How can I combine two tables and keep their original primary keys?

It sounds like you're mixing concepts here. You'd typically use a JOIN to get both IDs side by side, not a UNION. With UNION, you wouldn't have both IDs; instead, you'd see all the rows combined but only one ID from one table. If you want to keep both IDs to track their origins, a JOIN is the way to go.

Let’s clarify a bit. If you want to use UNION, your query might look like this: "SELECT ID AS OLD_ID, field2, field3 FROM Table1 UNION SELECT ID AS OLD_ID, field2, field3 FROM Table2." But remember, a UNION won’t create a direct link between the two different IDs unless you manage that linkage outside the query.

I totally agree! A UNION combines the rows of your tables. So if your first table has 10 rows and the second also has 10, you’ll end up with 20 rows. In contrast, a JOIN works to combine columns, giving you paired data based on matching keys. Your method of combining them depends on how related the data is!

Just to add, if you really want both IDs in the new table, you'd need to perform some sort of join operation. The way you’d do it is something like "SELECT table1.id AS table1_id, table2.id AS table2_id FROM table1 JOIN table2 ON [matching condition]." If the IDs are auto-incrementing, you don’t need to manually set them; the database will handle it for you during the insert.