摘要：
这是上面的userName。但事实上，当我们有多个表时，通常每个表都有一个POJO类。您还可以将StringuserName更改为NfUsernfUser，然后在mapper中将其更改为˂resultproperties=“nfUser.userName”column=“user_name”
1.多表查询，1对1的时候，最简单的做法
<resultMap id="postInfo" type="postInfoEntity"> <id property="postId" column="post_id"/> <result property="userName" column="user_name"/> <result property="postTitle" column="post_title"/> <result property="postContent" column="post_content"/> <result property="topPost" column="top_post"/> <result property="wonderfulPost" column="wonderful_post"/> <result property="createTime" column="create_time"/> <result property="count" column="num"/> </resultMap> <!--某个贴吧所有界面的帖子，通过置顶降序排序--> <select id="listAllPostInfos" resultMap="postInfo"> SELECT A.post_id, COUNT(*) num, post_title, post_content, top_post, wonderful_post, user_name, A.create_time FROM post_info A, nf_user B, reply_post_info C WHERE post_status = 1 AND C.reply_status = 1 AND post_bar_id = #{param1} AND A.user_id = B.user_id AND A.post_id = C.post_id GROUP BY post_id ORDER BY top_post DESC, create_time </select>
其中COUNT(*) 取了一个别名，目的是为了对应resultMap中的<result property="count" column="num"/>, 然而仅仅这样是不够的，因为虽然查得到，但是mybatis映射不出来，他底层的反射和动态代理还需要我们在实体类进行设置——我们加一个属性和property对应就可以了
@Data @Document(indexName = "post_info") public class PostInfoEntity { @Id private Long postId; @Field(type = FieldType.Long) private Long postBarId; @Field(type = FieldType.Text, analyzer = "ik_max_word") private String postTitle; @Field(type = FieldType.Text, analyzer = "ik_max_word") private String postContent; @Field(type = FieldType.Long) private Long userId; @Field(type = FieldType.Long) private Long topPost; @Field(type = FieldType.Long) private Long wonderfulPost; @Field(type = FieldType.Integer) private Integer audit; @Field(type = FieldType.Long) private Long visitCount; @Field(type = FieldType.Integer) private Integer postStatus; @Field(type = FieldType.Date) private Date createTime; private Integer count; private String userName; } // 除了@Data注解其他都无视，其他的是elasticsearch的
2.多表联合查询，可以像我们上面一样，和COUNT(*)同一种写法，直接写一个<result>标签然后实体类加一个属性对应。就是上面的userName.
但是实际上我们多表的时候一般每一张表都有POJO类的，也可以把String userName改成NfUser nfUser 然后mapper中对应修改成<result properties="nfUser.userName" colum="user_name"