Question

我正在尝试根据查询检索一些记录

Select distinct 
    tblAssessmentEcosystemCredit.AssessmentEcosystemCreditID,
    tblSpecies.CommonName 
from 
    tblAssessmentEcosystemCredit
left join 
    tblSpeciesVegTypeLink on tblAssessmentEcosystemCredit.VegTypeID = tblSpeciesVegTypeLink.VegTypeID 
left join 
    tblSpecies on tblSpecies.SpeciesID = tblSpeciesVegTypeLink.SpeciesID
where 
    tblAssessmentEcosystemCredit.SpeciesTGValue < 1

上面的查询返回17,000条记录但是当我删除tblSpecies.CommonName时，它只检索4200条（这实际上是正确的）。

我不知道如何仅区分tblAssessmentEcosystemCredit.AssessmentEcosystemCreditID列并检索查询中的所有其他表列。

Answer 1

请参阅此主题：mySQL select one column DISTINCT, with corresponding other columns

您可能需要停用ONLY_FULL_GROUP_BY，请参阅http://dev.mysql.com/doc/refman/5.7/en/sql-mode.html#sqlmode_only_full_group_by

Answer 2

此查询选择AssessmentEcosystemCreditID和CommonName的不同COMBINATION;如果你想要每个值的AssessmentEcosystemCreditID只有一行，那么你需要使用GROUP BY，如@JonasB所建议的那样;但是，在这种情况下，每个EvaluEcosystemCreditID值可能有几个CommonName值，因此SQL要求您指定所需的WHICH

Select  tblAssessmentEcosystemCredit.AssessmentEcosystemCreditID ,
         max(tblSpecies.CommonName) as CommonName,
         min(tblSpecies.CommonName) as CommonName2, -- so you can verify you only have one value
 from tblAssessmentEcosystemCredit
 left join tblSpeciesVegTypeLink 
      on tblAssessmentEcosystemCredit.VegTypeID = tblSpeciesVegTypeLink.VegTypeID 
 left join tblSpecies on tblSpecies.SpeciesID= tblSpeciesVegTypeLink.SpeciesID
 where tblAssessmentEcosystemCredit.SpeciesTGValue <1
 GROUP BY tblAssessmentEcosystemCredit.AssessmentEcosystemCreditID

在SQL Server中使用distinct的正确方法

2 个答案: