Merge Tables in SQL 2005

-- Create Database
CREATE DATABASE TUTORIAL
GO

-- create tables
CREATE TABLE [dbo].[Table1](
 [id] [int] NULL,
 [field1] [varchar](50) 
)
GO
CREATE TABLE [dbo].[Table2](
 [id] [int] NULL,
 [field2] [varchar](50) 
)
GO
CREATE TABLE [dbo].[Table3](
 [id] [int] NULL,
 [field3] [varchar](50) 
)
GO
CREATE TABLE [dbo].[destination](
 [id] [int] NULL,
 [field1] [varchar](50) NULL,
 [field2] [varchar](50) NULL,
 [field3] [varchar](50) NULL,
 [current] [bit] NULL
)
GO

-- Insert Data
INSERT INTO dbo.Table1(id, field1)
VALUES (1,'a')
GO
INSERT INTO dbo.Table1(id, field1)
VALUES (3,'g')
GO
INSERT INTO dbo.Table1(id, field1)
VALUES (4,'j')
GO
INSERT INTO dbo.Table2(id, field2)
VALUES (1,'b')
GO
INSERT INTO dbo.Table2(id, field2)
VALUES (2,'e')
GO
INSERT INTO dbo.Table2(id, field2)
VALUES (4,'k')
GO
INSERT INTO dbo.Table3(id, field3)
VALUES(1,'c')
GO
INSERT INTO dbo.Table3(id, field3)
VALUES(2,'f')
GO
INSERT INTO dbo.Table3(id, field3)
VALUES(3,'i')
GO
INSERT INTO dbo.destination (id, field1, field2, field3, [current])
VALUES (2,'d','e','f', 'True')
GO
INSERT INTO dbo.destination (id, field1, field2, field3, [current])
VALUES (3,'g','h','i', 'True')
GO
INSERT INTO dbo.destination (id, field1, field2, field3, [current])
VALUES (4,'j','k','l', 'True')
GO

 -- Also we need to create a function similar to a NVL2 function in ORACLE.
CREATE FUNCTION [dbo].[nvl2]
(
 @business_key varchar(100),
 @source_value varchar(100),
 @destination_value varchar(100)
)
RETURNS varchar(100)
AS
BEGIN
 DECLARE @return varchar(100);
 IF @business_key IS NULL
 SET @return = @destination_value
 ELSE
 SET @return = @source_value
 RETURN @return
END

 -- Now the source query to achieve this merge is:
with cte_keys
AS
(
select id
from table1
union
select id
from table2
union
select id
from table3
)

select
a.id
-- using isnull
,isnull(one.field1,dest.field1) as isnull_field1 
-- using coalesce
,coalesce(one.field1,dest.field1) as coalesce_field1
-- using function dbo.nvl2(src key, src field, dest field)
,dbo.nvl2(one.id, one.field1, dest.field1) as fnc_field1

,isnull(two.field2,dest.field2) as isnull_field2
,coalesce(two.field2,dest.field2) as coalesce_field2
,dbo.nvl2(two.id, two.field2, dest.field2) as fnc_field2

,isnull(three.field3,dest.field3) as isnull_field3
,coalesce(three.field3,dest.field3) as coalesce_field3
,dbo.nvl2(three.id, three.field3, dest.field3) as fnc_field3

from
cte_keys a
Left Join table1 one on a.id = one.id
left join table2 two on a.id = two.id
left join table3 three on a.id = three.id
left join destination dest on a.id = dest.id and [current] = 'True'