MySQL: get MAX or GREATEST of several columns, but with NULL fields

Question

I\'m trying to select the max date in three different fields in each record (MySQL) So, in each row, I have date1, date2 and date3: date1 is always filled, date2 and date3 c

Answer 1

If date1 can never be NULL, then the result should never be NULL, right? Then you could use this, if you want NULL dates be not counted in the calculations (or change the 1000-01-01 to 9999-12-31, if you want Nulls to count as the "end of time"):

GREATEST( date1
        , COALESCE(date2, '1000-01-01')
        , COALESCE(date3, '1000-01-01')
        ) AS datemax

Answer 2

COALESCE your date columns before you use them in GREATEST.

The way you handle them will depend on how you want to deal with NULL.. either high or low?

Answer 3

buuut, if all dates happen to be null? you still want to have null as an output, right? then you need this

select nullif(greatest(coalesce(<DATEA>, from_unixtime(0)), coalesce(<DATEB>, from_unixtime(0))), from_unixtime(0));

Now, if both are null you get null, if one of them is not null of both of them are not null, you get the greatest.

This is crazy, especially if you will use it multiple times, for this then you might want to create it as a function, like this:

delimiter //
drop function if exists cp_greatest_date//
create function cp_greatest_date ( dateA timestamp, dateB timestamp ) returns timestamp
  deterministic reads sql data
  begin

    # if both are null you get null, if one of them is not null of both of them are not null, you get the greatest
    set @output = nullif(greatest(coalesce(dateA, from_unixtime(0)), coalesce(dateB, from_unixtime(0))), from_unixtime(0));
    # santiago arizti

    return @output;
  end //
delimiter ;

Then you can use it like this

select cp_greatest_date(current_timestamp, null);
-- output is 2017-05-05 20:22:45

Answer 4

Use COALESCE

SELECT id, 
   GREATEST(date1, 
     COALESCE(date2, 0),
     COALESCE(date3, 0)) as datemax 
FROM mytable

Update: This answer previously used IFNULL which does work, but as Mike Chamberlain pointed out in the comments, COALESCE is actually the preferred method.