c# - 使用 2 个表之间的可选/条件联接进行查询

Question

ResourceTranslation
--------------------------------------------
ID                          binary(16)      |
ShortStringResourceID       binary(16)      |
LocaleName                  varchar(50)     |
TranslatedText              text            |
--------------------------------------------


ShortStringresource
--------------------------------------------
ID                          binary(16)      |
PrimaryLocaleName           varchar(50)     |
ContentText                 varchar(255)    |
--------------------------------------------

我想在 LINQ to SQL 中实现与以下 SQL 等效的功能:

SELECT (CASE p.PrimaryLocaleName WHEN 'en-GB' THEN p.ContentText ELSE t.ContentText END)
FROM shortstringresource p 
     LEFT OUTER JOIN resourcetranslation t ON t.ShortStringResourceID = p.ID 
WHERE p.ContentText = "Question 1 English Text"
AND (p.PrimaryLocaleName = 'en-GB' OR t.LocaleName = 'en-GB')
LIMIT 1;

或者将以下 2 个查询加入到 1 个查询中:

var qry1 = (from p in I18nObjects.ShortStringResources
            where (p.PrimaryLocaleName == "en-GB" && p.ContentText == "my text")
            select p.ContentText);

var qry2 = (from t in I18nObjects.ResourceTranslations
           where t.LocaleName == "en-GB" 
           join p in I18nObjects.ShortStringResources on t.ShortStringResourceID equals p.ID
           select t.TranslatedText);

Answer 1

您可能需要在选择之前考虑订购。

var results = (from p in ShortStringResources 
   join t in ResourceTranslations on p.ID equals t.ShortStringResourceID into xy
   from x in xy.DefaultIfEmpty()
   where p.ContentText == "Question 1 English Text" &&
        (p.PrimaryLocaleName == "en-GB" || x.LocaleName == "en-GB")
   select new {
      newField = p.PrimaryLocaleName == "en-GB"? p.ContentText : x.ContentText
   }).ToList().Take(1);