假设我们有三个数据表：学校、班级和学生。每个学校可以有多个班级，每个班级可以有多个学生。现在我们想要使用Android Room查询每个学校及其所包含的班级和学生。

首先，我们需要定义三个实体类：School、Class和Student。 School实体类应该包含主键和其他属性，而Class和Student实体类应该包含外键以引用School表中的主键。

@Entity(tableName = "school")
data class School(
    @PrimaryKey
    @ColumnInfo(name = "id") val id: Int,
    @ColumnInfo(name = "name") val name: String
)

@Entity(tableName = "class", foreignKeys = [
    ForeignKey(entity = School::class, parentColumns = ["id"], childColumns = ["school_id"])
])
data class Class(
    @PrimaryKey
    @ColumnInfo(name = "id") val id: Int,
    @ColumnInfo(name = "name") val name: String,
    @ColumnInfo(name = "school_id") val schoolId: Int
)

@Entity(tableName = "student", foreignKeys = [
    ForeignKey(entity = Class::class, parentColumns = ["id"], childColumns = ["class_id"])
])
data class Student(
    @PrimaryKey
    @ColumnInfo(name = "id") val id: Int,
    @ColumnInfo(name = "name") val name: String,
    @ColumnInfo(name = "class_id") val classId: Int
)

然后，我们需要创建一个包含所有三个实体类的数据访问对象(DAO)。我们可以使用@Transaction注释和自定义查询来获取与每个学校、其关联的班级和每个班级、其关联的学生相关的所有数据。

@Dao
interface SchoolDao {
    @Query("SELECT * FROM school")
    fun getSchoolsWithClassesAndStudents(): List

    @Transaction
    @Query("SELECT * FROM school")
    fun getSchoolsWithClassesAndStudentsLiveData(): LiveData>
}

我们还需要定义一个包含School、Class和Student实体的数据类SchoolWithClassesAndStudents，以便我们可以一次性获取所有请求的数据。

data class SchoolWithClassesAndStudents(
    @Embedded val school: School,
    @Relation(
        parentColumn = "id",
        entityColumn = "school_id"
    )
    val classes: List
)

data class ClassWithStudents(
    @Embedded val class: Class,