【Mybatis系列】Mybatis空值关联

一.问题描述

1.已知条件

2.关联条件

现在需要将 table_1 和 table_2 进行关联,关联条件是 order_no,community_id,post_id 这 3 个字段,但是 order_no 不为 null,不过 community_id,post_id 是可能为 null,也可能不为 null

3.初步解法

如以下 SQL 所示,发现结果为空,没有查询到数据,实际是有数据,问题在于 community_id 和 post_id 对于空值的处理。

<select id="totalPageInfo" resultType="com.kwan.springbootkwan.entity.dto.CsdnTotalIncomeDTO">
    SELECT t1.receiver_nick_name AS nickName
    , SUM(t1.received_money) AS amount
    FROM table_1 t1 left join table_2 t2
    on t1.order_no = t2.order_no
           AND t1.community_id=t2.community_id
           AND t1.post_id=t2.post_id
    WHERE 1 = 1
    <if test="query.startDate != null">
        AND t1.receive_time <![CDATA[>= #{query.startDate}]]>
        AND t2.create_time <![CDATA[>= #{query.startDate}]]>
    </if>
    <if test="query.endDate != null">
        AND t1.receive_time <![CDATA[<= #{query.endDate}]]>
        AND t2.create_time <![CDATA[<= #{query.endDate}]]>
    </if>
    GROUP BY nickName
    ORDER BY amount DESC
</select>

二.解决方案

1.SQL 如下

<select id="totalPageInfo" resultType="com.kwan.springbootkwan.entity.dto.CsdnTotalIncomeDTO">
    SELECT t1.receiver_nick_name AS nickName
    , SUM(t1.received_money) AS amount
    FROM table_1 t1 left join table_2 t2
    on t1.order_no = t2.order_no
    <![CDATA[
           AND t1.community_id   <=> t2.community_id
           AND t1.post_id<=> t2.post_id
    ]]>
    WHERE 1 = 1
    <if test="query.startDate != null">
        AND t1.receive_time <![CDATA[>= #{query.startDate}]]>
        AND t2.create_time <![CDATA[>= #{query.startDate}]]>
    </if>
    <if test="query.endDate != null">
        AND t1.receive_time <![CDATA[<= #{query.endDate}]]>
        AND t2.create_time <![CDATA[<= #{query.endDate}]]>
    </if>
    GROUP BY nickName
    ORDER BY amount DESC
</select>

2.解释说明

在这个 SQL 查询中，由于 community_id 和 post_id 可能为空，你可以通过使用 COALESCE 函数或 IFNULL 函数（具体取决于你使用的数据库系统）来处理可能的空值情况。

下面是一种修改方式，假设你使用的是 MySQL 数据库：

SELECT
    t1.receiver_nick_name AS nickName,
    SUM(t1.received_money) AS amount
FROM
    table_1 t1
LEFT JOIN
    table_2 t2 ON t1.order_no = t2.order_no
                        AND t1.community_id <=> t2.community_id
                        AND t1.post_id <=> t2.post_id
WHERE
    1 = 1
    <if test="query.startDate != null">
        AND t1.receive_time <![CDATA[>= #{query.startDate}]]>
        AND t2.create_time <![CDATA[>= #{query.startDate}]]>
    </if>
    <if test="query.endDate != null">
        AND t1.receive_time <![CDATA[<= #{query.endDate}]]>
        AND t2.create_time <![CDATA[<= #{query.endDate}]]>
    </if>
GROUP BY
    nickName
ORDER BY
    amount DESC

在这里，使用了 <=> 操作符，它在 MySQL 中用于处理 NULL 值的比较。如果 community_id 或 post_id 的其中一个是 NULL，那么 <=> 操作符会返回 true。

请根据你使用的数据库类型来调整语法。如果是其他数据库，可能会使用 COALESCE 或 IS NULL 等不同的语法。