mybatis如何使用注解实现一对多关联查询

mybatis 注解实现一对多关联查询

@Select("select id,mockexam_section as section,id as sectionId"
			+ " from t_p_qb_mockexam_section"
			+ " where mockexam_charpter_id = #{charpterId} and is_delete = 0"
			+ " order by mockexam_section_idx asc")
@Results({
@Result(property = "questionList",column = "sectionId",many = @Many(select = "com.zikaoshu.baseinfo.mapper.BaseinfoQuestionMapper.listQuestionResDto"))})
List<SectionQuestionDto> listSectionQuestionDto(@Param("charpterId") Integer charpterId);
	
	
@Select("select id,type,discuss_title as discussTitle,stem1,material,a,b,c,d,e,answer,analysis,mockeaxm_section_id as sectionId"
			+ " from t_p_qb_question_mockexam"
			+ " where mockeaxm_section_id = #{id} and is_delete = 0"
			+ " order by q_sequence,gmt_create asc")
List<QuestionResDto> listQuestionResDto(@Param("id") Integer id);

mybatis多对多查询(xml方式和注解方式）

前面总结了一对一，多对一和一对多的多表查询，今天总结一下多对多的mybatis多表查询。同样有xml方式和注解方式，步骤和前两种查询差不多，最主要的区别就在表和sql语句上了。

数据库表及关系

这里采用用户和角色的例子

一个用户可以有多个角色

一个角色可以赋予多个用户

在进行多表查询时，我们需要一张中间表，中间表中包含各自的主键，在中间表中是外键。

多对多查询（xml方式）

这次我们首先清理一下思路，我们先在数据库里把我们需要的数据查出来再写代码。

我们查询用户时要同时查出其对应的角色，借助中间表，根据UID查询RID，再根据RID查询角色表，中间表的数据我们不需要，所以不显示。

这里我们可以用左外连接来进行多表的查询，查询所有用户，用户有角色信息就连接到该用户后面，没有则为空。

select u.*,r.id as rid,r.ROLE_NAME,r.ROLE_DESC from user u
        left outer join user_role ur on u.id=ur.uid
        left outer join role r on ur.rid = r.id

当我们查询角色想要得到相应的用户时道理是一样的，SQL语句也只要换一下连接顺序。

select u.*,r.id as rid,r.ROLE_NAME,r.ROLE_DESC from role r
        left outer join user_role ur on r.id=ur.rid
        left outer join user u on ur.uid = u.id

查询出来结果后剩下的内容就很简单。

在User和role里加入多对多实体映射

public class Role implements Serializable {
    private String roleId;
    private String roleName;
    private String roleDesc;
    //多对多映射关系，一个角色有多个用户
    private List<User> users;
    public List<User> getUsers() {
        return users;
    }
    public void setUsers(List<User> users) {
        this.users = users;
    }
    public String getRoleId() {
        return roleId;
    }
    public void setRoleId(String roleId) {
        this.roleId = roleId;
    }
    public String getRoleName() {
        return roleName;
    }
    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }
    public String getRoleDesc() {
        return roleDesc;
    }
    public void setRoleDesc(String roleDesc) {
        this.roleDesc = roleDesc;
    }
    @Override
    public String toString() {
        return "role{" +
                "roleId="" + roleId + """ +
                ", roleName="" + roleName + """ +
                ", roleDesc="" + roleDesc + """ +
                "}";
    }
}

public class User implements Serializable{
    private Integer id;
    private String username;
    private String address;
    private String sex;
    private Date birthday;
    //多对多映射关系，一个用户具备多个角色
    private List<Role> roles;
    public List<Role> getRoles() {
        return roles;
    }
    public void setRoles(List<Role> roles) {
        this.roles = roles;
    }
    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", username="" + username + """ +
                ", address="" + address + """ +
                ", sex="" + sex + """ +
                ", birthday=" + birthday +
                "}";
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public String getAddress() {
        return address;
    }
    public void setAddress(String address) {
        this.address = address;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    public Date getBirthday() {
        return birthday;
    }
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
}

然后配置xml，配置映射封装和sql语句

<!--定义resultMap-->
    <resultMap id="userWithRole" type="user">
        <id property="id" column="id"></id>
        <result property="username" column="username"></result>
        <result property="address" column="address"></result>
        <result property="sex" column="sex"></result>
        <result property="birthday" column="birthday"></result>
        <!--配置角色映射-->
        <collection property="roles" ofType="role">
            <id property="roleId" column="rid"></id>
            <result property="roleName" column="role_name"></result>
            <result property="roleDesc" column="role_desc"></result>
        </collection>
    </resultMap>
    <!--查询所有用户信息-->
    <select id="findAll" resultMap="userWithRole">
        select u.*,r.id as rid,r.ROLE_NAME,r.ROLE_DESC from user u
        left outer join user_role ur on u.id=ur.uid
        left outer join role r on ur.rid = r.id
    </select>

<resultMap id="roleUserMap" type="role">
        <id property="roleId" column="rid"></id>
        <result property="roleName" column="role_name"></result>
        <result property="roleDesc" column="role_desc"></result>
        <collection property="users" ofType="user">
            <id property="id" column="id"></id>
            <result property="username" column="username"></result>
            <result property="address" column="address"></result>
            <result property="sex" column="sex"></result>
            <result property="birthday" column="birthday"></result>
        </collection>
    </resultMap>
    <!--查询所有角色信息-->
    <select id="findAll" resultMap="roleUserMap">
        select u.*,r.id as rid,r.ROLE_NAME,r.ROLE_DESC from role r
        left outer join user_role ur on r.id=ur.rid
        left outer join user u on ur.uid = u.id
    </select>

测试结果

注解方式

思路是一样的，但我们使用注解时，不能像xml方式一样只使用一条sql语句完成直接封装，所以这里要按上面说的思路完成分步查询。

public interface IUserDao {
    /**
     * 查询所有操作,并携带账户信息
     * @return
     */
    @Select("select * from user")
    @Results(id = "userRoleMap",value = {
            //id表示主键
            @Result(id = true,column = "id",property = "id"),
            @Result(column = "username",property = "username"),
            @Result(column = "address",property = "address"),
            @Result(column = "sex",property = "sex"),
            @Result(column = "birthday",property = "birthday"),
            @Result(property = "roles",column = "id",many = @Many(select = "com.itcc.dao.IRoleDao.findByUid",fetchType = FetchType.LAZY))
    })
    List<User> findAll();
    /**
     * 根据id查询一个用户
     * @param rid
     */
    @Select("select * from user where id in(select uid from user_role where rid = #{rid})")
    @Results({
            @Result(id = true,column = "id",property = "id"),
            @Result(column = "username",property = "username"),
            @Result(column = "address",property = "address"),
            @Result(column = "sex",property = "sex"),
            @Result(column = "birthday",property = "birthday")
    })
    List<User> findByRId(Integer rid);
}

public interface IRoleDao {
    /**
     * 查询所有角色信息
     * @return
     */
    @Select("select * from role")
    @Results({
            @Result(id = true,column = "id",property = "roleId"),
            @Result(column = "role_name",property = "roleName"),
            @Result(column = "role_desc",property = "roleDesc"),
            @Result(property = "users",column = "id",many = @Many(select = "com.itcc.dao.IUserDao.findByRId",fetchType = FetchType.LAZY))
    })
    List<Role> findAll();
    @Select("select * from role where ID in(select rid from user_role where uid = #{uid})")
    @Results({
            @Result(id = true,column = "id",property = "roleId"),
            @Result(column = "role_name",property = "roleName"),
            @Result(column = "role_desc",property = "roleDesc")
    })
    List<Role> findByUid(String uid);
}

最终的测试结果和上面一样。

以上为个人经验，希望能给大家一个参考，也希望大家多多支持服务器之家。

原文链接：https://blog.csdn.net/CoderYin/article/details/90768007