Java中Springboot项目执行sq查询时报错：Expected one result (or null) to be returned by selectOne(), but found: 2

Java中Springboot项目执行sq查询时报错：Expected one result (or null) to be returned by selectOne(), but found: 2

我出现以上错误的原因是：数据库表中的数据不一致，当时将主表中的数据清空了，但是关联表没清空，导致主表和关联表的数据不一致。