Problems with Postgresql CASE syntax - sql

Question

Here is my SQL query:
SELECT (CASE (elapsed_time_from_first_login IS NULL)
WHEN true THEN 0
ELSE elapsed_time_from_first_login END)
FROM (
SELECT (now()::ABSTIME::INT4 - min(AcctStartTime)::ABSTIME::INT4)
FROM radacct
WHERE UserName = 'test156') AS elapsed_time_from_first_login;
When I execute the above query, I get this error:
ERROR: CASE types record and integer cannot be matched
From the error message I understand that PostgreSQL take the second select, respectively elapsed_time_from_first_login as a row, even if it will always be a single value (because of the min() function).
Question: do you have some suggestions on how to deal with this query?

Answer 1

I suppose, what you are actually trying to do should look like this:
SELECT COALESCE((SELECT now() - min(acct_start_time)
FROM radacct
WHERE user_name = 'test156')
, interval '0s')
While there is an aggregate function in the top SELECT list of the subselect, it cannot return "no row". The aggregate function min() converts "no row" to NULL, and the simple form below also does the trick.
db<>fiddle here
Oldsqlfiddle
Other problems with your query have already been pointed out. But this is the much simpler solution. It returns an interval rather than an integer.
Convert to integer
Simplified with input from artaxerxe.
Simple form does the job without check for "no row":
SELECT COALESCE(EXTRACT(epoch FROM now() - min(acct_start_time))::int, 0)
FROM radacct
WHERE user_name = 'test156';
Details about EXTRACT(epoch FROM INTERVAL) in the manual.
Aggregate functions and NULL
If you had used the aggregate function count() instead of sum() as you had initially, the outcome would be different. count() is a special case among standard aggregate functions in that it never returns NULL. If no value (or row) is found, it returns 0 instead.
The manual on aggregate functions:
It should be noted that except for count, these functions return a
null value when no rows are selected. In particular, sum of no rows
returns null, not zero as one might expect, and array_agg returns
null rather than an empty array when there are no input rows. The
coalesce function can be used to substitute zero or an empty array for
null when necessary.

Answer 2

Postgres is complaining that 0 and elapsed_time_from_first_login are not the same type.
Try this (also simplifying your select):
select
coalesce(elapsed_time_from_first_login::INT4, 0)
from ...

Answer 3

Here is how I formatted the SQL and now is working:
SELECT coalesce(result, 0)
FROM (SELECT (now()::ABSTIME::INT4 - min(AcctStartTime)::ABSTIME::INT4) as result
FROM radacct WHERE UserName = 'test156') as elapsed_time_from_first_login;

Answer 4

The second SELECT is returning a table, named elapsed_time_from_first_login with one column and one row. You have to alias that column and use it in the CASE clause. You can't put a whole table (even if it is one column, one row only) where a value is expected.
SELECT (CASE (elapsed_time IS NULL)
WHEN true THEN 0
ELSE elapsed_time end)
FROM (SELECT (now()::ABSTIME::INT4 - min(AcctStartTime)::ABSTIME::INT4)
AS elapsed_time -- column alias
FROM radacct
WHERE UserName = 'test156'
) as elapsed_time_from_first_login; -- table alias
and you can shorten the CASE by using the COALESCE() function (and optionally add an alias for that column to be shown in the results):
SELECT COALESCE(elapsed_time, 0)
AS elapsed_time
FROM (SELECT (now()::ABSTIME::INT4 - min(AcctStartTime)::ABSTIME::INT4)
AS elapsed_time
FROM radacct
WHERE UserName = 'test156'
) as elapsed_time_from_first_login; -- table alias