mybatis-plus多表联查join的实现
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1、安装依赖
<dependency>
<groupId>com.github.yulichang</groupId>
<artifactId>mybatis-plus-join</artifactId>
<version>1.2.4</version>
</dependency>
2、定义三个基本实体类
2.1 User类
@Data
public class User {
//对应数据库的主键(uuid,自增id,雪花算法,redis,zookper)
@TableId(type = IdType.AUTO)
private Integer id;
@NotNull(message = "名称xxx不能为空")//空校验
@NotBlank //不能为空字符串
@Size(min = 1,max = 5,message = "ssssss")//字符串长度校验
private String name;
// @Min(value = 10,message = "最小值10")
// @Max(value = 90,message = "最大值为99")
@NotNull
// @Max(value = 50,message = "超过最大值")
// @Min(value = 10,message = "超过最小值")
@Range(min=10,max = 99,message = "错误")//数字范围校验
private Integer age;
@NotNull
private String email;
//逻辑删除字段
@TableLogic //逻辑删除注解
private Integer deleted;
//字段添加填充内容
//策略
// @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss")
@TableField(fill = FieldFill.INSERT)
@DateTimeFormat(pattern = "yyyy-MM-dd")
@JsonFormat(pattern = "yyyy-MM-dd", timezone = "GMT+8")
private LocalDateTime createTime;
//策略
// @JsonFormat(pattern = "yyyy-MM-dd HH:mm:ss")
@JsonFormat(pattern = "yyyy-MM-dd", timezone = "GMT+8")
@TableField(fill = FieldFill.INSERT_UPDATE)
private LocalDateTime updateTime;
@TableField(fill = FieldFill.INSERT)
private Integer createBy;
@TableField(fill = FieldFill.INSERT_UPDATE)
private Integer updateBy;
@Version//乐观锁注解
private Integer version;
}
2.2 UserScore类
@Data
public class UserScore {
@TableId(type = IdType.AUTO)
private long id;
private String course;
private String score;
private Long userId;
}
2.3 UserCourseTeacher类
@Data
public class UserCourseTeacher {
@TableId(type = IdType.AUTO)
private Integer id;
private Long teacher;
private Long userCourseId;
}
3、定义三个mapper
3.1 userMapper
@Repository
@Mapper
public interface UserMapper extends MPJBaseMapper<User> {
}
3.2 UserScoreMapper
@Repository
@Mapper
public interface UserScoreMapper extends MPJBaseMapper<UserScore> {
}
3.3 UserCourseTeacherMapper
@Mapper
@Repository
public interface UserCourseTeacherMapper extends MPJBaseMapper<UserCourseTeacher> {
}
4、定义返回值dto
@Data
public class UserTeacherDto extends User {
private String course;
private String score;
private String teacher;
}
5、开发业务层
例如对应的sql为
select a.*,b.score,b.course,c.teacher from user a left join user_score b on a.id=b.user_id left join user_course_teacher c on b.id=c.user_course_id
则代码为
public List<UserTeacherDto> GetAllUserTeacher(){
MPJLambdaWrapper<User> mPJLambdaWrapper=new MPJLambdaWrapper<User>()
.selectAll(User.class)
.select(UserScore::getScore,UserScore::getCourse)
.select(UserCourseTeacher::getTeacher)
.leftJoin(UserScore.class,UserScore::getUserId,User::getId)
.leftJoin(UserCourseTeacher.class,UserCourseTeacher::getUserCourseId,UserScore::getId);
List<UserTeacherDto> list=userMapper.selectJoinList(UserTeacherDto.class, mPJLambdaWrapper);
return list;
}
6、假如进行分页查询的话
@Override
public Map<String,Object> GetUserDtoByPage(){
Map<String,Object> result=new HashMap<>();
MPJLambdaWrapper<User> mPJLambdaWrapper=new MPJLambdaWrapper<User>()
.selectAll(User.class)
.select(UserScore::getScore,UserScore::getCourse)
.leftJoin(UserScore.class,UserScore::getUserId,User::getId);
IPage<UserDto> UserDtoPage=userMapper.selectJoinPage(
new Page<UserDto>(3,2),
UserDto.class,
mPJLambdaWrapper
);
result.put("list",UserDtoPage.getRecords());
result.put("count",UserDtoPage.getTotal());
return result;
}
7、注意:
- selectAll():查询指定实体类的全部字段
- select():查询指定的字段,支持可变长参数同时查询多个字段,但是在同一个select中只能查询相同表的字段,所以如果查询多张表的字段需要分开写
- selectAs():字段别名查询,用于数据库字段与接收结果的dto中属性名称不一致时转换
- leftJoin():左连接,其中第一个参数是参与联表的表对应的实体类,第二个参数是这张表联表的ON字段,第三个参数是参与联表的ON的另一个实体类属性
8、其他
MPJLambdaWrapper<User> mPJLambdaWrapper=new MPJLambdaWrapper<User>()
.selectAll(User.class)
中的User可以理解为主表
leftJoin函数参数顺序依次为:子表的类型、子表的字段(实体类属性)、主表实体类的属性,就是用于on关联的
9、如果针对条件进行动态查询
@Override
public List<UserTeacherDto> GetAllUserTeacher(Map<String,Long> params){
System.out.println("传递参数");
System.out.println(params.get("id"));
MPJLambdaWrapper<User> mPJLambdaWrapper=new MPJLambdaWrapper<User>()
.selectAll(User.class)
.select(UserScore::getScore,UserScore::getCourse)
.select(UserCourseTeacher::getTeacher)
.leftJoin(UserScore.class,UserScore::getUserId,User::getId)
.leftJoin(UserCourseTeacher.class,UserCourseTeacher::getUserCourseId,UserScore::getId);
if(params.get("id")!=null && params.get("id").toString()!=""){
mPJLambdaWrapper.eq(UserScore::getId,params.get("id"));
}
List<UserTeacherDto> list=userMapper.selectJoinList(UserTeacherDto.class, mPJLambdaWrapper);
return list;
}
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