嵌套查询

<resultMap type="com.foo.bean.BlogInfo" id="BlogInfo">    <id column="blog_id" property="blogId" />    <result column="title" property="title" />    <association property="author" column="blog_author_id"      javaType="com.foo.bean.Author" select="com.foo.bean.AuthorMapper.selectByPrimaryKey">    </association>    <collection property="posts" column="blog_id" ofType="com.foo.bean.Post"      select="com.foo.bean.PostMapper.selectByBlogId">    </collection>  </resultMap>  <select id="queryBlogInfoById" resultMap="BlogInfo" parameterType="java.math.BigDecimal">    SELECT    B.BLOG_ID,    B.TITLE,    B.AUTHOR_ID AS BLOG_AUTHOR_ID    FROM LOULUAN.BLOG B    where B.BLOG_ID = #{blogId,jdbcType=DECIMAL}  </select>此方法的缺点：要进行n+1将查询重新定义BlogInfo的结果映射 resultMap <resultMap type="com.foo.bean.BlogInfo" id="BlogInfo">    <id column="blog_id" property="blogId"/>    <result column="title" property="title"/>    <association property="author" column="blog_author_id" javaType="com.foo.bean.Author">      <id column="author_id" property="authorId"/>      <result column="user_name" property="userName"/>      <result column="password" property="password"/>      <result column="email" property="email"/>      <result column="biography" property="biography"/>    </association>    <collection property="posts" column="blog_post_id" ofType="com.foo.bean.Post">      <id column="post_id" property="postId"/>      <result column="blog_id" property="blogId"/>      <result column="create_time" property="createTime"/>      <result column="subject" property="subject"/>      <result column="body" property="body"/>      <result column="draft" property="draft"/>    </collection>  </resultMap>对应的sql语句如下：<select id="queryAllBlogInfo" resultMap="BlogInfo">      SELECT        B.BLOG_ID,       B.TITLE,       B.AUTHOR_ID AS BLOG_AUTHOR_ID,       A.AUTHOR_ID,       A.USER_NAME,       A.PASSWORD,       A.EMAIL,       A.BIOGRAPHY,       P.POST_ID,       P.BLOG_ID   AS BLOG_POST_ID ,     P.CREATE_TIME,       P.SUBJECT,       P.BODY,       P.DRAFT  FROM BLOG B  LEFT OUTER JOIN AUTHOR A    ON B.AUTHOR_ID = A.AUTHOR_ID  LEFT OUTER JOIN POST P    ON P.BLOG_ID = B.BLOG_ID  </select>此方法是从数据库中将数据全部查询出来，然后对返回的结果集在内存中进行组装、赋值