-- Make a DB to demo in
Create database EduTest
GO

Use EduTest
GO

-- Create a table - this is the application's table
Create Table Table1 (
ID int, 
col1 varchar(10),
col2 varchar(10)
);

-- Create a table - this is the imported data
Create Table Table2 (
ID int, 
col1 varchar(10),
col2 varchar(10)
);

-- insert data into the 'reaal' table.
insert into Table1 (ID, col1, col2) values (1, 'some data', 'some data');
insert into Table1 (ID, col1, col2) values (2, 'some data', 'some data');
insert into Table1 (ID, col1, col2) values (3, 'some data', 'some data');
insert into Table1 (ID, col1, col2) values (4, 'some data', 'some data');

-- insert data into the 'import' table.  
insert into Table2 (ID, col1, col2) values (1, 'some data', 'some data');
insert into Table2 (ID, col1, col2) values (2, 'some data', 'some data');
insert into Table2 (ID, col1, col2) values (3, 'some data', 'new data');
insert into Table2 (ID, col1, col2) values (4, 'some data', 'new data');
insert into Table2 (ID, col1, col2) values (5, 'more data', 'more data');
insert into Table2 (ID, col1, col2) values (6, 'more data', 'more data');
GO

-- Have a look at table1 - 4 rows, each field containing 'some data' 
select * from Table1;

-- Have a look at table1 - 6 rows.  Note that:
--	- the first two are the same as the real data
--	- the second two are in the main table, but have different values
--  - the final two are new
select * from Table2;

-- Here's the SQL that does the work
-- update existing rows
update Table1
set Table1.col1 = Table2.col1,
	Table1.col2 = Table2.col2
from Table1
    inner join Table2 on
        Table1.ID = Table2.ID;

-- insert the new rows
INSERT INTO Table1 (ID, col1, col2)
    select Table2.ID, Table2.col1, table2.col2
    FROM Table2
	Where Table2.ID not in (select Table1.ID from Table1);


-- now check table1 and see what happened:	
select * from Table1;

-- Deleting from the main table isn't necessarily something you'd want to do, 
-- depends on how your app deals with integrity - you could do so with 
--	Delete from table1 where table1.ID not in (select table2.ID from table2);