What and where to read about data storage?

Question

Database

Hello!
I have quite a few questions.
1) storage of letters.
Letters outgoing and incoming, it is necessary to display their sequence and relationships with each other. The relationship is parent-child, as I understand it? What does it look like in the database?
2) file storage.
For quick access, the database stores paths to files, their names, type. But sometimes it is necessary to extract one type from an ordinary array and display it. I have something like: type == 'Tech. task', for catching the desired document from the array. Is this correct?
Where can I read about the correct operation and device of the database?
Thanks in advance!

Answer 1