业务需求中，连表查询根据表的某个字段，这个字段可能为空
如果为空，根据这个表的另外一个字段查询，用union连接对结果去重即可

<select id="selectByNull" resultMap="BaseResultMap">
    SELECT DISTINCT b.SAMPLE_NO,
    b.ID,
    b.SPECIMEN_ID,
    b.RECEIVED_SPECIMEN_TIME,
    b.RECEIVED_SPECIMEN_PLACE,
    b.RECEIVER,
    b.SENDER,
    b.REMARK,
    b.RECORD_STATUS,
    b.CREATE_TIME,
    b.MODIFY_TIME,
    r.Name,
    t.ASSAY_NO
    FROM BIZ_SPECIMEN_LAB_RECEPTION b
    LEFT JOIN BIZ_ASSAY_ORDER_SAMPLE o on o.SAMPLE_NO=b.SAMPLE_NO
    LEFT JOIN RES_LABORATORY r on r.ID=o.SAMP_PREP_DEPT_CODE
    LEFT JOIN BIZ_ASSAY_TASK_INFO t ON t.SAMPLE_NO=b.SAMPLE_NO
    LEFT JOIN BIZ_ASSAY_ITEM_DATA d on d.SPECIMEN_ID=b.SPECIMEN_ID
    WHERE 1=1
    AND d.ASSAY_STATUS=0
    <if test="laboratoryId!=null and laboratoryId!=''">
        AND r.ID=#{laboratoryId}
    </if>
    <if test="sampleNo!=null and sampleNo!=''">
        AND b.SAMPLE_NO=#{sampleNo}
    </if>
    <if test="fromDate!=null and fromDate!=''">
        and <![CDATA[b.RECEIVED_SPECIMEN_TIME>=to_date(#{fromDate}, 'yyyy-mm-dd') ]]>
    </if>
    <if test="toDate!=null and toDate!=''">
        and <![CDATA[b.RECEIVED_SPECIMEN_TIME<=to_date(#{toDate}, 'yyyy-mm-dd') ]]>
    </if>
    UNION
    SELECT DISTINCT b.SAMPLE_NO,
    b.ID,
    b.SPECIMEN_ID,
    b.RECEIVED_SPECIMEN_TIME,
    b.RECEIVED_SPECIMEN_PLACE,
    b.RECEIVER,
    b.SENDER,
    b.REMARK,
    b.RECORD_STATUS,
    b.CREATE_TIME,
    b.MODIFY_TIME,
    r.Name,
    t.ASSAY_NO
    FROM BIZ_SPECIMEN_LAB_RECEPTION b
    LEFT JOIN BIZ_ASSAY_ORDER_SAMPLE o on o.SAMPLE_NO=b.SAMPLE_NO
    LEFT JOIN RES_LABORATORY r on r.ID=o.SAMP_PREP_DEPT_CODE
    LEFT JOIN BIZ_ASSAY_TASK_INFO t ON t.SAMPLE_NO=b.SAMPLE_NO
    LEFT JOIN BIZ_ASSAY_ITEM_DATA d on d.ASSAY_TASK_INFO_ID=t.ID
    WHERE 1=1
    AND d.ASSAY_STATUS=0
    <if test="laboratoryId!=null and laboratoryId!=''">
        AND r.ID=#{laboratoryId}
    </if>
    <if test="sampleNo!=null and sampleNo!=''">
        AND b.SAMPLE_NO=#{sampleNo}
    </if>
    <if test="fromDate!=null and fromDate!=''">
        and <![CDATA[b.RECEIVED_SPECIMEN_TIME>=to_date(#{fromDate}, 'yyyy-mm-dd') ]]>
    </if>
    <if test="toDate!=null and toDate!=''">
        and <![CDATA[b.RECEIVED_SPECIMEN_TIME<=to_date(#{toDate}, 'yyyy-mm-dd') ]]>
    </if>
</select>

union，默认为union distinct，已经对结果去重了，所以不用加distinct
union，返回去重后的数据
union all，返回所有的数据