That title might be a bit confusing but hopefully this clears it up. I currently have a timetable on my website that takes data from a mysql table and puts it into a simple HTML table on my website. Now to make it more user friendly, I wanted to add in a search box so the user can search for certain things like all flights leaving Boston. Even more in depth, I also have fields for arrival cities and flight numbers. This equates to something like this...
SELECT * FROM timetable WHERE $dep = ['dep'] AND $arr = ['arr'] AND $flighnum = ['flightnum']
Now my question is how can i make a variable like $dep a neutral value but keep all the other fields intact. So for example, a user plugs in they want all flights arriving in New York but they don't care what the departure airport or flight number is so they leave it blank in the search form. In other terms, is there a value that can be made as an option in a drop down box that selects all of one column in a mySQL column?