Detailed explanation of MyBatis one-to-many query configuration: in-depth analysis of mapping relationships

MyBatis is a popular Java persistence layer framework, and its flexible SQL mapping configuration enables the database to be Operation becomes simple and efficient. In actual development, we often encounter one-to-many query requirements, that is, one entity object is associated with multiple sub-entity objects. This article will delve into how to configure one-to-many query in MyBatis, parse the mapping relationship, and give specific code examples.

One-to-many relationship mapping

In the database, one-to-many relationships are usually established through foreign keys. For example, if a class has multiple students, the primary key in the class table can be used as a foreign key in the student table, thus establishing a one-to-many relationship. In MyBatis, we can implement one-to-many query by configuring the mapping file.

Entity class design

First, we need to design the corresponding entity class to map the structure of the database table. Take a class as an example. There are multiple students in a class. You can design the following Java class:

public class Class {
    private int id;
    private String name;
    private List<Student> students;
    // 省略getter和setter方法
}

public class Student {
    private int id;
    private String name;
    private int classId;
    // 省略getter和setter方法
}

In the Class class, we use a List type attribute to store the list of students in the class; in Student In a class, use the classId attribute to represent the foreign key relationship of the class to which it belongs.

Mapping file configuration

Next, we need to configure the MyBatis mapping file and define a one-to-many query relationship. In the Class mapping file, we can configure the association with the Student entity class through the tag:

<mapper namespace="com.example.ClassMapper">
    <select id="getClassById" resultType="Class" parameterType="int">
        SELECT * FROM class WHERE id = #{id}
    </select>

    <select id="getStudentsByClassId" resultType="List" parameterType="int">
        SELECT * FROM student WHERE class_id = #{classId}
    </select>
</mapper>

Here, we define two query statements respectively, one is to query the class based on the class id information method, and the other method is to query the student list based on class ID.

Implementation code example

Finally, let’s take a look at how to implement one-to-many query operations in Java code. First, define an interface ClassMapper and the corresponding implementation class ClassMapperImpl:

public interface ClassMapper {
    Class getClassById(int id);
    List<Student> getStudentsByClassId(int classId);
}

public class ClassMapperImpl {
    public Class getClassById(int id) {
        // 調(diào)用SQL查詢語句獲取班級信息
    }

    public List<Student> getStudentsByClassId(int classId) {
        // 調(diào)用SQL查詢語句獲取學(xué)生列表
    }
}

Then, call these methods in the business logic to complete the one-to-many query operation:

Class class = classMapper.getClassById(1);
List<Student> students = classMapper.getStudentsByClassId(1);
class.setStudents(students);
System.out.println(class.getName() + "的學(xué)生有：");
for (Student student : students) {
    System.out.println(student.getName());
}

Through the above operations, we Successfully implemented one-to-many query configuration and mapping operations. In practical applications, we can design more complex one-to-many relationships according to business needs, and flexibly use MyBatis' mapping configuration to implement related functions.

Summary

This article introduces in detail how to configure and implement one-to-many query operations in MyBatis. Through the steps of designing entity classes, configuring mapping files, and implementing Java code, it provides an in-depth analysis of Mapping relationship to many relationships. I hope this article will help readers deal with one-to-many query problems in MyBatis. It also encourages readers to conduct more practical exercises and attempts to deepen their understanding and application of the MyBatis framework.

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