SQL。列不存在时的左连接和空值

Question

我被困在一个查询中。希望有人可以在这里帮助我。

我有这 4 张桌子：

表格：项目

id_items
1

表：items_atr_currencies_match

id_items_atr_currencies_match | id_items_atr_currencies | id_items
1                             | 1                       | 1

表格：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   

表格：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

得到 0 结果而不是

我想我在最后一个 LEFT JOIN 中遗漏了类似“OR iact3.id_language_code 不存在”的内容，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 有翻译但有数据且翻译不存在时，它可以正常工作。

提前谢谢你

* 更新 *

最后这个查询正在做我想做的事：

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 移动到第一个位置并检查那里的语言代码。

Answer 1

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

如果您现在希望 currencies_translations 仅在它们映射到有效语言时才被考虑，那么您必须在“左连接”之前通过连接连接 currencies_translations 和 languages 以及总体结果：

...
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 

Answer 2

您有一系列left joins。当您在第一个表之外的任何表的where 子句中有条件时，它往往会将left join 变成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。