Hi,
I am developing a web based application, and I am expecting lots of users accessing the site simultaneously. I would like to know which database should i go for, SQL Server, ORACLE ,MySQL or any other db would suffice. The number of people I am expecting to be accessing simultaneously would be around 1 million users. It would be great if anyone can help me on this. Thanks

There are many criteria in decision which RDBMS to use.
- number o concurent users
- cardinality of database
- technologies supported by database
- hardware for database
- support (commercial)
- end product will be commercional or free
etc. etc.

In fact MySQL and Oracle (I don't know so good MSSQL to compare) are good datatabases but you need to consider according the above criteria which RDBMS you will be using.
My opinion is if you need database as website backend then choose MySQL. If you need database for enterprise purposes then choose Oracle.

Well, if you want to do web stuff, MySQL is the way to go. Your problems will most likely be related to concurrency so Oracle, DB2 and MySQL are the RDBMS you should evaluate... SQL Server will die with that amount of connections and its page locking...

DB2 offers an EEE versions supporting clustering and it works quite nicely (on SMP architecture too!). Oracle offers very good performance but is *very* ressource hungry... MySQL has the smallest memory footprint so that could help considering the number of simultaneous connections you'll have.

I'd suggest you send an email to Jeremy Zawodny on the MySQL mailing list... He's the guy that designed the setup of MySQL for Yahoo! Finance... He knows what big numbers and performance are about!

Hope this helps!