What does it mean to have jobs with a null stop date?

Question

I had used the following SQL to list out all jobs without a stop date. I thought that I could use this to find all active jobs. What I noticed is that I have a number of job

Answer 1

Each time the SQL Agent starts, it puts a new row in syssessions and subsequently any jobs run will get that session_id in sysjobactivity. For your jobs that have a null stop date, my guess is that they're not for the "current" session which would mean that they were still running when the agent was stopped.