Question

我陷入了疑问。希望有人可以帮助我。

我有这4张桌子：

表：项目

id_items
1

表：items_atr_currencies_match

id_items_atr_currencies_match | id_items_atr_currencies | id_items
1                             | 1                       | 1

表：items_atr_currencies_translations

id_items_atr_currencies_translations | id_items_atr_currencies | id_language_code | translation
1                                    | 1                       | 1                | $

表：items_atr_currencies

id_items_atr_currencies
1

表：语言

id_languages | language_code
1            | es
2            | en

我的查询：

SELECT
    id_items,
    iact3.translation AS currency

FROM
    items i

LEFT JOIN 
    items_atr_currencies_match AS iacm3 ON iacm3.id_items = i.id_items
LEFT JOIN 
    items_atr_currencies AS iac3 ON iac3.id_items_atr_currencies = iacm3.id_items_atr_currencies
LEFT JOIN 
    items_atr_currencies_translations AS iact3 ON iact3.id_items_atr_currencies = iacm3.id_items_atr_currencies 
LEFT JOIN 
    languages AS l ON l.id_languages = iact3.id_language_code OR iact3.id_language_code IS NULL

WHERE
    i.id_items = 1
    AND l.language_code = "en"

预期结果：

id_items | currency
1        | null

获得0结果

我想我错过了像＃34;或者iact3.id_language_code不存在＆＃34;在最后一次LEFT JOIN中，但我不知道该怎么做。

当表items_atr_currencies_match没有数据或表items_atr_currencies_translations有翻译但没有数据并且翻译不存在时，它可以正常工作。

提前谢谢

*更新*

最后这个查询正在做我想要的事情：

SELECT 
     i.id_items,            
     iact3.translation AS currency

FROM
     items i

LEFT JOIN 
     languages AS l ON l.language_code = 'en'
LEFT JOIN 
     items_atr_currencies_match AS iacm3 ON iacm3.id_items = i.id_items
LEFT JOIN 
     items_atr_currencies_translations AS iact3 ON iact3.id_items_atr_currencies = iacm3.id_items_atr_currencies 
     AND l.id_languages = iact3.id_language_code            
LEFT JOIN items_atr_currencies AS iac3 ON iac3.id_items_atr_currencies = iacm3.id_items_atr_currencies

WHERE
    i.id_items = 1

我所要做的就是将语言LEFT JOIN移到第一位并检查那里的语言代码。

Answer 1

你有一系列left join个。当您在第一个表以外的任何表上的where子句中有条件时，它往往会将left join转换为inner join。您需要将该条件放在on子句中：

SELECT id_items,iact3.translation AS currency FROM items i LEFT JOIN items_atr_currencies_match AS iacm3 ON iacm3.id_items = i.id_items LEFT JOIN items_atr_currencies AS iac3 ON iac3.id_items_atr_currencies = iacm3.id_items_atr_currencies LEFT JOIN items_atr_currencies_translations AS iact3 ON iact3.id_items_atr_currencies = iacm3.id_items_atr_currencies LEFT JOIN languages AS l ON (l.id_languages = iact3.id_language_code OR iact3.id_language_code IS NULL) AND l.language_code = 'en' WHERE i.id_items = 1;

第一个表的条件放在WHERE子句中。

注意：我认为最后一个条件可以简化为：

ON l.id_languages = iact3.id_language_code AND l.language_code = 'en'

即使没有匹配，LEFT JOIN也会包含该行。

Answer 2

您需要将条件AND l.language_code = "en"移动到左连接中。否则，将在评估完所有左连接后检查此条件，如果结果元组没有language_code = "en"，则将从整体结果中删除它。

如果你现在想要currencies_translations仅在他们映射到有效语言时才被考虑，那么你必须先通过联接连接currencies_translations和languages＆＃34; left-接合＆＃34;它与总体结果一起：

...
LEFT JOIN (select iact3.id_items_atr_currencies as iact3id from
    items_atr_currencies_translations AS iact3 JOIN 
    languages AS l ON (l.id_languages = iact3.id_language_code AND l.language_code = "en") OR iact3.id_language_code IS NULL) as iact3l 
    ON iact3l.iact3id = iacm3.id_items_atr_currencies

SQL。当列不存在时，左连接和空值

2 个答案: