mySQL選擇優先級 - 選擇一行或另一行（選擇默認或「後備」行）

Question

這應該相當簡單，但在搜索時找到合適的術語很困難。

在下面的查詢中，如果WHERE lang='nl_NL'什麼也沒有返回，我需要選擇WHERE lang='en'。換句話說，我想lang=nl_NL

SELECT * FROM international WHERE script='generic' AND (lang = 'nl_NL' OR lang = 'en') 

Answer 1

SELECT * FROM international WHERE script = 'generic' AND lang = CASE 
    WHEN EXISTS(
    SELECT 1 
    FROM international 
    WHERE script = 'generic' 
     AND lang = 'nl_NL' 
    LIMIT 1 
) THEN 'nl_NL' 
    ELSE 'en' 
END 

Answer 2

找到的所有記錄WHERE script='generic'，優先這裏你可以得到指定的結果集的一種方式的示例：

SELECT i.* 
    FROM international i 
WHERE i.lang = 'nl_NL' 
    OR (i.lang = 'en' 
     AND NOT EXISTS 
     (SELECT 1 FROM international d WHERE d.lang = 'nl_NL') 
     ) 

這不是不清楚您的問題何時需要應用「script='generic'」謂詞。

SELECT i.* 
    FROM international i 
WHERE (i.script = 'generic' AND i.lang = 'nl_NL') 
    OR (i.script = 'generic' AND i.lang = 'en' 
     AND NOT EXISTS 
      (SELECT 1 
       FROM international d 
       WHERE d.lang = 'nl_NL' 
       AND d.script='generic' 
      ) 
     )