How do I loop through a set of records from a select statement?
Say I have a few records that I wish to loop through and do something with each record. Here's a primitive version of my select statement:
select top 1000 * from dbo.tablewhere StatusID = 7Best Answer
By using T-SQL and cursors like this :
DECLARE @MyCursor CURSOR;DECLARE @MyField YourFieldDataType;BEGINSET @MyCursor = CURSOR FORselect top 1000 YourField from dbo.tablewhere StatusID = 7 OPEN @MyCursor FETCH NEXT FROM @MyCursor INTO @MyFieldWHILE @@FETCH_STATUS = 0BEGIN/*YOUR ALGORITHM GOES HERE */FETCH NEXT FROM @MyCursor INTO @MyField END; CLOSE @MyCursor ;DEALLOCATE @MyCursor;END;When it comes to iterating through rows in SQL, there are several approaches you can take. One common method is to use a loop construct, such as a WHILE or FOR loop, to iterate through each row in the result set. This allows you to perform operations on each row individually.
Another approach is to use a cursor, which is a database object that allows you to retrieve and manipulate rows from a result set one at a time. You can declare a cursor, open it, fetch the rows one by one, and then close the cursor when you're done.
Alternatively, you can use a SELECT statement with the FETCH clause to retrieve a specific number of rows from a result set at a time. This can be useful when dealing with large result sets, as it allows you to process the data in smaller chunks.
It's important to note that iterating through rows in SQL should be done carefully, as it can impact performance. Make sure to optimize your queries, use appropriate indexes, and limit the amount of data retrieved if possible. This will help ensure efficient row iteration and minimize any potential bottlenecks.
This is what I've been doing if you need to do something iterative... but it would be wise to look for set operations first. Also, do not do this because you don't want to learn cursors.
select top 1000 TableIDinto #ControlTable from dbo.tablewhere StatusID = 7declare @TableID intwhile exists (select * from #ControlTable)beginselect top 1 @TableID = TableIDfrom #ControlTableorder by TableID asc-- Do something with your TableIDdelete #ControlTablewhere TableID = @TableIDenddrop table #ControlTableSmall change to sam yi's answer (for better readability):
select top 1000 TableIDinto #ControlTable from dbo.tablewhere StatusID = 7declare @TableID intwhile exists (select * from #ControlTable)beginselect @TableID = (select top 1 TableIDfrom #ControlTableorder by TableID asc)-- Do something with your TableIDdelete #ControlTablewhere TableID = @TableIDenddrop table #ControlTableBy using cursor you can easily iterate through records individually and print records separately or as a single message including all the records.
DECLARE @CustomerID as INT;declare @msg varchar(max)DECLARE @BusinessCursor as CURSOR;SET @BusinessCursor = CURSOR FORSELECT CustomerID FROM Customer WHERE CustomerID IN ('3908745','3911122','3911128','3911421')OPEN @BusinessCursor;FETCH NEXT FROM @BusinessCursor INTO @CustomerID;WHILE @@FETCH_STATUS = 0BEGINSET @msg = '{"CustomerID": "'+CONVERT(varchar(10), @CustomerID)+'","Customer": {"LastName": "LastName-'+CONVERT(varchar(10), @CustomerID) +'","FirstName": "FirstName-'+CONVERT(varchar(10), @CustomerID)+'", }}|'print @msgFETCH NEXT FROM @BusinessCursor INTO @CustomerID;ENDJust another approach if you are fine using temp tables.I have personally tested this and it will not cause any exception (even if temp table does not have any data.)
CREATE TABLE #TempTable(ROWID int identity(1,1) primary key,HIERARCHY_ID_TO_UPDATE int,)--create some testing data--INSERT INTO #TempTable VALUES(1)--INSERT INTO #TempTable VALUES(2)--INSERT INTO #TempTable VALUES(4)--INSERT INTO #TempTable VALUES(6)--INSERT INTO #TempTable VALUES(8)DECLARE @MAXID INT, @Counter INTSET @COUNTER = 1SELECT @MAXID = COUNT(*) FROM #TempTableWHILE (@COUNTER <= @MAXID)BEGIN--DO THE PROCESSING HERE SELECT @HIERARCHY_ID_TO_UPDATE = PT.HIERARCHY_ID_TO_UPDATEFROM #TempTable AS PTWHERE ROWID = @COUNTERSET @COUNTER = @COUNTER + 1ENDIF (OBJECT_ID('tempdb..#TempTable') IS NOT NULL)BEGINDROP TABLE #TempTableENDYou could choose to rank your data and add a ROW_NUMBER and count down to zero while iterate your dataset.
-- Get your dataset and rank your dataset by adding a new row_numberSELECT TOP 1000 A.*, ROW_NUMBER() OVER(ORDER BY A.ID DESC) AS ROWINTO #TEMPTABLE FROM DBO.TABLE AS AWHERE STATUSID = 7;--Find the highest number to start withDECLARE @COUNTER INT = (SELECT MAX(ROW) FROM #TEMPTABLE);DECLARE @ROW INT;-- Loop true your data until you hit 0WHILE (@COUNTER != 0)BEGINSELECT @ROW = ROWFROM #TEMPTABLEWHERE ROW = @COUNTERORDER BY ROW DESC--DO SOMTHING COOL -- SET your counter to -1SET @COUNTER = @ROW -1ENDDROP TABLE #TEMPTABLEthis way we can iterate into table data.
DECLARE @_MinJobID INTDECLARE @_MaxJobID INTCREATE TABLE #Temp (JobID INT)INSERT INTO #Temp SELECT * FROM DBO.STRINGTOTABLE(@JobID,',')SELECT @_MinJID = MIN(JobID),@_MaxJID = MAX(JobID) FROM #TempWHILE @_MinJID <= @_MaxJIDBEGININSERT INTO Mytable ( JobID, ) VALUES ( @_MinJobID, ) SET @_MinJID = @_MinJID + 1;ENDDROP TABLE #TempSTRINGTOTABLE is user define function which will parse comma separated data and return table. thanks
I think this is the easy way example to iterate item.
declare @cateid intselect CateID into [#TempTable] from Category where GroupID = 'STOCKLIST'while (select count(*) from #TempTable) > 0beginselect top 1 @cateid = CateID from #TempTableprint(@cateid)--DO SOMETHING HEREdelete #TempTable where CateID = @cateidenddrop table #TempTable