title: how to subtract a value in a column when user enter a no of seats
body: <p>I am using php and mysql, My table like this </p>
  
  <p><img src="https://i.stack.imgur.com/rRghf.jpg" alt="enter image description here"></p>
  
  <p>if a user enter a time 09:00 and reserve 2 seats and 
  if a user enter a time 10:00 and reserve 1 seat so how to subtract value in seats available </p>
  
  <p><img src="https://i.stack.imgur.com/MuXnS.jpg" alt="enter image description here"></p>
  
  <p>like this</p>