在右表上加入两个特殊情况的表格？

Question

我有两个表：Candidate：{Id，Name}和Candidate_Education：{Id，CandidateId，Education，GraduationDate}

我想说明的候选人的名字和他过去的教育，我做了查询：

SELECT c.Name, ce.Education AS 'Last Education' 
FROM Candidate c 
LEFT JOIN Candidate_Education ce 
    ON c.Id = (SELECT TOP 1 CandidateID FROM Candidate_Education 
       ORDER BY GraduationDate DESC) 

但结果不正确，还有谁赋予的教育考生，他们没有与Candidate_Education关系

Answer 1

更多典型的方法可以做到你想要使用ROW_NUMBER()或OUTER APPLY什么：

SELECT c.Name, ce.Education 
FROM Candidate c OUTER APPLY 
    (SELECT TOP 1 ce.* 
     FROM Candidate_Education ce 
     WHERE ce.CandidateID = c.CandidateID 
     ORDER BY ce.GraduationDate DESC 
    ) ce; 

您的查询缺少FROM子句中的两个表之间的连接条件。但是，有一些更适合SQL Server的替代方案。

Answer 2

其他的解决办法

with OneCandidate as (
select * from (
       select ce.*, ROW_NUMBER() over(partition by ce.CandidateID order by ce.CandidateID desc) rang 
       from Candidate_Education ce 
       ) tmp 
where tmp.rang=1 
) 
SELECT c.Name, ce.Education 
FROM Candidate c left outer OneCandidate ce on ce.CandidateID = c.CandidateID 

Answer 3

WITH maxGraduationCTE AS 
(SELECT Id, MAX(GraduationDate) AS 'Last Education' FROM Candidate_Education 
GROUP BY Id), 

lastEducationCTE AS 
(SELECT [Last Education], ce.Id, ce.CandidateId, ce.Education FROM maxGraduationCTE 
INNER JOIN Candidate_Education ce ON maxGraduationCTE.Id = ce.Id); 

SELECT c.Name, 
CASE WHEN [Last Education] IS NULL THEN 'No Education' 
    ELSE CAST([Last Education] As Varchar) 
END As [Last Education] 
FROM Candidate c 
LEFT JOIN lastEducationCTE ce 
ON c.Id = ce.CandidateId 

您可以使用两个CTE表情和左与CASE语句加入到处理空值。我使用了一个案例，因为我不确定GraduationDate的数据类型...可能必须将其转换，因为如果它是日期，则不能混合使用varchar和日期数据类型