====== concatenating null strings ======
In Postgres, when using the || operator to concatenate fields into one string, if any of the fields contain a null value then the result will entirely be a null value.  To avoid this problem, we can use the coalesce function:
<code sql>
select 'Doe' || ', ' || null || 'John' as name
-- result will be null

select coalesce('Doe','') || coalesce(', ','') || coalesce(null,'') || coalesce('John','') as name
-- result will be 'Doe, John'
</code>