Database/ resource caching, how to avoid merge conflicts

So i'm making a 911 dispatch system of sorts for my security company. My officers which are sometimes in building do not have constant wifi/ cell network access.

I'm developing an application where officers can create "activity" entries. These "activity" entries need to be read and potentially modified from anyone else using the system. For example, a dispatcher and group of officers may need to update info, as the situation changes.

Problem: If an officer updates an existing record, and has no internet access; when they obtain internet access the program will automatically commit new or updated entries to the main database. What's preventing that officer from overwriting someone else's changes. In an ideal world, all changes should automatically merge.

How does source control detect and autoresolve merge conflicts among many changes. And how can I implement that in my program.

By main database (located on the net) is using MySQL. I was going to develop an offline cache database, which runs local on the android phone, until it has net access. I will likely be using SQLite.