本小节将要介绍一对一、一对多的查询【技术上】。
你有可能会问，多对多怎么查，这个问题就不该问！开玩笑的，哈哈。
其实在设计表时，一般多对多的关系就已经转化为一对多了。

一对多

就用一个学生有多本书籍来举例子吧!

image.png

table

create table `student`(
`id` int auto_increment primary key,
`name` varchar(30) not null,
`age` int
)auto_increment=10000;

create table `book`
(`id` int auto_increment primary key,
`bookName` varchar(20),
`author` varchar(10),
`studentID` int references `student`(`id`)
)auto_increment=10000;

model

public class Student {
    private int id;
    private String name;
    private int age;

    private List<Book> books;

    public Student() {//无参构造函数===>创建对象
    }
//getter和setter略
}

public class Book {
    private int bookID;
    private String bookName;
    private String author;

    public Book() {
    }
}
//getter和setter略

dao

public interface StudentMapper {
    Student findMyBooks(int id);
}

mapple.xml
方式一，直接嵌套

    <select id="findMyBooks" resultMap="studentMapper">
        select `student`.`id` `id`,
       `student`.`name` `name`,
       `student`.`age` `age`,
       `book`.`id` `bookID`,
       `book`.`bookName` `bookName`,
       `book`.`author` `author`
        from `student`,`book`
        where `student`.`id` = `book`.`studentID`
        and `student`.`id` = #{id}
    </select>

    <resultMap id="studentMapper" type="student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <collection property="books"  ofType="book">
            <id property="bookID" column="bookID"/>
            <result property="bookName" column="bookName"/>
            <result property="author" column="author"/>
        </collection>
    </resultMap>

方式二：resultMap复用

    <select id="findMyBooks" resultMap="studentMapper">
        select `student`.`id` `id`,
       `student`.`name` `name`,
       `student`.`age` `age`,
       `book`.`id` `bookID`,
       `book`.`bookName` `bookName`,
       `book`.`author` `author`
        from `student`,`book`
        where `student`.`id` = `book`.`studentID`
        and `student`.`id` = #{id}
    </select>

    <resultMap id="bookMapper" type="book">
        <id property="bookID" column="bookID"/>
        <result property="bookName" column="bookName"/>
        <result property="author" column="author"/>
    </resultMap>

    <resultMap id="studentMapper" type="student">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <collection property="books" resultMap="bookMapper"/>
    </resultMap>

关联查询必须用无参构造函数来创建对象，如果没有无参构造函数，就会报构造函数参数错误。希望能引起你的警觉。