- An inner join will only select records where the joined keys are in both specified tables.
- A left outer join will select all records from the first table, and any records in the second table that match the joined keys.
- A right outer join will select all records from the second table, and any records in the first table that match the joined keys.
That's something I fished out from someone else, but I know is correct from using it in recent lectures!
Unfortunately I haven't came across the 'UNION' term myself yet (I'm still learning!).
Perhaps this webpage can help:
SQL UNION Operator
Hope this helps to answer your question!