Re: How do I check for NULL

On 12/9/25 18:29, David G. Johnston wrote:
> On Tue, Dec 9, 2025 at 10:14 AM Thiemo Kellner 
> <thiemo@gelassene-pferde.biz> wrote:
>
>     I feel, you meant to say, the subquery does not return any record
>     which is not the same as returns NULL.
>
>
> For a scalar subquery the final output of a zero-row query is the null 
> value.
>
To me, it does not look like that (please note the empty line in the 
last example). Can you point me to the documentation saying that 0 rows 
is sometimes equal to 1 row?

postgres=# select * from pg_user;
  usename  | usesysid | usecreatedb | usesuper | userepl | usebypassrls 
|  passwd  | valuntil | useconfig
----------+----------+-------------+----------+---------+--------------+----------+----------+----------- 

  postgres |       10 | t           | t        | t       | t   | 
******** |          |
(1 row)

postgres=# select usename from pg_user where false;
  usename
---------
(0 rows)

postgres=# select null as usename from pg_user;
  usename
---------

(1 row)