在 SQL select 语句中未找到返回默认值

Question

I need to return default value in Select statement when WHERE clause is not met. So basically, this query returns valid guid:

SELECT COALESCE(w.guid , gen_random_uuid()) as guid
        FROM MyTable w
        WHERE w.id = 108
        AND w.company IN (27);

But this doesn't find id 109, so it should generate new random uuid:

SELECT COALESCE(w.guid , gen_random_uuid()) as guid
        FROM MyTable w
        WHERE w.id = 109
        AND w.company IN (27);

But instead it returns nothing. How I can solve this?

Answer 1

You get no uuid because no row is selected and nothing returned therefore. This might solve your problem:

  SELECT coalesce(
    (select w.guid FROM MyTable w
        WHERE w.id = 109 AND w.company IN (27)
    ), 
    gen_random_uuid()) as guid;

Didn't test it on real data, but you get the idea i'm sure.

Answer 2

Use a RIGHT JOIN on a sub select that creates the default value. Something like this (not tested, no data available):

SELECT 
    COALESCE( w.guid, G.guid ) AS guid 
FROM
    genius."attribute" w
    RIGHT JOIN ( SELECT gen_random_uuid()) G ( guid ) ON w.ID = 109 AND w.company IN ( 27 );