## Using Ranking Functions to Deduplicate Data

 Author Message Langston Montgomery SSC Rookie Group: General Forum Members Points: 42 Visits: 508 Very nice article. Thanks for sharing. My two cents are that I would use the dense_rank() function to return the rank in tandem order. The rank() function tends to skip a rank depending on how many members belong to a group.`declare @AlphaList table (AlphaKey char);insert into @AlphaList(AlphaKey) values ('A');insert into @AlphaList(AlphaKey) values ('A');insert into @AlphaList(AlphaKey) values ('B');insert into @AlphaList(AlphaKey) values ('B');insert into @AlphaList(AlphaKey) values ('C');insert into @AlphaList(AlphaKey) values ('D');insert into @AlphaList(AlphaKey) values ('D');insert into @AlphaList(AlphaKey) values ('E');select RANK() over (order by AlphaKey) as Rank, dense_RANK() over (order by AlphaKey) as [Rank by density], ROW_NUMBER() over (order by AlphaKey) as RowNumber, AlphaKeyfrom @AlphaList;``Rank Rank by density RowNumber AlphaKey1 1 1 A1 1 2 A3 2 3 B3 2 4 B5 3 5 C6 4 6 D6 4 7 D8 5 8 E` Andre Guerreiro Ten Centuries Group: General Forum Members Points: 1065 Visits: 1514 I'm still getting ranking functions into my head slowly. :-DNice article about a very useful way to use them. Best regards,Andre Guerreiro NetoDatabase Analysthttp://www.softplan.com.brMCITPx1/MCTSx2/MCSE/MCSA lgidon Grasshopper Group: General Forum Members Points: 22 Visits: 59 I'm disappointed. I've been doing this for a while with ROW_NUMBER and PARTION and it works gr8 in all the situations I've come across.I can't figure out what advantage I can gain using the RANK function.I'm not sure there is an advantage, but I'll be happy if someone corrects me :-) Jeff Moden SSC-Forever Group: General Forum Members Points: 45062 Visits: 39903 I'm torn... it's a nicely written article but the premise is at least twice as complicated as it needs to be and 2 times as slow as conventional methods, IMHO. Don't take my word for it, though... test it yourself... `--===========================================================================-- Create a multi-million row test table. This is not a measured-- part of the test.--===========================================================================--===== Conditionally drop the test table to make reruns easier IF OBJECT_ID('TempDB..#AlphaList','U') IS NOT NULL DROP TABLE #AlphaList;--===== Create and populate a multi-million row test table on the fly SELECT TOP 5000000 AlphaListID = IDENTITY(INT,1,1), AlphaKey = CHAR(ABS(CHECKSUM(NEWID()))%26+65) --A thru Z randomly INTO #AlphaList FROM Master.sys.All_Columns t1 CROSS JOIN Master.sys.All_Columns t2;--===== Create the expected indexes ALTER TABLE #AlphaList ADD PRIMARY KEY CLUSTERED (AlphaListID); CREATE NONCLUSTERED INDEX IX_#AlphaList_AlphaKey ON #AlphaList (AlphaKey);--===========================================================================-- Test 3 different methods with CPU and Duration measurements.--===========================================================================--===== "Clear the guns" PRINT REPLICATE('=',80); PRINT '========== Traditional RowNumber Method =========='; DBCC FREEPROCCACHE; DBCC DROPCLEANBUFFERS; SET STATISTICS TIME ON;--===== Test the codeselect AlphaKeyfrom ( select row_number() over(partition by AlphaKey order by AlphaKey) rownum, AlphaKey from #AlphaList ) alwhere rownum = 1 order by AlphaKey; SET STATISTICS TIME OFF;--===== "Clear the guns" PRINT REPLICATE('=',80); PRINT '========== Simple Distinct =========='; DBCC FREEPROCCACHE; DBCC DROPCLEANBUFFERS; SET STATISTICS TIME ON;--===== Test the codeSELECT DISTINCT AlphaKey FROM #AlphaListORDER BY AlphaKey; SET STATISTICS TIME OFF;--===== "Clear the guns" PRINT REPLICATE('=',80); PRINT '========== New Rank Method from Article =========='; DBCC FREEPROCCACHE; DBCC DROPCLEANBUFFERS; SET STATISTICS TIME ON;--===== Test the codewith AlphaRank(Rank, RowNumber, AlphaKey) as (select RANK() over (order by AlphaKey) as Rank, ROW_NUMBER() over (order by AlphaKey) as RowNumber, AlphaKeyfrom #AlphaList)select AlphaKeyfrom AlphaRankwhere Rank=RowNumber; SET STATISTICS TIME OFF;`Here are the results on my 8 year old desktop single p4 CPU running at 1.8Ghz with 1GB of ram on SQL Server 2005 Developer's Edition sp3.`(5000000 row(s) affected)========================================================================================== Traditional RowNumber Method ==========DBCC execution completed. If DBCC printed error messages, contact your system administrator.DBCC execution completed. If DBCC printed error messages, contact your system administrator.(26 row(s) affected)SQL Server Execution Times: CPU time = 2875 ms, elapsed time = 3005 ms.========================================================================================== Simple Distinct ==========DBCC execution completed. If DBCC printed error messages, contact your system administrator.DBCC execution completed. If DBCC printed error messages, contact your system administrator.(26 row(s) affected)SQL Server Execution Times: CPU time = 2985 ms, elapsed time = 3043 ms.========================================================================================== New Rank Method from Article ==========DBCC execution completed. If DBCC printed error messages, contact your system administrator.DBCC execution completed. If DBCC printed error messages, contact your system administrator.(26 row(s) affected)SQL Server Execution Times: CPU time = 7953 ms, elapsed time = 8037 ms.` --Jeff ModenRBAR is pronounced ree-bar and is a Modenism for Row-By-Agonizing-Row.First step towards the paradigm shift of writing Set Based code: Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column.Although they tell us that they want it real bad, our primary goal is to ensure that we dont actually give it to them that way.Although change is inevitable, change for the better is not.Just because you can do something in PowerShell, doesnt mean you should. Helpful Links:How to post code problemsHow to post performance problemsForum FAQs PravB4u SSC-Enthusiastic Group: General Forum Members Points: 147 Visits: 600 Hi,I think we can achieve the same without rank function.SELECT * FROM GetMissingItems WHERE RowNumber = 1 Praveen D'saMCITP - Database Administrator 2008http://sqlerrors.wordpress.com Jeff Moden SSC-Forever Group: General Forum Members Points: 45062 Visits: 39903 PravB4u (7/29/2010)Hi,I think we can achieve the same without rank function.SELECT * FROM GetMissingItems WHERE RowNumber = 1Ummm.... Not without the "Partition By" part of the OVER clause for ROW_NUMBER in what appears to be a missing CTE from your code. --Jeff ModenRBAR is pronounced ree-bar and is a Modenism for Row-By-Agonizing-Row.First step towards the paradigm shift of writing Set Based code: Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column.Although they tell us that they want it real bad, our primary goal is to ensure that we dont actually give it to them that way.Although change is inevitable, change for the better is not.Just because you can do something in PowerShell, doesnt mean you should. Helpful Links:How to post code problemsHow to post performance problemsForum FAQs