Postgres - using select statement inside CASE WHEN - postgresql

Question

Inside case when condition I am executing select statement & if returns anything then I need to get it's value but I am getting error
ERROR: missing FROM-clause entry for table "us"
query is..
SELECT u.user_id,
CASE
WHEN
(SELECT us.attr_value
FROM user_setting us
WHERE us.user_id = u.user_id) IS NOT NULL THEN us.attr_value
ELSE
(SELECT gus.attr_value
FROM global_user_setting gus
WHERE gus.attr_key='key')
END
FROM user u
WHERE u.user_id IN (1,
2,3)
Error comes at IS NOT NULL THEN us.attr_value I understood the issue but couldn't find how to get that value outside select statement?

Answer 1

Try:
COALESCE((SELECT us.attr_value
FROM user_setting us
WHERE us.user_id = u.user_id),
(SELECT us.attr_value
FROM global_user_setting gs
WHERE gus.attr_key='key'))
instead. The reason for the problem is, that the binding of the es alias is not visible outside of the sub-select (as it is used in a "scalar" context). The whole subselect is basically a single expression, which will yield a single value.
Another (IMHO better) approach would be to left-join on the enrollment_settings table:
SELECT u.user_id,
COALESCE(us.attr_value, (SELECT gus.attr_value
FROM global_user_setting gs
WHERE gus.attr_key='key'))
FROM user u LEFT JOIN user_settings es ON us.user_id = u.user_id
WHERE u.user_id IN (1, 2, 3)
I assume here, that this join would yield at most a single row per row of user.