CASE query in postgres does not return expected output - postgresql

Question

In my Postgres database, I am trying to execute the below query:
with
user_details as
(
SELECT username
FROM user_management WHERE username = 'admin'
)
select (case
when user_details.username is NOT NULL then 'user found'
else
'no user found' end) as username from user_details
The above query will return the username and works as expected. But if I pass the username that does not exist in the database, then I expect the CASE query to return 'no user found' but the query does not return anything at all.
But when I tried to execute the below query:
select (case
when (SELECT username
FROM user_management WHERE username = 'sadmin') is not null then 'user found'
else
'no user found' end) as username
It works as expected and returns the 'no user found'.
Why didn't I get the expected output when using the WITH query in the first case? Is there something with the WITH queries that I am missing?

Answer 1

When you're selecting the output from user_details , you're getting 0 rows , that is the reason you're not seeing any o/p in final result. And in the 2nd query , you're selecting the o/p from sub-query which is null then producing user not found. You might run into an error like this
ERROR: more than one row returned by a subquery used as an expression .
When there is more user having the same username. So your query should look like this
select (case
when exists(SELECT username
FROM user_management WHERE username = 'sadmin') then 'user found'
else
'no user found' end) as username

Answer 2

I assume that the column username is unique in the table user_management, so the CTE returns either 1 row when the username exists in the table or nothing if it does not exist.
Maybe you think that the CTE returns NULL in the case where the username does not exist in the table, but it is not.
So if the CTE returns 1 row (the username exists) then your code works fine, but if it returns nothing then your SELECT statement that selects from user_details will also return nothing, because there are no rows to select from.
Use aggregation so that you will get 1 row in the results even if the user does not exist:
SELECT COALESCE(MAX(username), 'no user found') username
FROM user_management
WHERE username = 'admin'
See the demo.