Question

oracle中in和exist的区别

Answer 1

in和exist的主要区别体现在对sql执行计划的影响上。
传统上认为，如果子查询的条件更具选择性（selective），就用in；而如果父查询（外层查询）的条件更具选择性（selective），就用exist。
具体的内容可以参考以下oracle原厂的手册，不好意思，oracle的原厂手册都是英文版的。

另外需要特别注意的是，in和exist的区别只在10.2.0.3及以前的版本中存在；而10.2.0.4及以后的版本中，in和exist的效果是完全一样的，手册中也删除了有关二者区别的说明。

以下是对手册的引用：
In certain circumstances, it is better to use IN rather than EXISTS. In general, if the selective predicate is in the subquery, then use IN. If the selective predicate is in the parent query, then use EXISTS.

Sometimes, Oracle can rewrite a subquery when used with an IN clause to take advantage of selectivity specified in the subquery. This is most beneficial when the most selective filter appears in the subquery and there are indexes on the join columns. Conversely, using EXISTS is beneficial when the most selective filter is in the parent query. This allows the selective predicates in the parent query to be applied before filtering the rows against the EXISTS criteria.