mybatis 查詢的數據返回爲null報空指針異常

解決方法：
使用IFNULL函數返回null時附默認值

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="tms">
    <!--真正項目開發過程中*要用每個字段替代-->

    <!--查詢經營收入表表數據-->

    <select id="income"  parameterType="String"  resultType="model.tms.BusinessIncome">
        select IFNULL(sum(income_money),0) sim FROM business_income
        where  waybill_id=#{waybill_id} and data_status=1;
    </select>
</mapper>