I have 2 simple tables -- Table1 and Table2

Both tables have identical columns -- id and value

Question:

if I query (SELECT * FROM Table1) UNION ALL (SELECT * FROM Table2) ORDER BY id

based on my query above, what order do the rows appear in result when there are duplicate id's in both tables?

will the row(s) in Table1 always and consistently appear first before Table2 or can the resulting order not be predicted.

The reason I ask is I truly need order because the value from table 1 is to serve as a default and the value from table2 (if present) is to serve as an override.

[I know I can add another column for sort order to ensure the result I am looking for but I am curious if there is a consistent way the MySQL engine handles the UNION ALL and ORDER BY when there are duplicate rows.

Thanks for the feedback.