Please note that selection of the database purely depends on your application requirements. You need to consider the following parameters:
1) How many transactions are you willing to process per day?
a] 10 to 1000 transactions
b] 1,000 to 10,000 transactions
c] 10,000 to 100,000 transactions
d] 100,000 + transactions
2) Do you have any choice of the scripting language or operating system?
a] Windows O/S and ASP.NET Scripting
b] Windows O/S and PHP Scripting
c] Linux O/S and PHP Scripting
d] Linux O/S and Perl Scripting
3) Your budget for this application for the hosting?
a] 0 - $4 /month
b] $4 - $10 /month
c] $10 - $30 /month
d] $30+ /month
Please answer the above queries and we can guide you further on choice of the database.