Spring Data JPA

Java Persistence API

JPA 是一种 Java 规范，定义了如何在 Java 对象和关系型数据库之间进行数据映射和管理（CRUD）。

JPA 使用 SQL 来进行数据库操作：当使用 JPA 进行数据访问时，JPA 实现（例如 Hibernate）会自动生成并执行 SQL 语句。例如，当调用 repository.save() 方法保存实体时，JPA 会自动生成相应的 INSERT SQL 语句来插入数据。

ORM: Object Relational Mapping

Hibernate 是 JPA 的一种实现, Spring Data JPA基于Hibernate

ORM 将编程语言中的对象（例如 Java 对象）映射到数据库表，每个对象的属性对应于数据库表中的字段。例如，一个 Student 类的实例可以对应于数据库中的一行记录。

1. config application.properties, start the application

2. connect to db

3. Define a Class and map it to db

4. CRUD: Extends Repositories

Repositories server as the Data Access Layer

PostgreSQL

Sequence是和Table平级的数据对象

Table

@Entity(name = "ST")

public class Student {

Attributes:

@Id

@SequenceGenerator(name = "st_seq_generator", sequenceName = "st_seq", allocationSize = 1)

@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "st_seq_generator")

private Long id;

@Column(name = "email", updatable = false, columnDefinition = "TEXT", unique = true)

private String email;

}

Aspect	Table-level Constraint	Field Attribute
Scope	Entire table, often involving multiple columns	Single column
Main Purpose	Defines relationships between columns (e.g., multi-column uniqueness) and table integrity	Defines storage characteristics of a column, such as type, length, uniqueness, etc.
Defined in	Typically defined in the `@Table` annotation in JPA	Typically defined in the `@Column` annotation in JPA
Typical Examples	`@UniqueConstraint`, `@ForeignKey`, etc.	`nullable`, `unique`, `length`, etc.
Applicable Scenarios	When constraints involving multiple columns or foreign keys are needed	When defining column-specific properties, like allowing null values, uniqueness, length, etc.

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SEQUENCE是在什么时候发挥作用的？是在插入数据的时候不用指明id吗？

Yes, you can delete this.id = xxx in constructor. No need to specify this attribute manually.