最近做项目遇到一个问题,外键关联查询,返回长度为1,但是值为 All elements are null没数据。 有两张表,分别为a表有数据,b表无数据。 select b.* from a as a left join b as b on b.a_id = a.id 发生异常 修正sql select b.* from a as a left join b as b on b.a_id = a.id where b.id is not null就可以避免这种情况。 select a.* from a as a left join b as b on b.a_id = a.id 无异常,