当列是空字符串时清空LINQ查询

Question

我对以下查询有疑问：

var checkDropdowns =
(from d in db.SIMS_Data
 where d.LanguageID == LanguageID
 join s in db.CL_Surveys on new { X1 = d.SurveyCodeID, X2 = d.LanguageID } equals new { X1 = s.SurveyCodeID, X2 = s.LanguageID }
 where areas == s.SubjectAreaTitle && surveys == s.SurveyTitle 
 join f in db.CL_Freq on new { Z1 = s.FrequencyID, Z2 = d.LanguageID } equals new { Z1 = f.ID, Z2 = f.LanguageID }
 where f.LanguageID == d.LanguageID && f.ID == s.FrequencyID
 join p in db.CL_FreqPer on new { Y1 = s.FrequencyID, Y2 = d.PeriodID } equals new { Y1 = p.FrequencyID, Y2 = p.PeriodID }
 where frequencyID == p.PeriodID
 join y in db.CL_ReferenceYear on d.YearID equals y.ID
 where referenceYear == y.ReferenceYear
 select new { d, s.SubjectAreaTitle, s.SurveyShortTitle, s.SurveyTitle, f.FrequencyCode });

问题是，当选择的 s.SurveyShortTitle 部分为空字符串时，查询会给出一个空列表，但是当选择内容写入时DB，一切都很好。什么似乎是问题？