Update table row with certain id while deleting the recurrent row - sql

Question

I have 2 tables
Table name: Attributes
attribute_id | attribute_name
1 attr_name_1
2 attr_name_2
3 attr_name_1
4 attr_name_2
Table name: Products
product_id | product_name | attribute_id
1 prod_name_1 1
2 prod_name_2 2
3 prod_name_3 3
4 prod_name_4 4
If you can see, attribute_id in the table Products has the following id's (1,2,3,4), instead of (1,2,1,2).
The problem is in the table Attributes, namely, there are repeating values(attribute_names) with different ID, so I want:
To pick One ID of the repeating, from the table Attributes
Update the table Products with that "picked" ID(only in cases that attribute_id has same name in the table Attributes)
And after that, delete the repeating values from the table Attributes witch has no use in the table Products
Output:
Table name: Attributes
attribute_id | attribute_name
1 attr_name_1
2 attr_name_2
Table name: Products
product_id | product_name | attribute_id
1 prod_name_1 1
2 prod_name_2 2
3 prod_name_3 1
4 prod_name_4 2
Demo on SQLFiddle
Note:
it will help me a lot if i use sql instead fixing this issue manually.

Answer 1

update Products
set attribute_id = (
select min(attribute_id)
from Attributes a
where a.attribute_name=(select attribute_name from Attributes a2 where a2.attribute_id=Products.attribute_id)
);
DELETE
FROM Attributes
WHERE attribute_id NOT IN
(
SELECT MIN(attribute_id)
FROM Attributes
GROUP BY attribute_name
);

Answer 2

The following may be faster than #Alexander Sigachov's suggestion, but it does require at least SQL Server 2005 to run it, while Alexander's solution would work on any (reasonable) version of SQL Server. Still, even if only for the sake of providing an alternative, here you go:
WITH Min_IDs AS (
SELECT
attribute_id,
min_attribute_id = MIN(attribute_id) OVER (PARTITION BY attribute_name)
FROM Attributes
)
UPDATE p
SET p.attribute_id = a.min_attribute_id
FROM Products p
JOIN Min_IDs a ON a.attribute_id = p.attribute_id
WHERE a.attribute_id <> a.min_attribute_id
;
DELETE FROM Attributes
WHERE attribute_id NOT IN (
SELECT attribute_id
FROM Products
WHERE attribute_id IS NOT NULL
)
;
The first statement's CTE returns a row set where every attribute_id is mapped to the minimum attribute_id for the same attribute_name. By joining to this mapping set, the UPDATE statement uses it to replace attribute_ids in the Products table.
When subsequently deleting from Attributes, it is enough just to check if Attributes.attribute_id is not found in the Products.attribute_id column, which is what the the second statement does. That is to say, grouping and aggregation, as in the other answer, is not needed at this point.
The WHERE attribute_id IS NOT NULL condition is added to the second query's subquery in case the column is nullable and may indeed contain NULLs. NULLs need to be filtered out in this case, or their presence would result in the NOT IN predicate's evaluation to UNKNOWN, which SQL Server would treat same as FALSE (and so no row would effectively be deleted). If there cannot be NULLs in Products.attribute_id, the condition may be dropped.