SELECT DISTINCT with ORDER BY 和 CASE WHEN

Question

我的查询是：

SELECT DISTINCT LanguageCode
       FROM myTable
       ORDER BY (CASE WHEN LanguageCode='GB' THEN 0 ELSE 1 END), LanguageCode

目标是获取语言，但首先要获取 GB 语言。 我收到此错误：

ORDER BY items must appear in the select list if SELECT DISTINCT is specified.

我看过这个问题的答案： Reason for - ORDER BY items must appear in the select list if SELECT DISTINCT is specified

但我仍然不明白为什么它适用于我（以及如何解决它），因为我只访问一个列。

Answer 1

不知何故，同事找到了一个解决方法：

SELECT DISTINCT (CASE WHEN LanguageCode='GB' THEN 0 ELSE 1 END) AS IsGB, LanguageCode
   FROM myTable
   ORDER BY IsGB ASC, LanguageCode

Answer 2

改为使用聚合：

SELECT LanguageCode
FROM myTable
GROUP BY LanguageCode
ORDER BY (CASE WHEN LanguageCode = 'GB' THEN 0 ELSE 1 END), 
         LanguageCode

有些数据库对 ORDER BY 中的非聚合表达式很挑剔。如果是这种情况，那么您可以使用：

SELECT LanguageCode
FROM myTable
GROUP BY LanguageCode
ORDER BY MAX(CASE WHEN LanguageCode = 'GB' THEN 0 ELSE 1 END), 
         LanguageCode