Question

I have two tables which both have the column DateTime.

How can I get the MAX() DateTime?

The shorter/simpler the better, because this is just part of a larger query.

Answer 1

You could use the GREATEST function:

SELECT GREATEST((SELECT MAX(column) 
                   FROM TABLE_1),
                (SELECT MAX(column) 
                   FROM TABLE_2))

Using UNIONs:

SELECT MAX(col)
  FROM (SELECT col FROM TABLE_1
        UNION ALL
        SELECT col FROM TABLE_2)

Use UNION ALL for this – it’s faster because it doesn’t remove duplicates, and it doesn’t matter if duplicates are returned by the subquery in this example.