Exists用于只能用于子查询，可以替代in，若匹配到结果，则退出内部查询，并将条件标志为true，传回全部结果资料,in不管匹配到匹配
以Oracle数据库为例
1 exists的使用
IN is often better if the results of the subquery are very small
When you write a query using the IN clause, you're telling the rule-based optimizer that you
want the inner query to drive the outer query.
When you write EXISTS in a where clause, you're telling the optimizer that you want the outer
query to be run first, using each value to fetch a value from the inner query.
In many cases, EXISTS is better because it requires you to specify a join condition, which can
invoke an INDEX scan. However, IN is often better if the results of the subquery are very
small. You usually want to run the query that returns the smaller set of results first.
匹配原则，拿最小记录匹配大记录。
在看下官网文档里怎么说：
首先看下语法，，语法很简单，一看例子大家都会明白，但一定要注意Operation里面的那句话很重要，因为这样关系到null的问题，是返回至少一行就返回true值。
