Sequential numbers randomly selected and added to table - sql

Question

The SO Question has lead me to the following question.
If a table has 16 rows I'd like to add a field to the table with the numbers 1,2,3,4,5,...,16 arranged randomly i.e in the 'RndVal' field for row 1 this could be 2, then for row 2 it could be 5 i.e each of the 16 integers needs to appear once without repetition.
Why doesn't the following work? Ideally I'd like to see this working then to see alternative solutions.
This creates the table ok:
IF OBJECT_ID('tempdb..#A') IS NOT NULL BEGIN DROP TABLE #A END
IF OBJECT_ID('tempdb..#B') IS NOT NULL BEGIN DROP TABLE #B END
IF OBJECT_ID('tempdb..#C') IS NOT NULL BEGIN DROP TABLE #C END
IF OBJECT_ID('tempdb..#myTable') IS NOT NULL BEGIN DROP TABLE #myTable END
CREATE TABLE #B (B_ID INT)
CREATE TABLE #C (C_ID INT)
INSERT INTO #B(B_ID) VALUES
(10),
(20),
(30),
(40)
INSERT INTO #C(C_ID)VALUES
(1),
(2),
(3),
(4)
CREATE TABLE #A
(
B_ID INT
, C_ID INT
, RndVal INT
)
INSERT INTO #A(B_ID, C_ID, RndVal)
SELECT
#B.B_ID
, #C.C_ID
, 0
FROM #B CROSS JOIN #C;
Then I'm attempting to add the random column using the following. The logic is to add random numbers between 1 and 16 > then to effectively overwrite any that are duplicated with other numbers > in a loop ...
SELECT
ROW_NUMBER() OVER(ORDER BY B_ID) AS Row
, B_ID
, C_ID
, RndVal
INTO #myTable
FROM #A
DECLARE #rowsRequired INT = (SELECT COUNT(*) CNT FROM #myTable)
DECLARE #i INT = (SELECT #rowsRequired - SUM(CASE WHEN RndVal > 0 THEN 1 ELSE 0 END) FROM #myTable)--0
DECLARE #end INT = 1
WHILE #end > 0
BEGIN
SELECT #i = #rowsRequired - SUM(CASE WHEN RndVal > 0 THEN 1 ELSE 0 END) FROM #myTable
WHILE #i>0
BEGIN
UPDATE x
SET x.RndVal = FLOOR(RAND()*#rowsRequired)
FROM #myTable x
WHERE x.RndVal = 0
SET #i = #i-1
END
--this is to remove possible duplicates
UPDATE c
SET c.RndVal = 0
FROM
#myTable c
INNER JOIN
(
SELECT RndVal
FROM #myTable
GROUP BY RndVal
HAVING COUNT(RndVal)>1
) t
ON
c.RndVal = t.RndVal
SET #end = ##ROWCOUNT
END
TRUNCATE TABLE #A
INSERT INTO #A
SELECT
B_ID
, C_ID
, RndVal
FROM #myTable
If the original table has 6 rows then the result should end up something like this
B_ID|C_ID|RndVal
----------------
| | 5
| | 4
| | 1
| | 6
| | 3
| | 2

Answer 1

I don't understand your code, frankly
This will update each row with a random number, non-repeated number between 1 and the number of rows in the table
UPDATE T
SET SomeCol = T2.X
FROM
MyTable T
JOIN
(
SELECT
KeyCol, ROW_NUMBER() OVER (ORDER BY NEWID()) AS X
FROM
MyTable
) T2 ON T.KeyCol = T2.KeyCol
This is more concise but can't test to see if it works as expected
UPDATE T
SET SomeCol = X
FROM
(
SELECT
SomeCol, ROW_NUMBER() OVER (ORDER BY NEWID()) AS X
FROM
MyTable
) T

Answer 2

When you add TOP (1) (because you need to update first RndVal=0 record) and +1 (because otherwise your zero mark means nothing) to your update, things will start to move. But extremely slowly (around 40 seconds on my rather outdated laptop). This is because, as #myTable gets filled with generated random numbers, it becomes less and less probable to get missing numbers - you usually get duplicate, and have to start again.
UPDATE top (1) x
SET x.RndVal = FLOOR(RAND()*#rowsRequired) + 1
FROM #myTable x
WHERE x.RndVal = 0
Of course, #gbn has perfectly valid solution.

Answer 3

This is basically the same as the previous answer, but specific to your code:
;WITH CTE As
(
SELECT B_ID, C_ID, RndVal,
ROW_NUMBER() OVER(ORDER BY NewID()) As NewOrder
FROM #A
)
UPDATE CTE
SET RndVal = NewOrder
SELECT * FROM #A ORDER BY RndVal