[英]SQL: Inner Join return one row based on criteria
这可能很简单,但我正在寻找原始SQL来执行INNER JOIN
但只根据条件返回第二个表中的一个匹配项。
给出两个表:
**TableOne**
ID Name
1 abc
2 def
**TableTwo**
ID Date
1 12/1/2014
1 12/2/2014
2 12/3/2014
2 12/4/2014
2 12/5/2014
我想加入但只返回第二个表中的最新日期:
Expected Result:
1 abc 12/2/2014
2 def 12/5/2014
我可以在LINQ中轻松完成此操作:
TableOne.Select(x=> new { x.ID, x.Name, Date = x.TableTwo.Max(y=>y.Date) });
换句话说,上面的LINQ语句在原始SQL中转化为什么?
有两种方法可以做到这一点:
使用GROUP BY
和MAX()
:
SELECT one.ID, one.Name, MAX(two.Date) FROM TableOne one INNER JOIN TableTwo two on one.ID = two.ID GROUP BY one.ID, one.Name
将ROW_NUMBER()
与CTE一起使用:
; WITH cte AS ( SELECT one.ID, one.Name, two.Date, ROW_NUMBER() OVER (PARTITION BY one.ID ORDER BY two.Date DESC) as rn FROM TableOne one INNER JOIN TableTwo two ON one.ID = two.ID ) SELECT ID, Name, Date FROM cte WHERE rn = 1
您可以使用聚合查询加入第一个表:
SELECT t1.id, d
FROM TableOne t1
JOIN (SELECT id, MAX[date] AS d
FROM TableTwo
GROUP BY id) t2 ON t1.id = t2.id
就像是:
SELECT TableOne.id, TableOne.name, MAX(TableTwo.Date)
FROM TableOne
LEFT JOIN TableTwo ON TableOne.id = TableTwo.id
GROUP BY TableOne.id, TableOne.name;
连接将生成一个包含TableTwo的行数的表,但group by将根据TableOne的行将其过滤为一行。
由于没有其他人已经覆盖了将执行您想要的任务的公用表表达式(CTE),我将把它放在这里:
with maxDates as (
select Id, max(Date)
from Table2
group by Id
)
select x.Id, x.Name, y.Date
from TableOne x
inner join maxDates y
on x.Id = y.id
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