MySQL的：選擇默認的行（不是默認值），如果其他行不提供

Question

編輯：

我已經包括了創建報表和一個小的測試數據集，爲您嘗試。因此，我已將示例id更改爲2而不是5以表示測試數據中現有的id。

/編輯

我必須保持局部的頁面信息爲三個MySQL表：

CREATE TABLE `locale` (
    `languageCode` char(3) NOT NULL, 
    `regionCode` char(3) NOT NULL default 'ZZ', 
    `isActive` enum('yes','no') NOT NULL default 'no', 
    PRIMARY KEY (`languageCode`,`regionCode`) 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

CREATE TABLE `page` (
    `id` int(10) unsigned NOT NULL auto_increment, 
    `parentId` int(10) unsigned default NULL, 
    PRIMARY KEY (`id`), 
    KEY `FK_page_page_1` (`parentId`), 
    CONSTRAINT `FK_page_page_1` FOREIGN KEY (`parentId`) REFERENCES `page` (`id`) ON DELETE CASCADE ON UPDATE CASCADE 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

CREATE TABLE `pageInfo` (
    `pageId` int(10) unsigned NOT NULL, 
    `languageCode` char(3) NOT NULL, 
    `regionCode` char(3) NOT NULL default 'ZZ', 
    PRIMARY KEY (`pageId`,`languageCode`,`regionCode`), 
    KEY `FK_pageInfo_locale_1` (`languageCode`,`regionCode`), 
    KEY `FK_pageInfo_page_1` (`pageId`), 
    CONSTRAINT `FK_pageInfo_locale_1` FOREIGN KEY (`languageCode`, `regionCode`) REFERENCES `locale` (`languageCode`, `regionCode`) ON DELETE CASCADE, 
    CONSTRAINT `FK_pageInfo_page_1` FOREIGN KEY (`pageId`) REFERENCES `page` (`id`) ON DELETE CASCADE ON UPDATE CASCADE 
) ENGINE=InnoDB DEFAULT CHARSET=utf8; 

這裏是一些測試數據：

/* locale */ 
INSERT INTO `locale` (languageCode,regionCode,isActive) VALUES ('de','ZZ','yes'); 
INSERT INTO `locale` (languageCode,regionCode,isActive) VALUES ('en','ZZ','yes'); 
INSERT INTO `locale` (languageCode,regionCode,isActive) VALUES ('nl','ZZ','yes'); 
/* page */ 
INSERT INTO `page` (id,parentId) VALUES (1,NULL); 
INSERT INTO `page` (id,parentId) VALUES (2,1); 
/* pageInfo */ 
INSERT INTO `pageInfo` (pageId,languageCode,regionCode) VALUES (1,'de','ZZ'); 
INSERT INTO `pageInfo` (pageId,languageCode,regionCode) VALUES (1,'en','ZZ'); 
INSERT INTO `pageInfo` (pageId,languageCode,regionCode) VALUES (1,'nl','ZZ'); 
INSERT INTO `pageInfo` (pageId,languageCode,regionCode) VALUES (2,'en','ZZ'); 
INSERT INTO `pageInfo` (pageId,languageCode,regionCode) VALUES (2,'nl','ZZ'); 

的困境：
用檢索頁面所有活動區域設置我發出以下SQL語句：

SELECT 
     p.*, pi.languageCode, pi.regionCode 
    FROM 
     page AS p 
     INNER JOIN pageInfo AS pi 
      ON pi.pageId = p.id 
     INNER JOIN locale AS l 
      ON l.languageCode = pi.languageCode AND l.regionCode = pi.regionCode 
    WHERE 
     (p.id = '2') 
     AND (l.isActive = 'yes') 

我將如何改變這個語句，所以，當id 2不適用於特定的語言環境中，它會自動回落爲區域根頁上（即：page.parentId IS NULL）？我的目標是讓MySQL爲我的任何一個，但不是兩個，爲活動的區域設置。

我想：

WHERE 
    (p.id = '2' OR (p.parentId IS NULL)) 

但是，當然，這給了我兩個記錄爲實際上有id 2也是語言環境。我很確定這是可能的（或者與UNION，子選擇，或頁面上的重複連接），但我在這裏有一個總的SQL編寫器塊。我會很感激你的幫助。

Answer 1

XOR如何？ 「一個或另一個但不是兩個」：

WHERE 
    (p.id = '5' XOR (p.parentId IS NULL)) 
       ^---here 

Answer 2

使用LEFT JOIN並將兩個表連接兩次。您第二次專門加入p.parentId IS NULL。

在SELECT列表中使用IFNULL。第一個參數是指定id，第二個參數是後備，即根頁面的值。

SELECT 
    p.*, 
    IFNULL(l.languageCode, lr.LanguageCode) AS languageCode, 
    IFNULL(l.regionCode, lr.regionCode) AS regionCode 
FROM 
    page AS p 
    LEFT JOIN pageInfo AS pi 
     ON pi.pageId = p.id 
    LEFT JOIN locale AS l 
     ON l.languageCode = pi.languageCode AND l.regionCode = pi.regionCode 
    LEFT JOIN pageInfo AS pir 
     ON pir.pageId = p.id AND p.parentId IS NULL 
    LEFT JOIN locale AS lr 
     ON lr.languageCode = pir.languageCode AND lr.regionCode = pir.regionCode 
WHERE 
    (p.id = '5' AND l.isActive = 'yes') 
    OR (p.parentId IS NULL AND lr.isActive = 'yes') 

Answer 3

您可以通過p.id = 5訂購和使用的限制1.這是一個黑客攻擊的一位，但它會工作。

SELECT 
     p.*, pi.languageCode, pi.regionCode 
    FROM 
     page AS p 
     INNER JOIN pageInfo AS pi 
      ON pi.pageId = p.id 
     INNER JOIN locale AS l 
      ON l.languageCode = pi.languageCode AND l.regionCode = pi.regionCode 
    WHERE 
     (p.id = '5' OR (p.parentId IS NULL)) 
     AND (l.isActive = 'yes') 
    ORDER BY p.id = '5' DESC 
    LIMIT 1