在ORACLE中相交（SQL不是PL/SQL）

我必须在两个select语句之间进行交集。条件是，只有当两个select语句都返回一些resultset时，才会发生交集。如果其中任何一个返回O行，则交集不起作用，并且最终结果必须是返回某些行的语句。这在

SQL

中可能吗（我不需要

pl/SQL

中的答案）

您可以使用这种构造来实现这一点

with t1 as (select level id from dual where 1 = 0 connect by level <= 3),
     t2 as (select level id from dual where 1 = 1 connect by level <= 5)
select distinct * 
  from t1 
  where exists (select null from t2 where id = t1.id)
    or (select count(1) from t2) = 0
union all
select distinct * from t2 where (select count(1) from t1) = 0

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很抱歉，刚才的评论。这是一个完美的解决方案，有很好的解释。这对我有用。非常感谢：-）

select * from t1 
intersect 
select * from t2 
union 
select * from t1 where (select count(1) from t2) = 0
union 
select * from t2 where (select count(1) from t1) = 0