从表中选择最大日期

Question

I have a table variable defined thus

DECLARE @DatesTable TABLE
(
    Id uniqueidentifier,
    FooId uniqueidentifier,
    Date date,
    Value decimal (26, 10)
)

Id is always unique but FooId is duplicated throughout the table. What I would like to do is to select * from this table for each unique FooId having the max(date). So, if there are 20 rows with 4 unique FooIds then I'd like 4 rows, picking the row for each FooId where the date is the largest.

I've tried using group by but I kept getting errors about various fields not being in the select clause etc.

Answer 1

Use a common table expression with row_number() :

;WITH cte AS
(
    SELECT Id, FooId, Date, Value,
           ROW_NUMBER() OVER(PARTITION BY FooId ORDER BY Date DESC) As rn
    FROM  @DatesTable
)

SELECT Id, FooId, Date, Value
FROM cte
WHERE rn = 1

Answer 2

Often the most efficient method is a correlated subquery:

select dt.*
from @DatesTable dt
where dt.date = (select max(dt2.date) from @DatesTable dt2 where dt2.fooid = dt.fooid);

However, for this to be efficient, you need an index on (fooid, date) . In more recent versions of SQL Server, you can have indexes on table variables. In earlier versions, you can do this using a primary key.