Be sure to use a text data type in your data base for the fields containing task IDs like 1.1.1.2, to avoid any attempts by MySQL to "fix" what it might think are numbers.
I am guessing that this is a project control app, and the tasks are identified by those IDs 1.1.2.1... to put them into a tree hierarchy, so you could take a child task like 1.1.2.1 and identify its parent task by splitting its ID at '.', cutting off the last number, and reassembling the parts with a JOIN USING Separator '.' block.
This is beyond the capabilities of SQL, so you might consider putting the parent ID as an additional field in each tasks' table row.
If you have a task with a projected completion date that you need to push back because of a delay, you may need to push back the estimated completion dates of its parent projects, right? (Unless the parent completion date has slack in it, and would not be affected?)
If your data base does not return immediate results, (like most don't), you would need to keep track of what you are doing somewhere outside that database so that when the database returns rows you know what to do with them.