SQL 选择 "best" 行

Question

我对 SQL 的了解很肤浅，我遇到了一个对我来说太复杂的查询。

我有 table 个带有名称和描述列的应用程序，它们是引用正确翻译的 ID。我有 table 个本地化字符串。

APPS（app_id、name_strid、description_strid）字符串（str_id、lang_id、字符串）

我需要一个 return 所有应用的查询，每个字符串都有最佳翻译。最好的语言顺序（让我说：it-it, it, en）

我找到了按语言排序所有应用程序的解决方案：

SELECT A.app_id, S1.string, S2.string
FROM APPS as A
JOIN STRINGS AS S1
ON A.name_strid = S1.str_id
JOIN STRINGS AS S2
ON A.description_strid = S2.str_id
WHERE S1.lang_id = S2.lang_id
AND S1.lang_id IN ("it-it", "it", "en")
ORDER BY
   CASE S1.lang_id
      WHEN "it-it" THEN 1
      WHEN "it" THEN 2
      WHEN "en" THEN 3
      ELSE 4
   END;

如何才能只获得最好的语言？

谢谢

Answer 1

在 SQL 的大多数方言中，您可以使用 row_number() 来解决此问题，但您没有该选项。解决这个问题的一种方法是使用相关子查询——这会为每个 str_id 提取第一个匹配的语言，然后您可以将其用于过滤。

SELECT A.app_id, S1.string, S2.string
FROM APPS as A JOIN
     STRINGS AS S1
     ON A.name_strid = S1.str_id JOIN
     STRINGS AS S2
     ON A.description_str_id = S2.str_id AND
        S1.lang_id = S2.lang_id
WHERE S1.lang_id IN ('it-it', 'it', 'en') AND
      S1.lang_id = (SELECT s3.LangId
                    FROM Strings s3
                    WHERE s3.str_id = S1.str_id
                    ORDER BY (CASE S3.lang_id
                                  WHEN 'it-it' THEN 1
                                  WHEN 'it' THEN 2
                                  WHEN 'en' THEN 3
                                  ELSE 4
                              END)
                   LIMIT 1
                  );

Answer 2

您可以使用 EXISTS 运算符来确保没有其他带有 "better score" 的翻译：

SELECT A.app_id, S1.string, S2.string
FROM APPS as A JOIN STRINGS AS S1 ON A.name_strid = S1.str_id
               JOIN STRINGS AS S2 ON A.description_strid = S2.str_id
WHERE S1.lang_id = S2.lang_id
AND S1.lang_id IN ("it-it", "it", "en")
AND NOT EXISTS (SELECT 1
                FROM STRINGS S3
                WHERE (CASE S3.lang_id
                      WHEN "it-it" THEN 1
                      WHEN "it" THEN 2
                      WHEN "en" THEN 3
                      ELSE 4 END) < (CASE S1.lang_id
                                     WHEN "it-it" THEN 1
                                     WHEN "it" THEN 2
                                     WHEN "en" THEN 3
                                     ELSE 4 END)

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