MySQL选择所有父数据和子数据(包括空值)

Question

我有 3 张 table :

• 单词:单词的 ID 和单词本身
• word_item:按单词 ID 的单词定义。
• word_item_translate:使用 de、fr、it 等语言代码翻译单词定义。

我当前的应用程序需要检索所请求单词的定义。示例:

西类牙语中的单词caballo

SELECT word.id_word, word_item.id_item, word_item.definition  
FROM word 
INNER JOIN word_item ON word.id_word = word_item.id_word 
WHERE word.lang = "es" 
AND word.word LIKE "caballo" 
ORDER BY word_item.votes DESC, word_item.id_item

http://sqlfiddle.com/#!9/d0a44/4

这很好用!但是应用程序还需要检索定义翻译，所以...

西类牙语单词 *caballo"及德语翻译

SELECT word.id_word, word_item.id_item, word_item.definition, word_item_translate.translation 
FROM word 
INNER JOIN word_item ON word.id_word = word_item.id_word 
LEFT JOIN word_item_translate 
ON word_item_translate.id_item = word_item.id_item 
AND (word_item_translate.id_item_translate OR word_item_translate.id_item_translate IS NULL) 
WHERE word.lang = "es" 
AND word.word LIKE "caballo" 
AND (word_item_translate.lang = "de" OR word_item_translate.lang IS NULL) 
ORDER BY word_item.votes DESC, word_item.id_item

http://sqlfiddle.com/#!9/d0a44/5

这效果不太好。正如你所看到的，原始单词“caballo”在 word_item 中有 11 种不同的定义。 。这 11 条记录中只有 8 条被翻译成德语。尽管如此，第二个查询应该显示 11 条记录，如果有 3 条记录没有翻译，则 translation列应显示值 NULL .

Answer 1

我认为您只是对如何使用LEFT JOIN感到困惑。以下内容应该有效:

SELECT word.id_word, word_item.id_item, word_item.definition, word_item_translate.translation  
FROM word 
INNER JOIN word_item ON word.id_word = word_item.id_word
LEFT JOIN word_item_translate ON
word_item_translate.id_item = word_item.id_item
AND word_item_translate.lang = 'de'
WHERE word.lang = 'es' 
AND word.word LIKE 'caballo' 
ORDER BY word_item.votes DESC, word_item.id_item