COALESCE function

[rausnitz]

It would be great if this package could handle queries like:

SELECT COALESCE(SUM(likes), 0) FROM posts WHERE likes > 500;

A recent change to Fluent allows a default value when a SUM() query would result in null: vapor/fluent#573. However, the change requires every SUM() query to be preceded by a separate COUNT() query. A more efficient and reliable solution would be to use COALESCE().

In the example of Fluent's sum() function, you could still pass a nil value for the default, and then the COALESCE() function would result in null if the SUM() is also null.