mybatis 一对多查询 按结果嵌套处理、按查询嵌套处理

最近用到一对多查询，记录一下

实体类

public class RegionEntity implements Serializable {private Long rid;private String regionName;private String addrImage;private String type;private List<FloorEntity> floorEntities;
}
public class FloorEntity implements Serializable {private Long id;private Long rid;private String floorName;
}

一个区域对应多个楼层

xml

按结果嵌套处理

把需要查询的结果，通过sql语句实现

    <select id="getRegionInfo" resultMap="regionMap">select f.id,f.floor_name,r.rid,r.region_name,r.addr_image,r.typefrom region r,floor fwhere f.rid=r.rid and r.rid=#{id}</select><resultMap id="regionMap" type="com.sf.entity.RegionEntity"><result property="rid" column="rid"/><result property="regionName" column="region_name"/><result property="addrImage" column="addr_image"/><result property="type" column="type"/><collection property="floorEntities" ofType="com.sf.entity.FloorEntity"><result property="id" column="id"/><result property="floorName" column="floor_name"/><result property="rid" column="rid"/></collection></resultMap>

按查询嵌套处理

把sql语句分开实现，通过一对多关联起来

    <select id="getRegionInfo2" resultMap="regionMap2">select * from region where rid = #{id}</select><resultMap id="regionMap2" type="com.sf.entity.RegionEntity"><result property="rid" column="rid"/><result property="regionName" column="region_name"/><result property="addrImage" column="addr_image"/><result property="type" column="type"/><collection property="floorEntities" javaType="ArrayList" ofType="com.sf.entity.FloorEntity" select="getFloorByRid" column="rid"/></resultMap><select id="getFloorByRid" resultType="com.sf.entity.FloorEntity">select * from floor where rid = #{id}</select>

<collection property="floorEntities" javaType="ArrayList" ofType="com.sf.entity.FloorEntity" select="getFloorByRid" column="rid"/>

property值为实体类中对象的值，javaType为对应的实体类，select为多方查询语句的命名id，column为两表关联id

dao层

RegionEntity getRegionInfo(Long id);

service、serviceImpl省略

controller层

@RequestMapping("/info/{rid}")
public R info(@PathVariable("rid") Long rid) {RegionEntity region =regionService.getRegionInfo(rid);return region;
}

浏览器测试
http://localhost:8089/info/26

{"rid": 26,"regionName": "401","addrImage": "default.png","deptId": 1,"type": "0","floorEntities": [{"id": 1,"rid": 26,"floorName": "一楼"}, {"id": 2,"rid": 26,"floorName": "二楼"}, {"id": 3,"rid": 26,"floorName": "三楼"}]

按结果嵌套处理、按查询嵌套处理两者的区别

按结果嵌套处理：若多的一方为空，则查询结果为空
按查询嵌套处理：若多的一方为空，依然可以输出查询结果，只是多的一项为null
例如：

{"rid": 27,"regionName": "402","addrImage": "default.png","deptId": 1,"type": "0","floorEntities": []
}