SELECT one column if the other is null

I want to select a2.date if it's there, but if it's NULL I want to select a1.date (a2 is being left-joined). This:

SELECT a2.date OR a1.date
       ...

Simply returns a boolean result (as one would expect), how do I get the actual value of the non-null column though? (a2.date is preferred, but if it's null then a1.date)

The ANSI means is to use COALESCE:

SELECT COALESCE(a2.date, a1.date) AS `date`
   ...

The MySQL native syntax is IFNULL:

SELECT IFNULL(a2.date, a1.date) AS `date`
   ...

Unlike COALESCE, IFNULL is not portable to other databases.

Another ANSI syntax, the CASE expression, is an option:

SELECT CASE
         WHEN a2.date IS NULL THEN a1.date
         ELSE a2.date
       END AS `date`
   ...

It requires more direction to work properly, but is more flexible if requirements change.

Use a CASE statement for the select.

SELECT CASE WHEN a2.date IS NULL THEN a1.date
    ELSE a2.date END AS mydate

Check out the COALESCE function.

Takes a variable number of arguments and returns the first non-null one. It works fine with joins.

SELECT COALESCE(a2.date, a1.date) ...