How to write a foreach in SQL Server?

Question

You seem to want to use a CURSOR. Though most of the times it's best to use a set based solution, there are some times where a CURSOR is the best solution. Without knowing more about your real problem, we can't help you more than that:

DECLARE @PractitionerId int

DECLARE MY_CURSOR CURSOR 
  LOCAL STATIC READ_ONLY FORWARD_ONLY
FOR 
SELECT DISTINCT PractitionerId 
FROM Practitioner

OPEN MY_CURSOR
FETCH NEXT FROM MY_CURSOR INTO @PractitionerId
WHILE @@FETCH_STATUS = 0
BEGIN 
    --Do something with Id here
    PRINT @PractitionerId
    FETCH NEXT FROM MY_CURSOR INTO @PractitionerId
END
CLOSE MY_CURSOR
DEALLOCATE MY_CURSOR

Suppose that the column PractitionerId is a unique, then you can use the following loop

DECLARE @PractitionerId int = 0
WHILE(1 = 1)
BEGIN
  SELECT @PractitionerId = MIN(PractitionerId)
  FROM dbo.Practitioner WHERE PractitionerId > @PractitionerId
  IF @PractitionerId IS NULL BREAK
  SELECT @PractitionerId
END

This generally (almost always) performs better than a cursor and is simpler:

DECLARE @PractitionerList TABLE(PracticionerID INT)
DECLARE @PracticionerID INT
    
INSERT @PractitionerList(PracticionerID)
SELECT PracticionerID
FROM Practitioner
    
WHILE(1 = 1)
BEGIN
            
    SET @PracticionerID = NULL
    SELECT TOP(1) @PracticionerID = PracticionerID
    FROM @PractitionerList
    
    IF @PracticionerID IS NULL
        BREAK
            
    PRINT 'DO STUFF'
    
    DELETE TOP(1) FROM @PractitionerList
    
END

Your select count and select max should be from your table variable instead of the actual table

DECLARE @i int
DECLARE @PractitionerId int
DECLARE @numrows int
DECLARE @Practitioner TABLE (
    idx smallint Primary Key IDENTITY(1,1)
    , PractitionerId int
)

INSERT @Practitioner
SELECT distinct PractitionerId FROM Practitioner

SET @i = 1
SET @numrows = (SELECT COUNT(*) FROM @Practitioner)
IF @numrows > 0
    WHILE (@i <= (SELECT MAX(idx) FROM @Practitioner))
    BEGIN

        SET @PractitionerId = (SELECT PractitionerId FROM @Practitioner WHERE idx = @i)

        --Do something with Id here
        PRINT @PractitionerId

        SET @i = @i + 1
    END

I would say everything probably works except that the column idx doesn't actually exist in the table you're selecting from. Maybe you meant to select from @Practitioner:

WHILE (@i <= (SELECT MAX(idx) FROM @Practitioner))

because that's defined in the code above like that:

DECLARE @Practitioner TABLE (
    idx smallint Primary Key IDENTITY(1,1)
    , PractitionerId int
)

The following line is wrong in your version:

WHILE (@i <= (SELECT MAX(idx) FROM @Practitioner))

(Missing the @)

Might be an idea to change your naming convention so that the tables are more different.

Although cursors usually considered horrible evil I believe this is a case for FAST_FORWARD cursor - the closest thing you can get to FOREACH in TSQL.