Union All but keep only duplicates from one table in T-SQL - sql

Question

I have two table which I would like to union. I need to keep only the duplicates from one of the two tables. I tried to find it, but could not find it anywhere. Hope somebody can help.
For example:
Table_1:
ID
Product
Amount
1
A
10
2
B
10
3
C
10
Table_2:
ID
Product
Amount
3
C
9
4
A
100
5
B
100
Desired result:
ID
Product
Amount
1
A
10
2
B
10
3
C
9
4
A
100
5
B
100
So always use the duplicates from table_2. In this example ID 3 is duplicate, so use the duplicate of table_2 with amount 9.
How to realize this with T-SQL? I used the code below:
Select * from Table_1 where Table_1.id != Table_2.id
Union All
Select * from Table_2
But then I receive the error:
'The multi-part identifier "Table_2.ID" could not be bound.'

Answer 1

Use not exists:
Select t1.*
from Table_1 t1
where not exists (select 1 from table_2 t2 where t2.id = t1.id)
Union All
Select t2.*
from Table_2 t2;

Answer 2

Try this:
SELECT T1.*
FROM #Table1 T1
WHERE T1.ID NOT IN (SELECT ID FROM #Table2)
UNION
SELECT T2.*
FROM #Table2 T2

Answer 3

I assume what you want is an EXISTS:
SELECT T1.ID,
T1.Product,
T1.Amount
FROM dbo.Table1 T1
WHERE NOT EXISTS (SELECT 1
FROM dbo.Table2 T2
WHERE T1.ID = T2.ID)
UNION ALL
SELECT T2.ID,
T2.Product,
T2.Amount
FROM dbo.Table2 T2;
A FULL OUTER JOIN, however, might also work if ID is unique in both tables:
SELECT ISNULL(T2.ID,T1.ID) AS ID,
ISNULL(T2.Product,T1.Product) AS Product,
ISNULL(T2.Amount,T1.Amount) AS Amount
FROM dbo.Table1 T1
FULL OUTER JOIN dbo.Table2 T2 ON T1.ID = T2.ID;

Answer 4

Union will give you the result. Union will always return unique values always. If you use union all you will get all with duplicates. Your answer would be to use union all.
SELECT
B.ID
,B.Product
,B.Amount
FROM
(
SELECT
A.ID
,A.Product
,A.Amount
,ROW_NUMBER() over (Partition BY ID, Product order by Amount ASC) AS [row_num]
FROM
(
SELECT
tb_1.*
FROM tb_1
UNION ALL
SELECT
tb_2.*
FROM tb_2
) AS A
) AS B
WHERE B.[row_num] = 1