SQL中的IF/CASE語句

Question

我真的很努力地找到這個問題的答案。我想寫一個if或case語句來改變另一列的值。如果「檢查」列是0，那麼列'點'也應該爲0。

SELECT club_name, COALESCE(club_winner,0) AS 'check', COUNT(*) AS points, 
    CASE 
    WHEN check = '0' 
    THEN points = '0' 
    ELSE check 
    END as Saleable 
    FROM clubs c 
    LEFT JOIN club_results cr ON c.club_id = cr.club_winner 
    GROUP BY club_name ORDER BY points DESC 

Answer 1

您不能在case語句中「SET」列，您可以返回值。將case語句看作可變列。返回'0'AS Saleable（或任何您想要命名的列）。

SELECT club_name, COALESCE(club_winner,0) AS 'check', COUNT(*) AS points, 
CASE 
WHEN COALESCE(club_winner,0) = 0 
THEN 0 
ELSE COUNT(*) 
END AS points_or_0_if_check_is_0 
FROM clubs c 
LEFT JOIN club_results cr ON c.club_id = cr.club_winner 
GROUP BY club_name ORDER BY points_or_0_if_check_is_0 DESC 

Answer 2

可以選擇只計算符合左連接，像這樣的行：

SELECT 
    club_name, 
    COUNT(
     CASE WHEN 
      club_winner IS NOT NULL 
     THEN 
      CR.id 
     ELSE 
      NULL 
     END 
    ) AS `points` 
FROM 
    clubs C 
LEFT JOIN 
    club_results CR ON C.club_id = CR.club_winner 
GROUP BY 
    C.club_id 
ORDER BY 
    points DESC