Iterating Over Result Sets in SQL Server

SQL Server, like many relational database systems, excels at set-based operations. This means performing actions on entire groups of data rather than processing rows individually. However, there are situations where you need to iterate over the rows of a result set to perform actions that aren’t easily expressed with standard SQL. This tutorial explores different approaches to achieve this iteration, along with their trade-offs.

Understanding the Need for Iteration

Typically, you’d use set-based operations like UPDATE, DELETE, or INSERT INTO ... SELECT to modify data in bulk. However, when the logic requires examining each row and performing a unique operation based on its values, iteration becomes necessary. Common scenarios include:

Performing complex calculations on each row.
Calling external procedures or services for each row.
Generating reports with row-specific formatting.

Methods for Iteration

Several methods can be employed to iterate over result sets in SQL Server. Let’s examine the most common ones:

1. While Loops with Table Variables

This approach involves loading the result set into a temporary table variable and then iterating over it using a WHILE loop. This is particularly useful when you need to modify the table variable as you iterate.

-- Create a table variable to hold the PractitionerIds
DECLARE @PractitionerList TABLE (
    RowNumber INT IDENTITY(1,1) PRIMARY KEY,
    PractitionerID INT
);

-- Insert the distinct PractitionerIds into the table variable
INSERT INTO @PractitionerList (PractitionerID)
SELECT DISTINCT PractitionerID
FROM Practitioner;

-- Declare variables for iteration
DECLARE @PractitionerID INT;
DECLARE @RowNumber INT;

-- Iterate over the table variable
WHILE (1 = 1)
BEGIN
    -- Get the next PractitionerID from the table variable
    SELECT TOP (1)
        @PractitionerID = PractitionerID,
        @RowNumber = RowNumber
    FROM @PractitionerList
    ORDER BY RowNumber; --Important to add an order by

    -- If no more rows, exit the loop
    IF @RowNumber IS NULL
        BREAK;

    -- Do something with the PractitionerId
    PRINT 'Processing PractitionerId: ' + CAST(@PractitionerID AS VARCHAR(10));

    -- Remove the processed row from the table variable
    DELETE FROM @PractitionerList WHERE RowNumber = @RowNumber;
END

Explanation:

A table variable @PractitionerList is created to store the distinct PractitionerID values.
The WHILE loop continues as long as there are rows in the table variable.
SELECT TOP(1) retrieves the next PractitionerID along with its RowNumber.
The loop terminates when RowNumber is NULL, indicating that there are no more rows.
The processed row is then deleted from the table variable to prevent infinite looping.

2. Cursors

Cursors provide a way to iterate through a result set row by row, similar to how you might iterate through an array in a programming language. While powerful, cursors are generally less efficient than set-based operations and should be used with caution.

-- Declare variables for the cursor
DECLARE @PractitionerId INT;

-- Declare the cursor
DECLARE MY_CURSOR CURSOR
LOCAL STATIC READ_ONLY FORWARD_ONLY
FOR
SELECT DISTINCT PractitionerId
FROM Practitioner;

-- Open the cursor
OPEN MY_CURSOR;

-- Fetch the first row
FETCH NEXT FROM MY_CURSOR INTO @PractitionerId;

-- Iterate through the cursor
WHILE @@FETCH_STATUS = 0
BEGIN
    -- Do something with the PractitionerId
    PRINT 'Processing PractitionerId: ' + CAST(@PractitionerId AS VARCHAR(10));

    -- Fetch the next row
    FETCH NEXT FROM MY_CURSOR INTO @PractitionerId;
END

-- Close and deallocate the cursor
CLOSE MY_CURSOR;
DEALLOCATE MY_CURSOR;

Explanation:

A cursor named MY_CURSOR is declared to iterate over the result of the SELECT statement.
The cursor is opened using OPEN MY_CURSOR.
The first row is fetched into the @PractitionerId variable using FETCH NEXT.
The WHILE loop continues as long as @@FETCH_STATUS is 0 (meaning the fetch was successful).
Within the loop, the PractitionerId is processed, and the next row is fetched.
Finally, the cursor is closed and deallocated to release resources.

3. While Loop with MIN Function

This method uses a WHILE loop along with the MIN function to find and process the next PractitionerId that hasn’t been processed yet. It is potentially more efficient than cursors for simple iteration tasks.

DECLARE @PractitionerId INT = 0;

WHILE (1 = 1)
BEGIN
    SELECT @PractitionerId = MIN(PractitionerId)
    FROM Practitioner
    WHERE PractitionerId > @PractitionerId;

    IF @PractitionerId IS NULL
        BREAK;

    -- Do something with the PractitionerId
    PRINT 'Processing PractitionerId: ' + CAST(@PractitionerId AS VARCHAR(10));

    -- Reset @PractitionerId for the next iteration
END

Explanation:

The WHILE loop continues as long as there are unprocessed PractitionerId values.
SELECT @PractitionerId = MIN(PractitionerId) finds the smallest PractitionerId that is greater than the current @PractitionerId value.
If no such PractitionerId exists, @PractitionerId becomes NULL, and the loop terminates.
Within the loop, the PractitionerId is processed, and the @PractitionerId variable is updated.

Performance Considerations

Set-Based Operations: Always prioritize set-based operations whenever possible. They are generally the most efficient way to process data in SQL Server.
Cursors: Cursors should be used as a last resort due to their performance overhead.
Table Variables: Table variables can be a good alternative to cursors for simple iteration tasks.
Indexing: Ensure that appropriate indexes are in place on the tables involved to improve query performance.

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