MySQL union query, order by 2 variables

Question

Each table (table1 & table2) has its own DATETIME field.
I\'m trying to catch id\'s of the two tables and order them by their DATETIME field.

Example:

Answer 1

I believe you are over-complicating a rather straight-forward task:

SELECT *
FROM (
    SELECT 1 AS selection, table1.id as id, table1.datetime1 as date FROM Table1 
    UNION ALL
    SELECT 2 AS selection, table2.id as id, table2.datetime2 as date FROM Table2 
) AS query
ORDER BY date DESC