select top 1 with a group by

问题:

I have two columns:

namecode name 050125  chris 050125  tof 050125  tof 050130  chris 050131  tof 

I want to group by namecode, and return only the name with the most number of occurrences. In this instance, the result would be

050125  tof 050130  chris 050131  tof 

This is with SQL Server 2000

回答1:

SELECT max_table.namecode, count_table2.name FROM     (SELECT namecode, MAX(count_name) AS max_count      FROM          (SELECT namecode, name, COUNT(name) AS count_name           FROM mytable           GROUP BY namecode, name) AS count_table1      GROUP BY namecode) AS max_table INNER JOIN     (SELECT namecode, COUNT(name) AS count_name, name      FROM mytable      GROUP BY namecode, name) count_table2 ON max_table.namecode = count_table2.namecode AND    count_table2.count_name = max_table.max_count 

回答2:

I usually use ROW_NUMBER() to achieve this. Not sure how it performs against various data sets, but we haven't had any performance issues as a result of using ROW_NUMBER.

The PARTITION BY clause specifies which value to "group" the row numbers by, and the ORDER BY clause specifies how the records within each "group" should be sorted. So partition the data set by NameCode, and get all records with a Row Number of 1 (that is, the first record in each partition, ordered by the ORDER BY clause).

SELECT      i.NameCode,      i.Name FROM  (     SELECT          RowNumber = ROW_NUMBER() OVER (PARTITION BY t.NameCode ORDER BY t.Name),         t.NameCode,         t.Name     FROM         MyTable t ) i WHERE     i.RowNumber = 1;  

回答3:

select distinct namecode , (      select top 1 name from              (              select namecode, name, count(*)               from myTable i              where i.namecode = o.namecode              group by namecode, name               order by count(*) desc             ) x ) as name from myTable o  

回答4:

I did not try but this should work,

select top 1 t2.* from ( select namecode, count(*) count from temp  group by namecode) t1 join temp t2 on t1.namecode = t2.namecode  order by t1.count desc 

回答5:

Here are to examples that you could use but the temp table use is more efficient than the view, but was done on a small data sample. You would want to check your own statistics.

    --Creating A View GO CREATE VIEW StateStoreSales AS SELECT t.state,t.stor_id,t.stor_name,SUM(s.qty) 'TotalSales' ,ROW_NUMBER() OVER (PARTITION BY t.state ORDER BY SUM(s.qty) DESC) AS 'Rank' FROM [dbo].[sales] s JOIN [dbo].[stores] t ON (s.stor_id = t.stor_id) GROUP BY t.state,t.stor_id,t.stor_name  GO SELECT * FROM StateStoreSales WHERE Rank <= 1 ORDER BY TotalSales Desc  DROP VIEW StateStoreSales   ---Using a Temp Table SELECT t.state,t.stor_id,t.stor_name,SUM(s.qty) 'TotalSales' ,ROW_NUMBER() OVER (PARTITION BY t.state ORDER BY SUM(s.qty) DESC) AS 'Rank'     INTO #TEMP FROM [dbo].[sales] s JOIN [dbo].[stores] t ON (s.stor_id = t.stor_id) GROUP BY t.state,t.stor_id,t.stor_name  SELECT * FROM #TEMP WHERE Rank <= 1 ORDER BY TotalSales Desc  DROP TABLE #TEMP 

文章来源: select top 1 with a group by