SQL UPDATE by groups - sql-server

Question

Consider following table:
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[Product](
[ProductID] [int] IDENTITY(1,1) NOT NULL,
[ProductCategory] [int] NOT NULL,
[ProductCategoryGuid] [uniqueidentifier] NULL,
CONSTRAINT [PK_Product] PRIMARY KEY CLUSTERED
(
[ProductID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
SET IDENTITY_INSERT [dbo].[Product] ON
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (1, 2, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (2, 2, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (3, 2, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (4, 3, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (5, 4, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (6, 2, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (7, 3, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (8, 4, NULL)
GO
INSERT [dbo].[Product] ([ProductID], [ProductCategory], [ProductCategoryGuid]) VALUES (9, 4, NULL)
GO
SET IDENTITY_INSERT [dbo].[Product] OFF
GO
Data looks like this:
ProductID ProductCategory ProductCategoryGuid
1 2 NULL
2 2 NULL
3 2 NULL
4 3 NULL
5 4 NULL
6 2 NULL
7 3 NULL
8 4 NULL
9 4 NULL
What I would like to achieve is to update [ProductCategoryGuid] column so that all rows with the same value for [ProductCategory] will have the same Guid value in [ProductCategoryGuid] column
To clarify:
Guid values will be generated as a part of the UPDATE query using NEWID() function
Rows with ProductID IN( 1, 2, 3, 6) will have Guid1
Rows with ProductID IN( 4,7) will have Guid2
Rows with ProductID IN( 5,8,9) will have Guid3

Answer 1

I would use following script that use a table variable to store list of distinct categories. The same table variable has a GUID column having as default value NEWID() function. At the end of the script there is an UPDATE statement using table variable as source and dbo.Product table as target:
DECLARE #Results TABLE (
[ProductCategory] [int] NOT NULL,
[ProductCategoryGuid] [uniqueidentifier] NOT NULL DEFAULT (NEWID())
)
INSERT #Results (ProductCategory)
SELECT DISTINCT p.ProductCategory
FROM dbo.Product p
UPDATE p
SET ProductCategoryGuid = r.ProductCategoryGuid
OUTPUT deleted.ProductCategoryGuid, inserted.ProductCategoryGuid
FROM dbo.Product p
INNER JOIN #Results r ON p.ProductCategory = r.ProductCategory
Comment OUTPUT clause if you don't want to see the old and new values.
Update: one statement solution (it requires SQL2012+)
;WITH CteUpdateProduct
AS (
SELECT *, FIRST_VALUE(NewGUID) OVER(PARTITION BY ProductCategory ORDER BY ProductID) AS NewProductCategoryGuid
FROM (
SELECT p.*, NEWID() AS NewGUID
FROM dbo.Product p
) x
)
UPDATE CteUpdateProduct
SET ProductCategoryGuid = NewProductCategoryGuid
OUTPUT inserted.ProductID, inserted.ProductCategory, inserted.ProductCategoryGuid;

Answer 2

WITH productCategories as (
SELECT DISTINCT ProductCategory
FROM product
), productCategoriesWithGuid as (
SELECT ProductCategory, NEWID() ProductCategoryGuid
From productCategories
)
UPDATE product
SET ProductCategoryGuid = pc.ProductCategoryGuid
FROM Product p
JOIN productCategoriesWithGuid pc on p.ProductCategory = pc.ProductCategory
This query gets the distinct ProductCategories,
Creates a GUID for each of them,
And lastly updates the product table with the GUIDs
All in one statement.