an anybody point me some books or online tutorials to write effective queries?

If you really want to find out why your queries don't perform well, you should learn about Execution Plans.  This book by Grant Fritchey should help you with that: SQL Server Execution Plans

ValentinoV,

Thanks for the reply.Let me go through on it.I want such books or links for Oracle DB as well.

Can't help you with Oracle, sorry...

I will wait for other oracle experts..

Thanks for the links.on which word(sql optimization or complex queries) do i need to google it?

some of the below strings/words can be searched and they get a lot of good content/urls. you need to spend time to review/pick up them which are good and store them for your frequent use etc...

oracle efficient sql statements
oracle sql optimization
oracle sql optimization techniques
oracle writing efficient sql
oracle writing efficient queries

what kind of oracle books other than online links do you refer mostly while writing complex queries?

sorry, not really using books these days but used them when i started with oracle stuff in 1990s/2000s and that was a long ago. i some times refer to library reference books but do not remember authors for them.