案例表达式不返回Postgres查询中的“ else”值

Question

I'm creating a SQL query with a lot of nested queries and I'm trying to use the CASE expression but it is behaving weirdly.

This is my query at the moment:

select t.fpl_id, t.team_name,
sum(pf.points)as gwpts,
(
    select sum(transfers_malus)
    from gameweeks
    where gameweeks.team_id = t.id and gameweeks.number = g.number
)as malus,
(
    select sum(points)
    from player_fixtures as pfix
    where gw_number = g.number and pfix.player_id = CASE WHEN minutes_played > 0 THEN g.captain_id ELSE g.vice_captain_id END
)
as cpts,
 (
    select max(web_name)
    from players
    join player_fixtures on players.id = player_fixtures.player_id and gw_number = g.number
    where players.id = CASE WHEN player_fixtures.minutes_played > 0 THEN g.captain_id ELSE g.vice_captain_id END
) as captain_name,
array_agg(p.id) as lineup
from teams as t
join gameweeks as g on t.id  = g.team_id
join gameweeks_players as gp on gp.gameweek_id = g.id
join players as p on gp.player_id = p.id
join player_fixtures as pf on p.id = pf.player_id  and pf.gw_number = g.number
where t.id = 1
group by t.id, g.id
order by g.number asc

The bit where I have a problem is when I'm doing the case statement to see if one of the player ( the captain ) hasn't played based on "minutes_played":

(select sum(points)
    from player_fixtures as pfix
    where gw_number = g.number and pfix.player_id = CASE WHEN minutes_played > 0 THEN g.captain_id ELSE g.vice_captain_id END
) as cpts

and

 (
    select max(web_name)
    from players
    join player_fixtures on players.id = player_fixtures.player_id and gw_number = g.number
    where players.id = CASE WHEN player_fixtures.minutes_played > 0 THEN g.captain_id ELSE g.vice_captain_id END
) as captain_name,

When minutes_played is indeed greater than 0, everything works fine. However, when it is equal to 0 it just doesn't "return" anything and I get a NULL in return in my row. minutes_played is an integer and when I try with the following script on the player_fixtures where minutes_played is = 0 my > evaluation works and I get the correct "BAD".

DO LANGUAGE plpgsql $$
    BEGIN
        IF (select minutes_played from player_fixtures where gw_number = 19 and player_id = 266 ) > 0 THEN 
            RAISE NOTICE 'GOOD';
        ELSE
            RAISE NOTICE 'BAD';
        END IF;
    END;
$$;

I'm pretty new at this whole DB thing so I'm probably making a rookie mistake but I've been trying for the past 6hours without any luck. Could someone point me in the right direction?

Answer 1

The problem is that when there are no rows that match, sum() returns a null, and no comparison with null is ever true.

Instead, use exists() , for example:

where exists(select * from player_fixtures
    where gw_number = 19
    and player_id = 266 
    and minutes_played > 0)

or you can use if exists(...) then in a stored proc.