users preferences. refer to https://towardsdatascience.com/3-approaches-to-build-a-recommendation-system-ce6a7a404576
mySQL : https://blogs.oracle.com/mysql/post/building-your-first-machine-learning-model-with-mysql-heatwave-ml
frequency of data engine. (training with new data/ recommendation). == Leave it to next level.
1.training set 2.test set 3. Evaluate
2.Relationship
Source : official / **Customized**
Connection Type : 1. main - main 2.main - Sub (Simple similar/Hard similar/Opposite)
Adopted : Check/Uncheck (like it or not)