EXISTS与IN的区别

 

 网上查了很多关于这两个单词的用法,说法很多,但大同小异,现在归纳如下: 关于EXISTS与IN的区别：EXISTS检查是否有结果，判断是否有记录，返回的是一个布尔型（TRUE/FALSE）。IN是对结果值进行比较，判断一个字段是否存在于几个值的范围中，所以 EXISTS 比 IN 快。主要区别是:exists主要用于片面的,有满足一个条件的即可,in主要用于具体的集合操作,有多少满足条件. exists是判断是否存在这样的记录，in是判断某个字段是否在指定的某个范围内。exists快一些吧 。in适合内外表都很大的情况，exists适合外表结果集很小的情况。在ASKTOM的讲解：Well, the two are processed very very differently.Select * from T1 where x in ( select y from T2 )is typically processed as:select * from t1, ( select distinct y from t2 ) t2where t1.x = t2.y;The subquery is evaluated, distinct'ed, indexed (or hashed or sorted) and then joined to the original table -- typically.As opposed to select * from t1 where exists ( select null from t2 where y = x )That is processed more like:for x in ( select * from t1 )loopif ( exists ( select null from t2 where y = x.x )then OUTPUT THE RECORDend ifend loopIt always results in a full scan of T1 whereas the first query can make use of an index on T1(x).So, when is where exists appropriate and in appropriate?Lets say the result of the subquery( select y from T2 )is "huge" and takes a long time. But the table T1 is relatively small and executing ( select null from t2 where y = x.x ) is very very fast (nice index on t2(y)). Then the exists will be faster as the time to full scan T1 and do the index probe into T2 could be less then the time to simply full scan T2 to build the subquery we need to distinct on.Lets say the result of the subquery is small -- then IN is typicaly more appropriate.If both the subquery and the outer table are huge -- either might work as well as the other -- depends on the indexes and othe* **ctors.