How to get max date from two columns, or null if either column is null?

Problem :

I have two date columns in a MySQL table representing tasks that are worked on by two users. The date columns record when user 1 agreed to the task and when user 2 agreed to the task, and can be NULL if that user hasn't agreed to the task yet.

I want to generate the date task agreed - which should be NULL unless both users have agreed to the task, or the maximum of the two dates if they've both agreed.

So I want a query that produces:

user_1_agreed  user_2_agreed  query_result
NULL           NULL           NULL
2014-01-02     NULL           NULL
NULL           2014-03-04     NULL
2014-01-02     2014-03-04     2014-03-04

I'm aware of MAX, COALESCE and IF, but I'm having trouble trying to work out how to combine them to give the above.

Solution :

The greatest function fits your bill perfectly:

SELECT GREATEST(user_1_agreed, user_2_agreed)
FROM   my_table

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