案例表达式在 Postgres 查询中不返回“else”值答案

【问题标题】：Case expression does not return the "else" value in Postgres query案例表达式在 Postgres 查询中不返回“else”值
【发布时间】：2014-01-02 23:31:06
【问题描述】：

我正在创建一个包含大量嵌套查询的 SQL 查询，我正在尝试使用 CASE 表达式，但它的行为很奇怪。

这是我目前的查询：

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

我遇到的问题是当我根据“minutes_played”进行案例陈述以查看其中一名球员（队长）是否没有上场时：

(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

和

 (
    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,

当 minutes_played 确实大于 0 时，一切正常。但是，当它等于 0 时，它不会“返回”任何东西，并且我在行中得到一个 NULL 作为回报。 minutes_played 是一个整数，当我在 player_fixtures 上尝试以下脚本时，minutes_played = 0 我的 > 评估有效，我得到正确的“BAD”。

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

我在整个数据库方面都很陌生，所以我可能犯了一个新手错误，但过去 6 小时我一直在尝试，但没有任何运气。有人能指出我正确的方向吗？

【问题讨论】：

如果您在案例语句中使用别名会发生什么，例如第一个：CASE WHEN pfix.minutes_played > 0 THEN g.captain_id ELSE g.vice_captain_id END
@cha 已经尝试过在任何地方使用别名或没有别名，也没有用:(

标签： sql postgresql rails-postgresql case-expression

【解决方案1】：

问题在于，当没有匹配的行时，sum() 返回 null，并且与 null 的比较永远不会为真。

改为使用exists()，例如：

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

或者您可以在存储过程中使用if exists(...) then。

【讨论】：