mybatis Result Maps對結果分組2--多層一對多 (譯)如何在sql中選取每一組的第一行/最後行/前幾行

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在以前的文章中說的是一個一對多狀況下mybatis的自動分組，這篇說下多個一對多的狀況。

好比qq空間裏的說說

能夠看到，說說和評論是一對多，評論又和回覆是一對多。

mood

public class Mood {
    private int mood_id;
    private String mood_content;
    private String mood_time;
    private User user;
    private List<MoodComment> moodComments;
       //getter,setter..
}

mood comment

public class MoodComment {
    private int moodcommentid;
    private String comment_content;
    private String comment_time;
    private User user;
    private Mood mood;
    private List<MoodCommentReply> moodCommentReplys;
        //getter,setter
}

mood reply

public class MoodCommentReply {
    private int moodreplyid;
    private String reply_content;
    private String reply_time;
    private User user;
    private MoodComment moodComment;
        //getter,setter
}

mood表

moodcomment表

moodcommentreply表

user表

表數據

作法很容易想到，就是把上一篇resultmap改一下就能夠了。

    <resultMap id="MoodResult" type="Mood">
        <id property="mood_id" column="mood_id" />
        <association property="user" javaType="User">
               <id property="id" column="mood_userid"/>
            <result property="name" column="mood_user"/>
        </association>
        <collection property="moodComments" ofType="MoodComment">
            <id property="moodcommentid" column="moodcommentid" />
            <association property="user" javaType="User">
                <id property="id" column="comment_userid" />
                <result property="name" column="comment_user"/>
            </association>
            <collection property="moodCommentReplys" ofType="MoodCommentReply">
                <association property="user" javaType="User">
                    <id property="id" column="reply_userid" />
                    <result property="name" column="reply_user"/>
                </association>
            </collection>
        </collection>
    </resultMap>

能夠看到在<collection>裏面再加一個<collection>.不過，要注意在裏面的<collection>加上<id>,不然mybatis不會自動進行第二次分組。

至於sql，仍是把三個表數據一併取出就能夠了，mybatis會自動分組。

        SELECT u1.name AS mood_user,u2.name AS comment_user,u3.name AS reply_user,
        mood.mood_id,mood.id AS mood_userid,mood_content,mood_time,
        moodcomment.moodcommentid,moodcomment.id AS comment_userid,moodcomment.comment_content,moodcomment.comment_time,
        moodcommentreply.moodcommentid,moodcommentreply.id AS reply_userid,reply_content,reply_time FROM mood 
        LEFT JOIN moodcomment ON moodcomment.mood_id=mood.mood_id 
        LEFT JOIN moodcommentreply ON moodcommentreply.moodcommentid=moodcomment.moodcommentid 
        LEFT JOIN USER AS u1 ON mood.id=u1.id 
        LEFT JOIN USER AS u2 ON moodcomment.id=u2.id 
        LEFT JOIN USER AS u3 ON moodcommentreply.id=u3.id 
        ORDER BY mood_time DESC,moodcomment.comment_time DESC,reply_time DESC

問題有來了，若是隻要評論的前2條，可是那2條評論的全部回覆要所有選出。

好比

仍是用(譯)如何在sql中選取每一組的第一行/最後行/前幾行裏面提到的user variables方法。

        SET @num := 0, @type := '';
        SELECT u1.name AS mood_user,u2.name AS comment_user,u3.name AS reply_user,
        mood.mood_id,mood.id AS mood_userid,mood_content,mood_time,
        moodcomment2.moodcommentid,moodcomment2.id AS comment_userid,moodcomment2.comment_content,moodcomment2.comment_time,
        moodcommentreply.moodcommentid,moodcommentreply.id AS reply_userid,reply_content,reply_time FROM mood 
        LEFT JOIN 
        (SELECT * 
        FROM (
          SELECT moodcommentid,comment_content,comment_time,id,mood_id,
              @num := IF(@type = mood_id, @num + 1, 1) AS row_number,
              @type := mood_id AS dummy
          FROM moodcomment
          ORDER BY mood_id
        ) AS moodcomment1 WHERE moodcomment1.row_number <= 2) AS moodcomment2
        ON moodcomment2.mood_id=mood.mood_id 
        LEFT JOIN moodcommentreply ON moodcommentreply.moodcommentid=moodcomment2.moodcommentid 
        LEFT JOIN USER AS u1 ON mood.id=u1.id 
        LEFT JOIN USER AS u2 ON moodcomment2.id=u2.id 
        LEFT JOIN USER AS u3 ON moodcommentreply.id=u3.id 
        ORDER BY mood_time DESC,moodcomment2.comment_time DESC,reply_time DESC

注意幾個moodcomment的命名空間。還有須要在jdbcUrl中加上allowMultiQueries=true，由於set variable也是一次查詢。