SELECT one column if the other is null

Question

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:

Answer 1

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.

Answer 2

Check out the COALESCE function.

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

Answer 3

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

Answer 4

Use a CASE statement for the select.

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