Update with random number

Posted on 2013-02-06
Last Modified: 2013-02-11

I would like to randomize values in a table.
The table we call MainTable It has columns UserId, Username, ColM (int).

Then I have another table with the random values.
The table we call PoolTable. It has columns Id, PoolCol (int)

I would like to find random values for each UserId. I would like to replace ColM with the value of PoolCol.

Id=1, PoolCol=55
Id=2, PoolCol=73
Id=3, PoolCol=32

Id=1, UserName="John", ColM=5
Id=2, UserName="Bill",ColM=5
Id=3, UserName="Howard", ColM=5
Id=4, UserName="Michael",ColM=5

One example of result could be
Id=1, UserName="John", ColM=73
Id=2, UserName="Bill",ColM=32
Id=3, UserName="Howard", ColM=55
Id=4, UserName="Michael",ColM=73

How is best to do this?
Question by:johnkainn
  • 2
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LVL 50

Expert Comment

ID: 38861990
update u
  set colm = x.poolcol
  from (select x.*,row_number() over (order by newid()) as rn
                from maintable as x) as U
  inner join pooltable as x
 where u.colm<>x.poolcol

one method use the windowing function over to calculate a random row_number for each row in the main table by using newid()  to achieve a random order to the rows....
then join the row number to the id of the pool table and update...

Author Comment

ID: 38863422
Thank you. How would I modify this so that when there are more items in MainTable then in PoolTable each user would get a random item from the Pool table. (It could also be the other way around, i.e. more items in pool table then in MainTable).
I created temporary table so you can run this. I added 2 names to the Maintable . Below is also the result that comes when I run it so you understand what I mean.

DECLARE @MainTable table(UserId int, Username varchar(50),ColM int)
INSERT INTO @MainTable(UserId,Username,ColM)values(1,'John',5)
INSERT INTO @MainTable(UserId,Username,ColM)values(2,'Bill',5)
INSERT INTO @MainTable(UserId,Username,ColM)values(3,'Howard',5)
INSERT INTO @MainTable(UserId,Username,ColM)values(4,'Michael',5)
INSERT INTO @MainTable(UserId,Username,ColM)values(5,'Anne',5)
INSERT INTO @MainTable(UserId,Username,ColM)values(6,'Mary',5)

DECLARE @PoolTable table(Id int, PoolCol int)
INSERT INTO @PoolTable(Id,PoolCol)values(1,55)
INSERT INTO @PoolTable(Id,PoolCol)values(2,73)
INSERT INTO @PoolTable(Id,PoolCol)values(3,32)

update u
  set colm = x.poolcol
  from (select x.*,row_number() over (order by newid()) as rn
                from @MainTable as x) as U
  inner join @PoolTable as x
 where u.colm<>x.poolcol

SELECT * FROM @MainTable

Open in new window

The result if I run this is

1      John      5
2      Bill      73
3      Howard      32
4      Michael      55
5      Anne      5
6      Mary      5

You see that the random number is only updated for 3 users.
LVL 50

Accepted Solution

Lowfatspread earned 500 total points
ID: 38863931
a simple method would be to use the modulus of the number of rows in the pool table to conform the ranges...

update u
  set colm = x.poolcol
  from (select x.*
              ,((row_number() over (order by newid()))
                    % ((select count(*) from @pooltable) - 1))+1 as rn
                from @MainTable as x) as U
  inner join @PoolTable as x
 where u.colm<>x.poolcol

so rn generates numbers in the range 1 - number of rows in @pooltable

you may need to go a row_number() over (order by id) as rid  on the pooltable
and join rn to rid if there is the possibility that the pooltable ids would be sequential from 1..

Author Closing Comment

ID: 38876197
Thank you very much:-)

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