1、selectById 根据主键查询一个对象，如果没有查到，则返回null：

GxySchoolDto isExist = gxySchoolMapper.selectById(schoolEntity.getSchoolId());

2、selectOne 根据 查询对象 queryWrapper 查询一个 对象， 没有查到则返回 null：

QueryWrapper queryWrapper = new QueryWrapper();
queryWrapper.eq("is_deleted",0);
queryWrapper.eq("school_code",schoolEntity.getSchoolCode());
GxySchoolDto isByCode = gxySchoolMapper.selectOne(queryWrapper);

@Mapper
public interface GxySchoolMapper extends BaseMapper {
}

@Data
@TableName("gxy_school")
public class GxySchoolDto extends BaseDto implements Serializable {
private static final long serialVersionUID = 1L;

/\*\*  
 \* 唯一主键  
 \*/  
@TableId(value = "school\_id", type = IdType.UUID)  
private String schoolId;  
/\*\*  
 \* 学校名称  
 \*/  
private String schoolName;  
/\*\*  
 \* 学校代码  
 \*/  
private String schoolCode;

3. list 查询列表：

QueryWrapper queryWrapper = new QueryWrapper();
queryWrapper.eq("plan_temp_id",planTemplateEntity.getPlanTemplateId());
queryWrapper.eq("is_deleted",0);

List planTempAppraiseItemDtos = gxyPlanTempAppraiseItemService.list(queryWrapper);

对应的sql：
SELECT plan_temp_appraise_item_id,is_deleted,create_time,plan_temp_id,appraise_item_id,prop,is_point,item_name,snow_flake_id FROM gxy_plan_temp_appraise_item WHERE plan_temp_id = ? AND is_deleted = ? 
Parameters: de42308f1617a837f98ba2507ac58d87(String), 0(Integer)

4. SQL 语句：
   SELECT school_id,school_name,is_deleted,school_code FROM gxy_school WHERE is_deleted=0 AND is_deleted = ? AND ( school_name = ? OR school_code = ? ) 
   对应的java 代码：

   QueryWrapper queryWrapper = new QueryWrapper<>();
   queryWrapper.eq("is_deleted",0);
   queryWrapper.and(wrapper -> wrapper.eq("school_name",schoolEntity.getSchoolName()).or().eq("school_code",schoolEntity.getSchoolCode()));
   GxySchoolDto isByName = gxySchoolMapper.selectOne(queryWrapper);

5. in 语句：

    QueryWrapper<NoticeTodoDto> queryWrapper = new QueryWrapper();  
    queryWrapper.eq("snow\_flake\_id",gxyTodoDto.getSnowFlakeId());  
    queryWrapper.eq("object\_id",gxyTodoDto.getObjectId());  
    queryWrapper.in("state",1,2);  
    NoticeTodoDto   todoDto=  noticeTodoMapper.selectOne(queryWrapper);  
    return todoDto;

6. 刪除

接口：

@Mapper
public interface GxyPlanDepMapper extends BaseMapper {

}

Bean：

@Data
@TableName("gxy_plan_dep")
public class GxyPlanDepDto implements Serializable {
private static final long serialVersionUID = 1L;

private int isDeleted;  
/\*\*  
 \* 创建时间  
 \*/  
private Date createTime;  
/\*\*  
 \* 唯一主键  
 \*/  
@TableId(value = "plan\_dep\_id",type = IdType.UUID)  
private String planDepId;  
/\*\*  
 \* 计划Id  
 \*/  
private String planId;  
/\*\*  
 \* 系部id  
 \*/  
private String depId;  

}

实现层：

planDepMapper.delete(new QueryWrapper().eq("plan_id",teacherStudentEntity.getPlanId()));

SQL：

DELETE FROM gxy_plan_dep
WHERE plan_id = ? ::: [a7637ddc0cafd26278ffc4be12262054]

7 修改

GxyPlanTeacherStudentDto dto = new GxyPlanTeacherStudentDto();
dto.setBackup(teacherStudentEntity.getPlanId());
dto.setPlanId(teacherStudentEntity.getNewPlanId());
dto.setTeacherId(teacherStudentDto.getTeacherId());
dto.setJobId(teacherStudentDto.getJobId());
dto.setCompanyName(teacherStudentDto.getCompanyName());

            QueryWrapper wrapper = new QueryWrapper<GxyPlanTeacherStudentDto>().eq("plan\_teacher\_student\_id", teacherStudentDto.getPlanTeacherStudentId()).eq("snow\_flake\_id", teacherStudentEntity.getSnowFlakeId());  
            planTeacherStudentMapper.update(dto,wrapper);

根据  wrapper  的条件，查出记录， 修改 dto 中的字段。

当在 bean 中加了 @TableField(strategy= FieldStrategy.IGNORED) 注解，当 dto 中 该字段是null 时，update 修改该字段， 表中该字段也改成null 。 如果不加这个注解，当dto 中 该字段是null 时， update 语句不修改该字段。

@TableField(strategy= FieldStrategy.IGNORED)
private String jobId;

8: 分页查询

Page page = new Page(teacherStudentEntity.getCurrPage(),teacherStudentEntity.getPageSize());

    IPage<GxyMultiStudentPlanTmpDto> page1 =  
    multiStudentPlanTmpMapper.selectPage(page, new QueryWrapper<GxyMultiStudentPlanTmpDto>()  
            .eq("create\_by",teacherStudentEntity.getUserId())  
            .eq("is\_deleted",0));

    List<GxyMultiStudentPlanTmpDto> list =    page1.getRecords();  
    return ListResponse.resList(list,page1.getTotal());

9： mybatis  查询可以返回Set 或者List：

　　Mapper：

Set selectMyStudent(@Param("params") GxyPlanTeacherStudentEntity planTeacherStudentEntity);

XML：

SELECT pstu.student_id
from gxy_plan_teacher_student AS pstu

LEFT JOIN gxy\_plan AS p ON pstu.plan\_id = p.plan\_id  
WHERE pstu.is\_deleted = 0  
<choose>  
    <when test="params.planId != null and params.planId != ''">  
        AND pstu.plan\_id = #{params.planId}  
    </when>  
    <otherwise>  
        and p.plan\_state = 1  
    </otherwise>  
</choose>

<if test="params.teacherId !='' and params.teacherId!=null">  
    AND pstu.teacher\_id = #{params.teacherId}  
</if>

AND FIND_IN_SET(#{params.position},t.position)

AND position LIKE concat('%',#{roleName},'%')

10. 修改 ， 根据 QueryWrapper 条件查询出所有的记录 后 修改 dto 实体中设置的字段

    　　　　　　MoguDeliverDto dto= new MoguDeliverDto();
    QueryWrapper queryWrapper = new QueryWrapper();
    queryWrapper.eq("user_id",deliver.getUserId());
    dto.setAcceptOffer(0);
    moguDeliverMapper.update(dto,queryWrapper);

UPDATE mogu_deliver SET accept_offer=?, modified_time=? WHERE is_deleted=0 AND user_id = ?

dto