前提介绍

在mybatis如何进行多对一、一对多（一对一）的多表查询呢？本章带你认识如何非常顺滑的解决！

基础使用篇

一对一

association

association通常用来映射一对一的关系，例如，有个类user,对应的实体类如下：

@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Student {
    private int id;
    private String name;
    /**
     * 学生要关联一个老师
     */
    private Teacher teacher;
}
@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Teacher {
    private int id;
    private String name;
}

Dao层进行Mapper查询操作

public interface TeacherMapper {
    Teacher getTeacher(@Param("tid") int id);
    Teacher getTeacher2(@Param("tid") int id);
}

Dao层进行Mapper.xml文件

<resultMap id="StudentTeacher" type="com.sunreal.pojo.Student">
        <result column="id" property="id"></result>
        <result column="name" property="name"></result>
        <association property="teacher" column="id"  javaType="com.sunreal.pojo.Teacher" select="getTeacher"/>
    </resultMap>
    <select id="getStudent" resultMap="StudentTeacher">
        select *
        from student
    </select>
    <select id="getTeacher" resultType="com.sunreal.pojo.Teacher">
        select *
        from teacher
        where id = #{id}
    </select>
    <resultMap id="StudentTeacher2" type="com.sunreal.pojo.Student">
        <result column="sid" property="id"></result>
        <result column="sname" property="name"></result>
        <association property="teacher" javaType="com.sunreal.pojo.Teacher">
            <result property="name" column="tname"></result>
        </association>
    </resultMap>
    <select id="getStudent2" resultMap="StudentTeacher2">
        select s.id sid, s.name sname, t.name tname
        from student s,
             teacher t
        where s.tid = t.id
    </select>

• assocication：可以指定联合的JavaBean对象

• select：指定相关查询结果sqlid

• property="role“：指定哪个属性是联合的对象

• javaType:指定这个属性对象的类型

• column="{javabean熟悉=数据库字段,Javabean属性=数据库字段}"

<association property="role" javaType="com.queen.mybatis.bean.Role">
    <id column="role_id" property="id"/>
    <result column="roleName" property="roleName"/>
</association>

以上如果跨越命名空间的情况下：select：需要用namespace.selectId进行指定。

collection

@Alias("Student")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Student {
    private int id;
    private String name;
    private int tid;
}
@Alias("Teacher")
@Data
@AllArgsConstructor
@NoArgsConstructor
@Accessors(chain = true)
public class Teacher {
    private int id;
    private String name;
    /**
     * 一个老师包含多个学生
     */
    private List<Student> studentList;
}

Dao层进行Mapper查询操作

public interface TeacherMapper {
    Teacher getTeacher(@Param("tid") int id);
    Teacher getTeacher2(@Param("tid") int id);
}

Dao层进行Mapper.xml文件

<resultMap id="TeacherStudent" type="Teacher">
        <result property="id" column="tid"></result>
        <result property="name" column="tname"></result>
        <collection property="studentList" ofType="Student">
            <result property="id" column="sid"></result>
            <result property="name" column="sname"></result>
            <result property="tid" column="tid"></result>
        </collection>
    </resultMap>
    <select id="getTeacher" resultMap="TeacherStudent">
        select s.id sid, s.name sname, t.name name, t.id tid
        from student s,
             teacher t
        where s.tid = t.id
          and t.id = #{tid}
    </select>
    <resultMap id="TeacherStudent2" type="Teacher">
        <collection property="studentList" javaType="ArrayList" ofType="Student"
                    select="getStudentByTeacherId" column="id"/>
    </resultMap>
    <select id="getTeacher2" resultMap="TeacherStudent2">
        select *
        from teacher
        where id = #{tid}
    </select>
    <select id="getStudentByTeacherId" resultType="Student">
        select *
        from student
        where tid = #{tid}
    </select>

注意：各个表之间尽量不要有重名字段，包括主键id，不然可能会造成数据混乱错误；

• JavaType和ofType都是用来指定对象类型的

• property="指的是对象内部（List类型）的属性信息字段名称"

• JavaType是用来指定pojo中属性的类型

• ofType指定的是映射到list集合属性中pojo的类型。

• column="{javabean熟悉=数据库字段,Javabean属性=数据库字段}"

• select：指定相关查询结果sqlid

”特叔“使用篇

一对一映射

实体列 class Tb_blog/TbBlog

private long blogId;
    private String blogTitle;
    private String blogContent;
    private Date createTime;
    private  String blogType;
    private String sId;
    private Tb_author author;
    List<TbAuthor> tbAuthorList;

实体类 class TbAuthor

private long id;
    private String username;
    private String password;
    private String email;
    private String address;
    private String phone;
    private TbBlog tbBlog;
    private List<TbBlog> tbBlogList;

resultMap标签配置

<resultMap id="blogMap" type="Tb_blog"  >
            <id column="blogId" property="blogId"/>
            <result column="blogTitle" property="blogTitle"/>
            <result column="blogContent"  property="blogContent"/>
            <result column="blogType"  property="blogType"/>
            <result column="createTime"  property="createTime"/>
             <result column="sId"    property="sId"/>
            <result column="id"    property="author.id"/> <!-- 映射第二张表的实体类属性 -->
                <result column="username"    property="author.username"/>
             <result column="password"    property="author.password"/>
            <result column="email"    property="author.email"/>
</resultMap>
  <select id="selectBlogAndAuthor" resultMap="blogMap">
        select * from tb_blog g inner join tb_author r
        on g.blogId = r.id 
  </select>

在sql加入别名alias与field属性字段一样，也可以自动注入进入。

association标签配置

<resultMap id="blogMap" type="Tb_blog"  >
            <id column="blogId" property="blogId"/>
            <result column="blogTitle" property="blogTitle"/>
            <result column="blogContent"  property="blogContent"/>
            <result column="blogType"  property="blogType"/>
            <result column="createTime"  property="createTime"/>
            <!-- 一对一高效率写法 association一对一关联  property属性为实体类中的第二张表的属性名   -->
            <association property="tb_author" javaType="TbAuthor"><!--javaType属性为 返回的实体类对象 -->
                <id column="id"  property="id"/>
                <result column="username" property="username"/>
                <result column="password" property="password"/>
                <result column="email" property="email"/>
                <result column="address" property="address"/>
            </association>
</resultMap>
    <select id="selectBlogAndAuthor" resultMap="blogMap">
        select * from tb_blog g inner join tb_author r on g.blogId = r.id 
    </select>

collection标签配置

mapper接口定义

AuthorMapper.interface

//!通过id 和映射文件中 association的column属性的值sId关联 来嵌套查询     嵌套查询的第二条sql语句都要写条件来关联第一张表
List<TbAuthor> selectAuthorandBlogAssociation(int id);

BlogMapper.interface

List<TbBlog> selectBlogAndAuthorAssociation();

AuthorMapper.xml

<select id="selectAuthorandBlogAssociation" resultType="com.xqh.pojo.TbAuthor">
        select * from tb_author where id=#{id}
</select>
 <resultMap id="mapCollection" type="TbAuthor">
        <id property="id" column="id"/>
        <result property="username" column="username"/>
        <result property="password" column="password"/>
        <result property="email" column="email"/>
        <result property="phone" column="phone"/>
        <result property="address" column="address"/>
        <collection property="tbBlogList" column="id"
            select="com.xqh.mapper.BlogMapper.selectBlogAndAuthor"
                    fetchType="lazy">
        </collection>
    </resultMap>
    <select id="selectAuthor_BlogList" resultMap="mapCollection">
        select * from tb_author
    </select>

BlogMapper.xml

<select id="selectBlogAndAuthor" resultType="com.xqh.pojo.TbBlog">
        select * from tb_blog where sId = #{id}
    </select>

总结

多表查询一对一映射

association标签

不嵌套 property=当前实体类中的第二种表的属性名 javaType=返回的实体类

嵌套 多加两个属性 column=当前实体类 关联的 第二张表 的外键字段 select=“第二条查询语句” (必须给第二条sql语句写参数限制 不然会获得所有值)

多表查询一对多

collection标签

不嵌套 property=当前实体类中的第二种表的属性名 ofType=返回是实体类

property=当前实体类中的第二种表的属性名 javaType=返回的实体类嵌套 多加两个属性 column=当前实体类 关联的 第二张表 的外键字段 select=“第二条查询语句” (必须给第二条sql语句写参数限制 不然会获得所有值)2.多表查询一对多collection标签

不嵌套 property=当前实体类中的第二种表的属性名 ofType=返回是实体类

嵌套 多加一个属性 column=当前实体类 关联的 第二张表 的外键字段 select=“第二条查询语句” (必须给第二条sql语句写参数限制 不然会获得所有值) [ofType = collection一对多嵌套查询 嵌套查询所有结果 不需写返回类型因为 select已经映射]

原文: https://www.cnblogs.com/liboware/p/15220433.html