Using of variables in SQL query

Hello, I found when I use SEF URL (domain/category/id-alias), it is not working. If I put e.g. ?id=123 after it is working. Is it possible to use it with SEF URL?

I have tried it, but when I use vchart module e.g. in article with id=123

select x, y
from #__table
where id = {id}

does not work (nothing shows). If I set

select x, y
from #__table
where id = 123

it is working. Are there some restrictions or should I use it spcial way-

Hello, you write

I don't understand the usage of the variables - what exactly are these variables? I would like to use SQL select like

select x, y
from #__table
where id = {articleid}

{articleid} should be the ID of the actual article. Can I use it this way? The purpose is to show different chart in every article.

Jan