wordpress database relations and point ways

i'm trying to create an application for my wordpress website. and to do this i have to know how wordpress database is working.. i mean how the tables are pointing to each other!?
i want to fetch some data like my post titles, descriptions, their feature images and their categories..
according to the mess database i don't know how the post titles and descriptions are in relation with their categories! also i couldn't find my solution in wordpress database flowcharts.....

Accept my preThanks for helping me!!