Update SQL table with random value from other table

On Microsoft SQL Server 2008, I have a table with Products:

Id | Name | DefaultImageId

And one with Images:

Id | ProductId | Bytes

I want to run an Update statement, that updates the DefaultImageId on all records in the Products table with a random Id from the Images table that is related to the Product via the ProductId column.

Can anyone help out? Should be simple for any SQL Champ (Which is obviously not me)..

You can do an order by on a NEWID to get a random number for every row of your update.

UPDATE
    Products
SET
    DefaultImageId =
    (
        SELECT TOP 1
            Id
        FROM
            Images
        WHERE
            Images.ProductId = Products.Id
        ORDER BY
            NEWID()
    )

This has been down marked and comments added indicating it does not solve the problem. I think the confusion has come from where people have not realised the original question requests a random image be selected for each product, hence the where clause with Product Id. Have provided a full script with data set below. It adds five products and three images for each product. Then randomly sets the default image id for each product.

CREATE TABLE Products(Id INT, Name NVARCHAR(100), DefaultImageId INT NULL)

CREATE TABLE Images (Id INT, ProductId INT, Bytes VARBINARY(100))

INSERT INTO Products (Id, NAME, DefaultImageId) VALUES(1, 'A', NULL)
INSERT INTO Products (Id, NAME, DefaultImageId) VALUES(2, 'B', NULL)
INSERT INTO Products (Id, NAME, DefaultImageId) VALUES(3, 'C', NULL)
INSERT INTO Products (Id, NAME, DefaultImageId) VALUES(4, 'D', NULL)
INSERT INTO Products (Id, NAME, DefaultImageId) VALUES(5, 'E', NULL)

INSERT INTO Images (Id, ProductId, Bytes) VALUES(1, 1, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(2, 1, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(3, 1, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(4, 2, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(5, 2, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(6, 2, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(7, 3, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(8, 3, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(9, 3, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(10, 4, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(11, 4, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(12, 4, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(13, 5, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(14, 5, NULL)
INSERT INTO Images (Id, ProductId, Bytes) VALUES(15, 5, NULL)

UPDATE
    Products
SET
    DefaultImageId =
    (
        SELECT TOP 1
            Id
        FROM
            Images
        WHERE
            Images.ProductId = Products.Id
        ORDER BY
            NEWID()
    )

SELECT * FROM Products

Addressing @philreed’s issue with the selected answer:

Is there a way to assign each row being updated with a different value
randomly chosen from the source table?

UPDATE Products
SET DefaultImageId = t2.Id
FROM Products t1
CROSS APPLY (
    SELECT TOP 1 Id
    FROM Images
    WHERE t1.Id = t1.Id
    ORDER BY newid()
    ) t2    

Another possible solution

UPDATE
    Products
SET
    DefaultImageId =
    (
        SELECT TOP 1
            Id
        FROM
            Images
        ORDER BY
            NEWID(), Products.Id
    )

Try this one (on AdventureWorks):

It updates all rows of the person table with a random name from the product table.

/*select rows from both tables in random order and match records by row number */
update person.person
set 
FirstName = otherTable.Name
from 
(select BusinessEntityID, row_number() over (order by newid()) as row_num from person.person) tableWithRownum
,(select ProductId, Name, row_number() over (order by newid()) as row_num from production.product) as otherTable
where 
person.person.BusinessEntityID=tableWithRownum.BusinessEntityID 
and (tableWithRownum.row_num%500)=otherTable.row_num /*match by row number;*/
/* optional: correct for different table sizes with modulo operation*/

Notes:

  • The new values are not sampled independently for each row.
  • Since the random order is determined only once, all entries occur at roughly the same rate (depending on table sizes: If both tables have the same number of rows, each entry should occur exactly once).
  • This behaviour might not fit every use case, but it should help performance…

Check this out.

Update Products
Set DefaultImageId = (
SELECT top 1 Id 
From Images 
Where 1=1
and Products.Id = Images.ProductId
ORDER BY NEWID()
)

declare @carg_id as int
DECLARE cursor_cargo CURSOR FOR 
    SELECT carg_id FROM cargo

OPEN cursor_cargo

FETCH NEXT FROM cursor_cargo
INTO @carg_id
WHILE  @@FETCH_STATUS =0
BEGIN

    update cargo set ciud_id = (
    select top 1 ciud_id from ciudad 
    inner join Pais on Pais.pais_id = ciudad.pais_id
    where ciudad.pais_id is not null and empr_id is  null and pais_status =1 order by  NEWID()) WHERE CARG_ID = @CARG_ID

    FETCH NEXT FROM cursor_cargo
        INTO @carg_id
END
CLOSE cursor_cargo
DEALLOCATE cursor_cargo

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