MySQL check two columns for value but with a preferred column result

I have a MariaDB SQL table, with two different ID rows.

Id's have been imported from a previous DB version (old_id reference is from another table), as a transitional measure searches need to be done to find id's, with a preference for the older id value. Only ONE row can ever be returned .

Table:

new_id(PK)  |  old_id  |   data   |   more_data 
---------------------------------------------
   1        |    34    |   harge  |     spalt
   2        |     7    |   greet  |    sausages
   4        |   852    |  humbug  |    gertrude
   6        |    13    |  bloody  |   festivalz
   7        |   412    |   hello  |   fiddlests
   8        |     3    |  fraggo  |  piddlebiscs

new_id is the Primary Key.

So:

When the page is loaded with ID=852 it needs to return row 4
When the page is loaded with ID=7 it needs to return row 2
When the page is loaded with ID=8 it returns row 8 (because 8 does not exist in the old_id column)
When the page is called with ID=5 it returns nothing (no match in either column)

What have I tried:

I have tried various ways of qualifying this but can't find the right syntax:

(first attempt is silly)

Attempted:

WHERE 
    table.old_id = :id 
    OR (table.new_id = :id AND table.old_id != :id) #bad one. 


WHERE 
    table.old_id = :id 
    OR (table.new_id = :id AND :id NOT IN (SELECT old_id FROM table)) 

WHERE 
    table.old_id = :id 
    OR (table.new_id = :id AND table.new_id NOT IN (SELECT old_id FROM table)) 
-- I think equivalent to the above 

WHERE CASE WHEN table.old_id = :id THEN true ELSE table.new_id = :id END

WHERE IF(table.old_id = :id, true, table.new_id = :id) 
-- I think equivalent to the above

My Issue:

When an ID is found the SQL returns one row when it is found in the new_id only but otherwise it return both rows each time, when it should stop after a successful find in the old_id.

What am I missing; how can I get the SQL to check the old_id column and only if not found, then check the new_id column and only ever return one result?

What I've checked

MySQL get rows but prefer one column value over another and
using CASE in the WHERE clause
and various others that don't fit the concept; there are two successful matches, but the matches should be proritised by column.

Solution

Assuming that your query should always return just one record (which is how I understand your question), you can do condition ordering and limit 1:

select *
from mytable 
where :id in (old_id, new_id)
order by case when old_id = :id then 0 else 1 end
limit 1

If two records match, the conditional order by clause puts the record that matched on old_id on top. Then limit 1 eliminates the other match. If only one record matches, ordering does not matter, and it is retained.