PostgreSQL - How to replace empty field with any string?

Question

I'm trying to replace some empty (NULL) fields, which I get as a result of my query, with any string I want. Those empty fields are placed in a "timestamp without timezone" column. So I tried to use COALESCE function, but no result (I got error: invalid input syntax for timestamp: "any_string":

select column1, coalesce(date_trunc('seconds', min(date)), 'any_string') as column2

What could be wrong?

Table:

╔════╦═════════════════════╦═════════════════════╗
║ id ║        date         ║        date2        ║
╠════╬═════════════════════╬═════════════════════╣
║  1 ║ 2013-12-17 13:54:59 ║ 2013-12-17 09:03:31 ║
║  2 ║ 2013-12-17 13:55:07 ║ 2013-12-17 09:59:11 ║
║  3 ║ 2013-12-17 13:55:56 ║ empty field         ║
║  4 ║ 2013-12-17 13:38:37 ║ 2013-12-17 09:14:01 ║
║  5 ║ 2013-12-17 13:54:46 ║ empty field         ║
║  6 ║ 2013-12-17 13:54:46 ║ empty field         ║
║  7 ║ 2013-12-17 13:55:40 ║ empty field         ║
╚════╩═════════════════════╩═════════════════════╝

Sample query:

select q1.id, q2.date, q3.date2
from (select distinct id from table1) q1
left join (select id, date_trunc('seconds', max(time)) as date from table2 where time::date = now()::date group by id) q2 on q1.id = q2.id
left join (select id, date_trunc('seconds', min(time2)) as date2 from table1 where time2:date = now()::date group by id) q3 on q1.id = q3.id
order by 1

And the matter is to replace those empty field above with any string I imagine.

Answer 1

You can simply cast timestamp to text using ::text

select column1, coalesce(date_trunc('seconds', min(date))::text, 'any_string') as column2

Answer 2

The date_trunc() function returns a timestamp, thus you cannot fit a string like any_string in the same column.

You'll have to pick a format and convert the resulting date to string, though of course it'll no longer be usable as date.