Register form - autocomplete select options from a table/column in the database
This may be a standard select attribute, or possibly a custom attribute you will have to code. All depends on your data.
For this kind of attribute I usually inherit from a text or numeric attribute - depends on whether I want to store a database id or a label for the selected value. The attribute form would then populate a select input from your database table.
That discusses developing a complete attribute type from scratch. You can likely start by inheriting/extending an existing attribute such as text or number. Doing so is usually a lot easier because most of the difficult work is inherited and you don't need to provide a value entity.
Also: is this what you are referring to?
Think of what you need for data:
- list of options - comes from your table, presented in a select dropdown
- storage for what is selected - a string or id
You don't need all the Select attribute complexity for storing lists of options in secondary tables.
You need to describe your data and overall problem in more detail.