I have a built in form I created on my WP website .
I want to store the inputs to a table on my DB. 
All tutorials that explane how to create a talke include the fact that I have to create a plugin for that. 

All my functions are written in functions.php and I prefer to write the function that creates the DB there too. 

If I write the function that creates the table in functions.php - will it make multiple tables or something like that? or does it work only once?