copy on condition

i have two tables A and B with the same fields,

If the id field of table B equals id field in Table A i need to update th edata for that id row.
If the id field doesn;t match then i need to insert a new record in tale A for that id

that is i need to perform insertion or updation into table A depending on table B data
Can anyone give me some idea how to start?

You need two statements.
UPDATE A SET col1 = b.col1, col2 = b.col2, ... FROM A INNER JOIN B on A.pkcol = B.pkcol
INERT INTO A (pkcol, col1, col2, col3)
SELECT pkcol, col1, col2, col3
FROM B
WHERE pkcol NOT IN (SELECT pkcol FROM A)|||

Thank you very much Alex!
One more question, I have millions of data in the table, will the 2 queries work well for such huge data
.I mean performance wise. I have created indexes .

|||You'll just have to try it to find out. No way of guessing that without knowing what the tables are, how the indexes are, hardware, load, etc ...