Update columns in Table A based on columns in Table B for more than 500K rows

Question

Guys,
I need to update 9 columns in table A based on value from table B for for more than 500K rows.
So what is best way to achieve this. I am thinking of writing a Procedure with cursor to update the rows of table A.
When i googled about it, they say cursor will decrease the performance. So i have no clue how to go for this.
Rough code which i though
1) Procedure  with no parameter
2) Will declare 9 variable to store value from cursor
3) cursor will fetch row by row based on join condition between table a and table b
4) i will pass column values from table B to variables
5) will make an update statement for table A
Please let me know if above method is correct or is there any other way to do this without using cursor.

Answer 1

Guys,
Below is the rough code i wrote as per my requirement. Does it look correct? As of now i dont have any platform to test it so any help with the below code is highly appreciated.  As i said i need to update more than 500K rows by matching Table
A and Table B.  One more thing which i would like to add in below code, is to get log of all the rows that are in table B but not exist in table A.  Table A already has more than million data in it.
Also not sure how the loop in below code willl run when @rowcount is become to zero?
Please let me know if i need to consider performance related impact while running the script.
GO
SET SERVEROUTPUT ON
CREATE PROCEDURE ONETIMEUPDATE
 DECLARE @cnt INT;
 SET @cnt = 1;
 DECLARE @MSG varchar(255);
 DECLARE @COUNT_VAR INT;
 SET @COUNT_VAR=0;
 WHILE @cnt > 0
    BEGIN
  Update TOP (50000) A
  Set A.Col1=B.Col1,
      A.COL2=B.COL2,
      A.COL3=B.COL3,
      A.COL4=B.COL4,
      A.COL5=B.COL5,
      A.COL6=B.COL6,
      A.COL7=B.COL7
  From TableA A
         Inner Join TableB B
         on A.ID = B.ID--ID
         WHERE A.Col1 <> B.Col1
                OR A.Col2 <> B.Col2;
          SET @cnt = @@ROWCOUNT;
         IF @@ROWCOUNT=25000
           @COUNT_VAR=@COUNT_VAR + @@ROWCOUNT
           SELECT @MSG = CONVERT(varchar, @COUNT_VAR) + "Rows Updated" -- I WANT TO DISPLAY UPDATE after EVERY 25000 ROWS
          PRINT @MSG
  IF @@ROWCOUNT=0
     BEGIN    
           COMMIT
                   END
                WAITFOR DELAY '00:00:01'  --wait for a second before the next update
            END;
 END;