What is best way to implement SQL support in multi-tiered distributed application ??

Recently I have encountered this question and answer is ......... ok .. first tell me what comes in your mind for  solution. Something like me when I first encountered with this question  :-).

Initially I thought as a solution that we can prepare multiple Query.xml and Query.properties or SQLMap( Ibatis) to supports multiple databases and can cache those during startup and would use @ runtime in application ...

But .. then I found another best way to do .... one of my friend suggested me to write Open SQL which supports all the databases . So single file and query plan which suites to all database helps to improve the scalabilty of the applciation with less coding and less maintainance.

SAP is implemented it .. check it out this link :
http://help.sap.com/saphelp_nw04/helpdata/EN/fc/eb3969358411d1829f0000e829fbfe/content.htm

Cheers.....