Hi @Mark Powell,
It's a pleasure to help. Please don't worry, this is a very reasonable question, especially when you're just getting started with Access. I'd be happy to share what I know in case it helps point you in the right direction.
Since both tables share a common ID field, the easiest way in Microsoft Access is to use the Find Unmatched Query Wizard, which is specifically designed to find records in one table that do not have matching records in another table.
For your scenario, you can try the following:
- Go to Create > Query Wizard.

- Select Find Unmatched Query Wizard.

- Choose Lead as the table containing the records you want returned.
- Choose Activity as the table containing the related records.
- Select the common ID field to match the tables.
- Choose any Lead fields you want displayed in the results.
- Finish the wizard.
The resulting query will show all Lead records that do not have a corresponding Activity record.
For more detail, please refer: Compare two tables and find records without matches | Microsoft Support
I hope this helps. If anything is unclear, or if you run into any difficulties while setting up the query, feel free to reply to me, I'll be happy to share any additional thoughts that might help.
Best of luck with your Access project!
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