mybatisplus

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<GxySchoolDto> {
}

@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<GxyPlanTempAppraiseItemDto> 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<GxySchoolDto> 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);

mybatisplus

原文：https://www.cnblogs.com/z360519549/p/11545559.html