新手才接触数据库，用的是SQL Server2008写的简单查询
SELECT DISTINCT JNO
       FROM SPJ
       WHERE SNO IN
            (SELECT SNO
             FROM   S
             WHERE  CITY = '天津');

SELECT DISTINCT JNO
       FROM SPJ
       WHERE EXISTS
            (SELECT SNO
             FROM   S
             WHERE  CITY = '天津');
很想知道这两个查询应该是等价的为什么运行结果不一样呢
看了好久都找不出问题，谢谢大神了