You can follow the below steps if you have a requirement of creating a Site Collection in to a Specific Database when you have multiple Content Db for a Particular web application .
- Under the Application Management section access the “Content Databases” link.
- In the upper right hand corner make sure you are in the correct Web Application. If you aren’t then change to the correct one.
- Click the link for each Content Database you have listed under the “Database Name” heading.
- When the “Manage Content Database Settings” page opens set the “Database Status” to “Offline” and then click “OK”. After you click ok you’ll be taken back to the “Manage Content Databases page where the database you just changed should now show as “Stopped”.
- Click the “Add a Content Database” link and create a new content database. Do not make any changes to the “Database Status” setting. It will show as “Started” in the list of Content Databases. You will also notice that the “Current Number of Sites” will be equal to 0.
- Navigate to the SharePoint Site Management section and create your new site collection.
- Go back to the Manage Content Databases list and you will now see that the Content Database you just created now shows that the Current Number of Sites is equal to 1.
You have just created a new site collection in the content database of your choice.