Mybatis查询到数据但是返回结果为null

╰半夏微凉° 2022-12-04 01:20 439阅读 0赞

**问题描述：**  
查看控制台打印的日志  
![在这里插入图片描述][20200904203308175.png_pic_center]  
可以看出执行了sql语句，但是也查询到了2条记录，但是返回的结果为null，说明没有设置成功数据  
代码:  
Position.java

public class Position implements Serializable { 
        @Id
    // @GeneratedValue(strategy = GenerationType.IDENTITY)
        @Column(name = "position_id")
        private Integer id;
        @Column(name = "position_name", length = 15)
        private String name;
      }

PositionMapper.xml

<?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="org.lieying.dao.PositionMapper">
        <select id="selectPositionByPositionCategoryId" resultType="Position" parameterType="int">
            select * from position where position_category_id = #{position_category_id}
        </select>
    </mapper>

**原因:**  
由于java的字段名称和数据库的字段名称不一致，所有set方法无法设置成功数据  
**解决方法:**  
（1）使用sql语句修改字段名称  
即

<select id="selectPositionByPositionCategoryId" resultType="Position" parameterType="int">
        select position_id as id,position_name as name from position where position_category_id = #{position_category_id}
        </select>

![在这里插入图片描述][20200904203929968.png_pic_center]  
成功查询到数据

(2)在配置文件中配置resultMap映射数据库属性和java类属性，设置select的返回类型为resultMap的id，即

<select id="selectPositionByPositionCategoryId" resultMap="positionResultMap" parameterType="int">
     --select position_id as id,position_name as name from position 
        select position_id as id,position_name as name from position where position_category_id = #{position_category_id}
        </select>
    
        <resultMap id="positionResultMap" type="Position">
            <id column="position_id" property="id"/>
            <result column="position_name" property="name"/>
            <result column="position_address" property="address"/>
        </resultMap>

![在这里插入图片描述][20200904204418738.png_pic_center]  
同样成功查询到数据

[20200904203308175.png_pic_center]: /images/20221123/12a2a2e3055c4c919b245a3964f10f9d.png
[20200904203929968.png_pic_center]: /images/20221123/d3647e9055834bd2a60f0e7b73b3259d.png
[20200904204418738.png_pic_center]: /images/20221123/848bd9acace5411392fc1ec8664e2da1.png