I was wondering how big sites like Youtube do this particular thing.

There you can rate video once and then if you try to rate again it does not allow.

was wondering how their table structure would be to log who voted for which video

is it as simple as Userid,VideoId but wont this get really big soon because they have so many visitors?
Is there a better database design for this ?

yes it is and yes it will

besides having the script for it, they also have the disk space

i thought the tables may have some row limits , isnt it so ?
in terms of efficiency is it better to store one user id and one video id per row or just one userid and many comma separated video ids in second column

no, it isn't so

the former, by a very large margin of difference in performance

Thank you very much for your answers

Does youtube do something as simple as setting a cookie when you vote? That way, the storage requirement falls on the user's machine...

yes, they do set a cookie, go have a look on your C drive (holler if you don't know where to look, and please mention what browser you're using)

the only thing stored in my youtube cookie (i do not sign on when i visit) is data in the following categories:

- VISITOR_INFO1_LIVE
- LOCALE_PREFERENCE
- GEO

you do realize that youtube lets you vote on thousands of images, and yet there is a limit to both the size and number of cookies that your browser will support