No problems. I'm happy it worked.
It was probaly very slow, because as you had missed the outer join condition, so it was matching every rows of each table to every rows of the outer join table.

It's called an cartesian product, or cross join.
You just usually don't want to see them, except on particular and very specific occasions.

This is a topic I thought I had sorted out but now looking at the queries I made to Insert the data entered by the user - then to get the newly ID inserted ID for the data - and then to insert the ID into the mapping table

I used three queries, but I feel the query that gets the newly inserted data isn't reliable and there is probably a better way of doing it.

At the moment it finds the newly inserted 'action' by finding the date, dueDate, and the notes fields with the data passed by $_POST but if there is a record created on the same day with exactly the same notes it might SELECT the other record

Would I need to create an extra ID? Or a temporary ID that is used just for inserting the data into the mapping table?

I found out that you can use a timestamp to find the newly inserted record which is more reliable than finding it using other data that could be the same for other records