This article applies to 123-reg Shared Hosting accounts.
You can add a database to your 123-reg hosting package through your 123-reg control panel:

2
Scroll down to the Web hosting section.
3
Select the relevant domain name from the drop down box and click on the Manage button.
4
Click on the Manage database option.
5
Click on the Add database button.
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You will then be presented with your database username and password.