQL commands to create the mock data.<\/p>\n
\n-- Create Database\nCREATE DATABASE test;\n\n-- Create Employee Table with Single Table Inheritance\nCREATE TABLE employee (\n    id BIGSERIAL PRIMARY KEY,            -- Unique identifier with auto-increment (BIGSERIAL)\n    name VARCHAR(100) NOT NULL,          -- Common attribute for all employees\n    salary DOUBLE PRECISION,             -- Specific to FullTimeEmployee\n    hourly_rate DOUBLE PRECISION,        -- Specific to PartTimeEmployee\n    employee_type VARCHAR(20) NOT NULL   -- Discriminator column (FULL_TIME, PART_TIME)\n);\n\n-- Insert FullTimeEmployee\nINSERT INTO employee (name, salary, employee_type) VALUES ('John Doe', 60000, 'FULL_TIME');\nINSERT INTO employee (name, salary, employee_type) VALUES ('Emily Carter', 75000, 'FULL_TIME');\n\n-- Insert PartTimeEmployee\nINSERT INTO employee (name, hourly_rate, employee_type) VALUES ('Jane Smith', 15, 'PART_TIME');\nINSERT INTO employee (name, hourly_rate, employee_type) VALUES ('Tom Brown', 18, 'PART_TIME');\n<\/pre>\n
2.3 Define the Java Subclasses<\/h3>\n
Let\u2019s start with defining the base class: Employee<\/code> with shared properties (id<\/code> and name<\/code>) for the subclasses.
 
<\/div> <\/div><\/p>\n
\n@Entity\n@Inheritance(strategy = InheritanceType.SINGLE_TABLE)\n@DiscriminatorColumn(name = \"employee_type\", discriminatorType = DiscriminatoryType.STRING)\npublic abstract class Employee {\n\n    private Long id;\n\n    private String name;\n\n    \/\/ Constructors, Getters, and Setters\n}\n<\/pre>\n
Now, we will define two subclasses: FullTimeEmployee<\/code> and PartTimeEmployee<\/code>. Each will have its unique properties followed by the setter and getter methods. In both the below classes, employee_type<\/code> is the discriminator column, which will contain FULL_TIME<\/code> for full-time employees and PART_TIME<\/code> for part-time employees.<\/p>\n
2.3.1 FullTimeEmployee Model class<\/h4>\n
\n@Entity\n@DiscriminatorValue(\"FULL_TIME\")\npublic class FullTimeEmployee extends Employee {\n    private double salary;\n\n    \/\/ Constructors, Getters, and Setters\n}\n<\/pre>\n
The given code defines a subclass of Employee<\/code> called FullTimeEmployee<\/code>, which is an entity in a Java Persistence API (JPA) context. <\/p>\n