A. Utilize PowerApps to do this by creating a custom form, and pulling the choices from a sql table. This requires PowerApps full licenses since SQL is a premium connector. B. Setup Business Data Connectivity to connect to the sql source, which will allow you to pick data from defined table/view you connect to. See: https://docs.microsoft.com/en-us/sharepoint/manage-business-connectivity-service-applications
From a SharePoint pure point of view, BCS is the answer to this question...but: (1) It requires the SQL Server you need to connect is accesible on the Internet (of course, SQL Azure is OK for that scenario) (2) Microsoft has not invested in BCS for years so I would think carefully if BCS is your option here