The Join in IDEA makes for a convenient look up between two files in a defined project folder within IDEA.
Through the Join, users can identify matches or unreconciled items / mismatches.
Take an example - you have been provided with two files - one - employee attendance and two - employee system access log.
You take the employee attendance file as the primary file and the employee system access log file as the secondary file.
Now your requirement is to identify employees who were present on a given date but did not access the system - sort of like disguised unemployment.
By matching the two files based on the Date and Employee ID, you can check the option 'Records with No Secondary Match' to identify employees who attended work (in primary file) but did not access the system (not in secondary file).
On the flip side - a grave red flag - employees who have accessed the system but were not present at work on a given date.
By matching the very same two fields, you can check the option 'Records with No Primary Match' to identify employees who have accessed the system (in secondary file) but who have not attended work on the given date (not in primary file).
So the two options 'Records with No Secondary Match' and 'Records with No Primary Match' are instrumental in helping you identify mismatches in two databases.