关联查询(一对一)
场景:一个作者(Author)有一个博客(Blog)
Blog.java
public class Blog {
private String id;
private String authorId;
private Author author;
//get & set
}
BlogMapper.xml
<resultMap id="blogResult" type="Blog">
<association property="author" column="author_id" javaType="Author" select="authorMapper.selectAuthor"/>
</resultMap>
<select id="selectBlog" resultMap="blogResult">
SELECT * FROM BLOG WHERE ID = #{id}
</select>
AuthorMapper.xml
<mapper namespace="authorMapper" >
<select id="selectAuthor" resultType="Author">
SELECT * FROM AUTHOR WHERE ID = #{id}
</select>
</mapper>
调用BlogMapper.xml中selectBlog 就可以查询关联后的结果。
集合查询(一对多)
场景:一个博客(Blog)有多篇文章(Post)
Blog.java
public class Blog {
private String id;
private List<Post> posts;
//get & set
}
BlogMapper.xml
<resultMap id="blogResult" type="Blog">
<collection property="posts" javaType="ArrayList" column="id" ofType="Post" select="postMapper.selectPostsForBlog"/>
</resultMap>
<select id="selectBlog" resultMap="blogResult">
SELECT * FROM BLOG WHERE ID = #{id}
</select>
PostMapper.xml
<mapper namespace="postMapper" >
<select id="selectPostsForBlog" resultType="Post">
SELECT * FROM POST WHERE BLOG_ID = #{id}
</select>
</mapper>
调用BlogMapper.xml中selectBlog就可以查询集合后的结果。