SQL JOIN与子查询问题：查询无结果（No Rows Selected）求助

错误原因 #

1. 子查询筛选逻辑错误：subject_name不可能同时等于两个不同的字符串，原代码中用AND连接两个相等条件会导致子查询返回空结果集，主查询自然无法匹配到数据。需将AND替换为OR，或用IN来匹配多个目标科目名称。
2. 缺少排序语句：需求要求按科目名称降序排序，原代码未添加对应的ORDER BY子句。

修正后的代码 #

方式一：修正子查询逻辑

select s.subject_name, min(m.value) as MIN_MARK
from mark m
join subject s on m.subject_id = s.subject_id 
where m.subject_id in (select subject_id from subject
                       where subject_name = 'Software Engineering' 
                       OR subject_name='Computer Programming') 
group by s.subject_name
order by s.subject_name DESC;

方式二：简化查询（直接过滤科目名称）

无需嵌套子查询，直接在主查询中匹配目标科目名称，代码更简洁：

select s.subject_name, min(m.value) as MIN_MARK
from mark m
join subject s on m.subject_id = s.subject_id 
where s.subject_name IN ('Software Engineering', 'Computer Programming')
group by s.subject_name
order by s.subject_name DESC;

内容的提问来源于stack exchange，提问作者Levi