Issue
Multiple Photos Libraries exist and you want to merge those libraries into one.
Environment
This article applies to users with Photos 4.0 on macOS 10.14.4 or older.
Cause
Multiple Photos Libraries have been created and are located on a single Mac. Instead of using the 'Option'-key at Photos starting up to select the specific Photos Library to access, you want to have all Libraries merged into one.
Resolution
Work through the steps at the Apple Communities article
Notes on Merging Photos Libraries, 2019 Version
https://discussions.apple.com/docs/DOC-250000662
:
See Related Articles to the right for more information.