具有默认关系的左连接-数据库问题

Left join with default relation(具有默认关系的左连接)

本文介绍了具有默认关系的左连接的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

需要找到所有带有意大利语翻译的单词，如果意大利语不存在，则需要使用西班牙语(默认语言).我不能使用多个查询，并且条件存在(技术限制)

Need to find all words with their Italian translation, if Italian doesn't exist, then need with Spanish (default language). I can`t use more than one query, and where exists condition(technical limitations)

词

id|name
-------
 1|Dog
 2|Cat

翻译

id|word_id|translation|language
-------------------------------
 1|      1|      Perro|es
 2|      1|      Cane |it
 3|      2|      Gatto|es

结果:

id|name|translation|language
 1| Dog|       Cane|it
 2| Cat|      Gatto|es

<小时>

SELECT * FROM words LEFT JOIN translation ON words.id = translation.word_id WHERE language = 'it' OR (language = 'es' AND NOT EXISTS(SELECT * FROM translation WHERE word_id = words.id AND language = 'it'))

此代码返回我需要的所有内容，但我无法在我的情况下使用 where exists 条件

This code return all I need, but I can't use where exists conditions in my situation

推荐答案

我会在 translations 表中加入 words 表两次，每种语言一次:

I'd join the words table on the translations table twice, once for each language:

SELECT    w.id, 
          w.name,
          COALESCE(it.translation, es.translation) AS translation,
          COALESCE(it.language, es.language) AS language
FROM      words w
LEFT JOIN translation it ON w.id = it.word_id AND it.language = 'it'
LEFT JOIN translation es ON w.id = es.word_id AND es.language = 'es'

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