i want to take a database certification,i don't know which one.
You have got at least Oracle, DB2 and SQL Server. Others are probably less popular.
what are the benfits that i have from this membership?
I do not know but I think you get discounts on Oracle courses. Very often these courses may offer free exam vouhers. So you can wrap up the course with certification. BTW currently courses attendance are a pre-requisite for Oracle Certification starting 9i.
what does the certification cost altogether under this circumstances?
Normally each exam costs $125 and with some discount code (offered with free membership with
http://otn.oracle.com you may get it slashed to $100. But with Oracle corporate membership you need to find details.
does i get a certificate already for the associate level from oracle or only one for the whole dba?
You get certificate for each of the levels but it may not be worth much compared to full DBA Certification.
at which time have i to take courses? more precisely, can i pass the associate level and get a certificate without this funny courses?
I think you can get the lower level certifications without course participation. But for DBA certification you get to provide proof of course attendance. If you still want to avoid course attendance altogether, you have got the 8i track. But 8i has more exams (five) and upgrades to 9i and 10i. But 8i exam delivery expires 12/04. It would still save you lot of money if you have to pay for courses yourself.
Has the oracle certificates a timeout?
It doesn't. But it is a version specific. So if nobody uses the version you are certified, you would rather not mention it.
has anybody pass the mysql-certifications? what are these certifications worth?
Sorry, I do not know. I am familiar with only Oracle.
Best wishes