如何组合两行并计算MySQL中两个时间戳值之间的时差？

I have a situation that I'm sure is quite common and it's really bothering me that I can't figure out how to do it or what to search for to find a relevant example/solution. I'm relatively new to MySQL (have been using MSSQL and PostgreSQL earlier) and every approach I can think of is blocked by some feature lacking in MySQL.I have a situation that I'm sure is quite commo