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Looping through table records in Sql Server

This article lists out extensive list of example scripts for looping through table records one row at a time. This article covers the examples for the following scenario’s for looping through table rows
  1. Looping column having no gaps/duplicate values
  2. Looping column having gaps
  3. Looping column having duplicates
To understand the looping of the table records in the above listed scenarios, let us first create a temporary table #Student as shown in the below image with sample data using the following script.
USE TEMPDB
GO
CREATE TABLE #Student
(Id INT, Name NVARCHAR(100), Status TINYINT)
GO
INSERT INTO #Student ( Id, Name, Status)
Values (1, 'Munesh sharma', 0),
        (2, 'Rahul sharma', 0),
        (3, 'Roy len', 0)
GO



The below examples illustrates how we can loop through table records in various ways. And also highlights the problem if any. Please go through all the examples before deciding on using one particular approach.
Example 1: Looping column having no gaps/duplicate values
Approach 1: Looping through table records with static loop counter initialization
DECLARE @LoopNo INT = 1, @MaxStudentId INT = 3 ,
        @StudentName NVARCHAR(100)

WHILE(@LoopNo <= @MaxStudentId)
BEGIN
   SELECT @StudentName = Name
   FROM #Student WHERE Id = @LoopNo

   PRINT @StudentName 
   SET @LoopNo  = @LoopNo  + 1       
END



In this example the loop running variable @LoopNo and the maximum loop counter variable @MaxStudentId values are initialized with a static value.
Note: This approach of looping through table rows doesn’t work if the looping column (i.e. in this case Id column of the #Student table) values have gaps or if it has duplicate values
Approach 2: Looping through table records with dynamic loop counter initialization
DECLARE @LoopNo INT , @MaxStudentId INT,
        @StudentName NVARCHAR(100)
SELECT @LoopNo = min(id) , @MaxStudentId = max(Id)
FROM #Student

WHILE(@LoopNo IS NOT NULL
      AND @LoopNo <= @MaxStudentId)
BEGIN
   SELECT @StudentName = Name
   FROM #Student WHERE Id = @LoopNo
   
   PRINT @StudentName 
   SET @LoopNo  = @LoopNo  + 1       
END



In this example the loop running variable @LoopNo and the maximum loop No variable @MaxStudentId values are initialized dynamically.
Note: This approach of looping through table rows doesn’t work if the looping column (i.e. in this case Id column of the #Student table) values have gaps or if it has duplicate values

Example 2: Looping through table records where looping column has gaps

Issue with example 1’s approach 1 and 2: These example approaches are assuming that looping column values doesn’t have any gap in it. Let us see what is the output of the example 1’s approach 1 and 2 if we have gaps in the looping column value.
To create a gap, delete Student record from the #Student table with id = 2 by the following script:
DELETE FROM #Student WHERE Id = 2
GO
select * from #Student


Now let us run the example 1’s approach 1 and 2 script on #Student table which is having gap in the Id column value (i.e. record with id column value 2 is missing).



From the above result it is clear that the example 1’s approach 1 and 2 script will not work in the scenarios where we have gap in the looping tables column values.
This problem can solved in multiple ways, below are two such example approaches. I would prefer the first approach.
Example-2
Approach 1: Looping through table records where looping column has gaps in the value

DECLARE @LoopNo INT , @MaxStudentId INT,
        @StudentName NVARCHAR(100)
SELECT @LoopNo = min(id) , @MaxStudentId = max(Id)
FROM #Student

WHILE ( @LoopNo IS NOT NULL
        AND  @LoopNo <= @MaxStudentId)
BEGIN
   SELECT @StudentName = Name FROM #Student
   WHERE Id = @LoopNo
   PRINT @StudentName 
   SELECT @LoopNo  = min(id) FROM #Student
   WHERE Id > @LoopNo
END



From the above result it is clear that this script works even when we have gaps in the looping column values.
Note: This approach of looping through table rows doesn’t work if the looping column (i.e. in this case Id column of the #Student table) has duplicate values
Approach 2: Looping through table records where looping column has gaps in the value
DECLARE @LoopNo INT , @MaxStudentId INT,
        @StudentName NVARCHAR(100)
SELECT @LoopNo = min(id) , @MaxStudentId = max(Id)
FROM #Student
WHILE ( @LoopNo IS NOT NULL
        AND  @LoopNo <= @MaxStudentId)
BEGIN
   SELECT @StudentName = Name
   FROM #Student WHERE Id = @LoopNo
   --To handle gaps in the looping column value
   IF(@@ROWCOUNT = 0 )
   BEGIN
     SET @LoopNo  = @LoopNo  + 1
     CONTINUE
   END

   PRINT @StudentName 
   SET @LoopNo  = @LoopNo  + 1       
END




From the above result it is clear that this script works even when we have gaps in the looping column values.
Note: This approach of looping through table rows doesn’t work if the looping column (i.e. in this case Id column of the #Student table) has duplicate values

Example 3: Looping through table records where looping column having duplicates

To create a duplicate record, insert one more Student record to the #Student table with id = 1 by the following script:
INSERT INTO #Student ( Id, Name, Status)
Values (1, 'Anshuman Sharma', 0)

Go
select * from #Student



Now let us run the example 2’s approach 1 and 2 script on #Student table which is having duplicate Id column values (i.e. there are two records with with Id column value as 1)

Print ' example 2 - approach 1'
DECLARE @LoopNo INT , @MaxStudentId INT,
        @StudentName NVARCHAR(100)
SELECT @LoopNo = min(id) , @MaxStudentId = max(Id)
FROM #Student

WHILE ( @LoopNo IS NOT NULL
        AND  @LoopNo <= @MaxStudentId)
BEGIN
   SELECT @StudentName = Name FROM #Student
   WHERE Id = @LoopNo
   PRINT @StudentName 
   SELECT @LoopNo  = min(id) FROM #Student
   WHERE Id > @LoopNo
END

Go

Print ' example 2 - approach 2'
DECLARE @LoopNo INT , @MaxStudentId INT,
        @StudentName NVARCHAR(100)
SELECT @LoopNo = min(id) , @MaxStudentId = max(Id)
FROM #Student
WHILE ( @LoopNo IS NOT NULL
        AND  @LoopNo <= @MaxStudentId)
BEGIN
   SELECT @StudentName = Name
   FROM #Student WHERE Id = @LoopNo
   --To handle gaps in the looping column value
   IF(@@ROWCOUNT = 0 )
   BEGIN
     SET @LoopNo  = @LoopNo  + 1
     CONTINUE
   END

   PRINT @StudentName 
   SET @LoopNo  = @LoopNo  + 1       
END





From the above result it is clear that the example 2’s approach 1 and 2 script will not work in the scenarios where we have duplicates in the looping column. Here only one record of the Student with id =1 is displayed and other record is skipped. This problem can solved in multiple ways, below are two such example approaches.
Approach 1: Looping through table records where looping column has duplicate values

SET NOCOUNT ON
DECLARE @LoopNo INT , @MaxStudentId INT,
        @StudentName NVARCHAR(100)
SELECT @LoopNo = min(id) , @MaxStudentId = max(Id)
FROM #Student
 
WHILE  ( @LoopNo IS NOT NULL
         AND  @LoopNo <= @MaxStudentId)
BEGIN
   UPDATE TOP(1) #Student
   SET  Status = 1, @StudentName = Name
   WHERE Id = @LoopNo  AND Status = 0
 
   PRINT @StudentName 
 
   SELECT @LoopNo  = min(id) FROM #Student 
   WHERE Id >= @LoopNo AND Status = 0
END



In this approach using the Status column to mark the records which are already processed. And also the update statement is used to update the status and also get the row values and one more thing is in Update using the TOP statement to update only one record at a time.
Approach 2: Looping through table records where looping column has duplicate values by inserting records into another temp table with identity column

--Create another temp table with identity column
CREATE TABLE #StudentCopy (LoopId INT IDENTITY(1,1),
  Id INT, Name NVARCHAR(100), Status TINYINT)
--Copy data to the table with identity column
INSERT INTO #StudentCopy(Id, Name, Status)
SELECT Id, Name, Status FROM #Student

DECLARE @LoopNo INT , @MaxStudentId INT,
        @StudentName NVARCHAR(100)
SELECT @LoopNo = min(LoopId),@MaxStudentId = max(LoopId)
FROM #StudentCopy
WHILE ( @LoopNo IS NOT NULL
        AND  @LoopNo <= @MaxStudentId)
BEGIN
   SELECT @StudentName = Name
   FROM #StudentCopy  WHERE LoopId = @LoopNo
   PRINT @StudentName 
   SELECT @LoopNo  = min(LoopId)
   FROM #StudentCopy  WHERE LoopId > @LoopNo
END






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