使用EXISTS引入子查询

我有一个长查询，执行时会引发异常。查询：

SELECT HostID,HostName,RackID,HostTypeID,DomainName,RackNumberOfHeightUnits,RackStartHeightUnits 
FROM tHosts, tDomains   
WHERE tHosts.DomainID=tDomains.DomainID AND (RackID IN ( SELECT tRacks.Name,tRacks.RackID,tRacks.SiteID,tRacks.Description,NumberOfHeightUnits   
                                                         FROM tDomains, tSites, tRacks   
                                                         WHERE   tDomains.AccountID= tSites.AccountID    
                                                         AND tSites.SiteID = tRacks.SiteID    
                                                         AND tSites.SiteID = 2  
                                                         AND tDomains.AccountID=1 ) 
AND  SiteID IN (SELECT SiteID FROM tSites WHERE SiteID IN (SELECT SiteID FROM tSites WHERE AccountID=1)))AND AccountID=1

在这里完成查询：

SELECT tRacks.Name,tRacks.RackID,tRacks.SiteID,tRacks.Description,NumberOfHeightUnits 
                                                         FROM tDomains, tSites, tRacks  
                                                         WHERE tDomains.AccountID= tSites.AccountID  
                                                         AND tSites.SiteID = tRacks.SiteID  
                                                         AND tSites.SiteID = 2  
                                                         AND tDomains.AccountID=1 

**错误：**如果EXISTS未引入子查询，则只能在选择列表中指定一个表达式。
提前感谢。