Select from many rows where is value or is another value based on a column

Question

In a table like this:

id_fk   | valid | val
----------------------
1       | 0     | val1A
1       | 1     | val1B
2       | 0     | val2A
3       | 1     | val3A
3       | 0     | val3B

How dould I select those that are valid=1, or if a valid one does not exist for a id_fk then one that is valid=0?

Basically from this table

id_fk   | valid | val
----------------------
1       | 0     | val1A
1       | 1     | val1B
2       | 0     | val2A
3       | 1     | val3A
3       | 0     | val3B

I want to select these (distinct id_fk per row):

id_fk   | valid | val
----------------------
1       | 1     | val1B
2       | 0     | val2A
3       | 1     | val3A

Answer 1

Exactly as you describe it:

SELECT *
FROM myTable a
WHERE valid = 1
OR NOT EXISTS
(
    SELECT 1
    FROM myTable b
    WHERE b.id_fk = a.id_fk
    AND b.valid = 1
)

Answer 2

Assuming your valid columns having 1 and 0 only :

select * from (SELECT *
FROM Table1 a order by id_fk asc , valid desc) abc
group by id_fk;

fiddle