order result of query by on of two columns

I have this table:

CREATE TABLE Foo
(
	id_key INTEGER PRIMARY KEY,
	key1 INTEGER NOT NULL,
    seq_num1 INTEGER NOT NULL,
    key2 INTEGER,
    seq_num2 INTEGER,
    data BLOB NOT NULL
);

And I want to select data where key1 or key2 match,
this is simple:

SELECT data 
FROM Foo 
WHERE (key1 = ? OR key2 = ?)

but then I want to order results of this query by seq_num1 or seq_num2, depending on what matched in the table's row - key1 or key2 respectively.

I think that something like this should work:

SELECT data 
FROM Foo 
WHERE (key1 = ? OR key2 = ?) 
ORDER BY 
    CASE 
        WHEN key1 = ? 
            THEN seq_num1 
            ELSE seq_num2 
    END

but I'm not sure is this optimal solution? Because of in this query I compare key1 twice, while it should be enough compare once.

I use sqlite3.

Could this be what you want? I don't use sqlite, but this mysql command seems to be understood by sqlite.

select
  data, seq_num1 as seq
from Foo
  where key1 = ?
union
select
  data, seq_num2 as seq
from Foo
  where key2 = ?
order by seq;