Mybatis关联查询方式

1.嵌套结果

比如我们想通过查询一个文章,返回这个文章和这个文章的作者的信息

 <!--pojo  BlogAndAuthor.java-->
public class BlogAndAuthor implements Serializable {
    Integer bid; // 文章ID
    String name; // 文章标题
    Author author; // 作者
  <!--省略set get tostring,不在这展示了-->
}

 <!--pojo  Author.java-->
public class Author implements Serializable {
    Integer authorId; // 作者ID
    String authorName; // 作者名称

    public Integer getAuthorId() {
        return authorId;
    }

    public void setAuthorId(Integer authorId) {
        this.authorId = authorId;
    }

    public String getAuthorName() {
        return authorName;
    }

    public void setAuthorName(String authorName) {
        this.authorName = authorName;
    }
}

封装一个结果集,内部对对象属性对一个结果映射

<!-- 根据文章查询作者，一对一查询的结果，嵌套结果 -->
    <resultMap id="BlogWithAuthorResultMap" type="BlogAndAuthor">
        <id column="bid" property="bid" jdbcType="INTEGER"/>
        <result column="name" property="name" jdbcType="VARCHAR"/>
        <!-- 联合查询，将author的属性映射到ResultMap -->
        <association property="author" javaType="Author">
            <id column="author_id" property="authorId"/>
            <result column="author_name" property="authorName"/>
        </association>
    </resultMap>

查询语句--结果集映射为我们上面自己定义的

 <!-- 根据文章查询作者，一对一，嵌套结果，无N+1问题 -->
    <select id="selectBlogWithAuthorResult" resultMap="BlogWithAuthorResultMap" >
        select b.bid, b.name, b.author_id, a.author_id , a.author_name
        from blog b
        left join author a
        on b.author_id=a.author_id
        where b.bid = #{bid, jdbcType=INTEGER}
    </select>

2.嵌套查询

    <!-- 另一种联合查询(一对一)的实现，但是这种方式有“N+1”的问题 -->
    <resultMap id="BlogWithAuthorQueryMap" type="BlogAndAuthor">
        <id column="bid" property="bid" jdbcType="INTEGER"/>
        <result column="name" property="name" jdbcType="VARCHAR"/>
        <association property="author" javaType="Author"
                     column="author_id" select="selectAuthor"/> 
    </resultMap>

 <!-- 嵌套查询 -->
    <select id="selectAuthor" parameterType="int" resultType="Author">
        select author_id authorId, author_name authorName
        from author where author_id = #{authorId}
    </select>

<select id="selectBlogWithAuthorQuery" resultMap="BlogWithAuthorQueryMap" >
       select b.bid, b.name, b.author_id, a.author_id , a.author_name
       from blog b
       left join author a
       on b.author_id=a.author_id
       where b.bid = #{bid, jdbcType=INTEGER}
   </select>

这种情况会有N+1的问题,我们可以开启来加载来解决(原理动态代理)

    <!-- 延迟加载的全局开关。当开启时，所有关联对象都会延迟加载。默认 false  -->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!-- 当开启时，任何方法的调用都会加载该对象的所有属性。默认 false，可通过select标签的 fetchType来覆盖-->
        <setting name="aggressiveLazyLoading" value="false"/>
        <!--  Mybatis 创建具有延迟加载能力的对象所用到的代理工具，默认JAVASSIST -->
        <!--<setting name="proxyFactory" value="CGLIB" />-->