MyBatis一对多查询：
有联合查询和嵌套查询
联合查询是几个表联合查询，只查询一次，通过在resultMap中配置collection节点配置一对多的类即可；
嵌套查询是先查一个表，根据这个表中的结果的外键id，再去另一个表中查询数据，也是通过collection，但是另一个表的查询通过selec节点配置
1.1 联合查询
<resultMap type="Post" id="postResultMap">
<id column="id" property="id"/>
<collection column="id" property="commentList" javaType="ArrayList" ofType="Comment">
</collection>
</resultMap>
<select id="selectPostById" parameterType="int" resultMap="postResultMap">
select * from post left join comment on post.id = comment.post_id where post.id = #{id}
</select>
console：
==> Preparing: select * from post left join comment on post.id = comment.post_id where post.id = ? ==> Parameters: 1(Integer) <== Total: 3 com.roxy.mybatis.pojo.Post@4abdb505
1.2 嵌套查询
<resultMap type="Post" id="postResultMap">
<id column="id" property="id"/>
<collection column="id" property="commentList" javaType="ArrayList" ofType="Comment"
select="com.roxy.mybatis.mapper.CommentMapper.selectCommentByPostId">
</collection>
</resultMap>
<select id="selectPostById" parameterType="int" resultMap="postResultMap">
select * from post where id = #{id}
</select>
<select id="selectCommentByPostId" parameterType="int" resultMap="commentResultMap">
select * from comment where post_id = #{postId}
</select>
console：
==> Preparing: select * from post where id = ?
==> Parameters: 1(Integer)
<== Total: 1
==> Preparing: select * from comment where post_id = ?
==> Parameters: 1(Integer)
<== Total: 3
com.roxy.mybatis.pojo.Post$$EnhancerByCGLIB$$ac65a46b@2d127a61
