具有 3 个表的复杂 IF 语句

Question

这是this question from yesterday 的延续。

这是我的三张桌子：

格斗桌

fighter_id |  name
-----------------------
1          | John
2          | Steve
3          | Bill
4          | Bobby

事件表

event_id  |  event_name  |  event_date
-------------------------------------------
1          | MMA         |  01/01/2010
2          | Cool        |  02/20/2010
3          | Yeaa!       |  04/15/2010

格斗桌

fight_id  |  fighter_a  |  fighter_b  |  winner  |  method  |  event
-----------------------------------------------------------------------
1         | 1           |  2          |  1       | Sub      | 1
2         | 4           |  1          |  4       | KO       | 2
3         | 1           |  3          |  NULL    | Draw     | 3

尝试获取的结果

result  |  opponent  |  method  |  event  |  date        
----------------------------------------------------------
Draw    | Bill       |  Draw    |  Yeaa!  |  04/15/2010  
Loss    | Bobby      |  KO      |  Cool   |  02/20/2010 
Win     | Steve      |  Sub     |  MMA    |  01/01/2010

所以在fights表中，fighter_a、fighter_b和winner是对应于Fighters表中的fighter_id的整数。

我基本上是在基于 fighter_id ($fighter_id) 检索数据的页面上。

我正在尝试为该战士的每场战斗创建行，其中包括他的对手的名字、结果（胜利、失败、平局或 nc）、方法、事件名称和事件日期。挑战在于获胜者可以在 fighter_a 或 fighter_b 中。它并不总是在同一列中。我很感激能得到任何帮助。

select 
    fight_id,
    CASE
      WHEN winner is not null and winner=fighter_id then 'win'
      WHEN winner is not null and winner<>fighter_id then 'loss'
      WHEN winner is null and method='Draw' then 'draw'
      WHEN winner is null and method = 'No Contest' then 'no contest'
      ELSE ''
    END as match_result,
    participant.name 'participant',
    opponent.name 'opponent'
FROM fights
    INNER JOIN fighters as participant on participant.fighter_id = fights.fighter_a
    INNER JOIN fighters as oppoent on opponent.fighter_id = fights.fighter_b
WHERE 
    fighter_a=$fighter_id OR fighter_b=$fighter_id
ORDER BY
    event_date DESC

Answer 1

使用带有条件的子选择将您要查找的 fighter_id 切换到 column_a（如果它在 column_b 中），这样可以简化您的操作并加入外部查询：

SELECT
    (
        CASE
            WHEN a.winner = a.f_a THEN 'Win'
            WHEN a.winner = a.f_b THEN 'Loss'
            WHEN a.winner IS NULL THEN a.method
        END
    ) AS result,
    b.name AS opponent,
    a.method AS method,
    c.event_name AS event,
    c.event_date AS date
FROM
    (
        SELECT 
            IF(fighter_b = $fighter_id, fighter_b, fighter_a) AS f_a,
            IF(fighter_b = $fighter_id, fighter_a, fighter_b) AS f_b,
            winner,
            method,
            event
        FROM 
            fights
        WHERE
            $fighter_id IN (fighter_a, fighter_b)
    ) a
INNER JOIN
    fighters b ON a.f_b = b.fighter_id
INNER JOIN
    events c ON a.event = c.event_id
ORDER BY
    c.event_date DESC

此外，如果获胜者字段为空，则只需回显方法字段。这样，当您想在系统中添加另一种获胜者为空的方法时，您不必继续对 CASE 语句进行更多条件检查。

Answer 2

你可以使用联合：

SELECT CASE WHEN winner = participant_id THEN 'Win'
            WHEN winner = opponent_id then 'Loss'
            ELSE method
       END AS result, fighters.name AS opponent, method, event_name, event_date
  FROM fights
  INNER JOIN fighters ON fighter_id = opponent_id
  INNER JOIN events ON event = event_id
  WHERE participant_id = $fighter_id
UNION ALL
SELECT CASE WHEN winner = participant_id THEN 'Loss'
            WHEN winner = opponent_id then 'Win'
            ELSE method
       END AS result, fighters.name AS opponent, method, event_name, event_date
  FROM fights
  INNER JOIN fighters ON fighter_id = participant_id
  INNER JOIN events ON event = event_id
  WHERE opponent_id = $fighter_id
ORDER BY event_date DESC

Answer 3

您可以修改 CASE 语句以匹配 $fighter_id。

所以你的 CASE 语句将是 -

select fight_id,
       CASE
           WHEN winner is not null and winner = f.fighter_id then 'win'
           WHEN winner is not null and winner <> f.fighter_id then 'loss'
           WHEN winner is null and method='Draw' then 'draw'
           WHEN winner is null and method = 'No Contest' then 'no contest'
           ELSE ''
        END as match_result,
        f.name AS participant,
        (SELECT o.name FROM fighters o WHERE o.fighter_id = IF(f.fighter_id = fs.fighter_a, fs.fighter_b, fs.fighter_a)) AS opponent
FROM fights fs,
     fighters f
WHERE f.fighter_id = $fighter_id
AND (fs.fighter_a = f.fighter_id OR fs.fighter_b = f.fighter_id)

Answer 4

SELECT
    CASE
      WHEN winner is not null and winner=fighter_id then 'win'
      WHEN winner is not null and winner<>fighter_id then 'loss'
      WHEN winner is null and method='Draw' then 'draw'
      WHEN winner is null and method = 'No Contest' then 'no contest'
      ELSE ''
    END result,
    f2.name opponent,
    method,
    event_name event,
    event_date date
FROM fights f1
INNER JOIN fighters f2
ON f1.fighter_b = f2.fighter_id
INNER JOIN events e
ON e.event_id = f1.event
ORDER BY date DESC