解决mybatis，sql后端查询成功但postman测试返回的data却为null，空问题

http://localhost:8080/Sell/vi?begin=2023-06-04&end=2023-06-06
//接口get查询地址

查询成功，但是postman测试返回data为空

查询成功，但是postman测试返回data为空
{
    "code": 1,
    "msg": "success",
    "data": [
        null
    ]
}

这时到后端查看mybatis查询情况却发现查询成功返回数据正常

SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6a2e1306] was not registered for synchronization because synchronization is not active
JDBC Connection [HikariProxyConnection@2076509092 wrapping com.mysql.cj.jdbc.ConnectionImpl@64dc86c6] will not be managed by Spring
==>  Preparing: SELECT SUM(pricesum) FROM tb_sellinfo WHERE date BETWEEN ? AND ?
==> Parameters: 2023-06-04(LocalDate), 2023-06-06(LocalDate)
<==    Columns: SUM(pricesum)
<==        Row: 1400
<==      Total: 1
Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@6a2e1306]

排查问题发现实际上是实体类中没有查询返回结果中的 “SUM(pricesum)” getset方法

public class SellInfo {
    private Integer id;
    //规整时间返回格式@JsonFormat(shape=JsonFormat.Shape.STRING,pattern="yyyy-MM-dd HH:mm:ss",timezone="GMT+8")
    @JsonFormat(shape=JsonFormat.Shape.STRING,pattern="yyyy-MM-dd",timezone="GMT+8")
    private Date date;
    private String ISBN;
    private Integer num;
    private String price;
    private Integer pricesum;
    private String SUM;//加上对应名称的实体类，并修改返回结果的列名
}

在sql查询中修改：

--这是原sql查询        
        SELECT SUM(pricesum)
        FROM tb_sellinfo
        <where>
            <if test="isbn != null">
                AND ISBN = #{isbn}
            </if>
            <if test="begin != null and end != null">
                AND date BETWEEN #{begin} AND #{end}
            </if>
        </where>

         --修改返回列名为实体类中对应的SUM  
        SELECT SUM(pricesum) AS SUM
        FROM tb_sellinfo
        <where>
            <if test="isbn != null">
                AND ISBN = #{isbn}
            </if>
            <if test="begin != null and end != null">
                AND date BETWEEN #{begin} AND #{end}
            </if>
        </where>

重启后端之后，此时同样的get请求查询sum返回已经正确封装在data中

{
    "code": 1,
    "msg": "success",
    "data": [
        {
            "id": null,
            "date": null,
            "num": null,
            "price": null,
            "pricesum": null,
            "isbn": null,
            "sum": "1400"
        }
    ]
}

到后端查看发现列名已经变成与实体类对应的SUM

SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@55c2be89] was not registered for synchronization because synchronization is not active
JDBC Connection [HikariProxyConnection@805059805 wrapping com.mysql.cj.jdbc.ConnectionImpl@5f3b84bd] will not be managed by Spring
==>  Preparing: SELECT SUM(pricesum) AS SUM FROM tb_sellinfo WHERE date BETWEEN ? AND ?
==> Parameters: 2023-06-04(LocalDate), 2023-06-06(LocalDate)
<==    Columns: SUM
<==        Row: 1400
<==      Total: 1

总结：实际上就是查询时数据库返回的列名在实体类找不到对应的字段，无法封装要查询的这个列和数据，所以get请求无法得到相应的data数据。