Returning Random Row based on Subset of Data within Table

Question

Hi,
Please see below.  Running SQL Server 2008 R2.
Sample DDL:
CREATE TABLE [dbo].[TestPersons]
[TestPersonID] [int] NOT NULL IDENTITY(1,1),
[FirstName] [varchar](50) NULL,
[LastName] [varchar](50) NULL,
[AreaID] [varchar](50) NULL,
CONSTRAINT [PK_TestPersons_TestPersonID] PRIMARY KEY CLUSTERED ([TestPersonID] 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
Sample Data:
INSERT INTO
[dbo].[TestPersons] ([FirstName], [LastName], [AreaID])
VALUES
('Carlos', 'Matlock', 'A0009'),
('William', 'Rivas', 'A0001'),
('Kathryn', 'Rice', 'A0008'),
('John', 'Ball', 'A0009'),
('Robert', 'Barnhill', 'A0009'),
('Timothy', 'Stein', 'A0009'),
('Christopher', 'Smith', 'A0001'),
('Brian', 'Speakman', 'A0001'),
('Harold', 'Clark', 'A0009'),
('Tim', 'Henson', 'A0009'),
('Victor', 'Chilson', 'A0009')
The above insert statement is a small example of the data contained in this table.  Normally the table would contain several thousand rows.  We use the following query to replace actual data with random rows from our test table:
UPDATE
[P]
SET
[P].[FirstName] = [T].[FirstName],
[P].[LastName] = [T].[LastName],
[P].[AreaID] = [T].[AreaID]
FROM
[dbo].[Persons] [P]
INNER LOOP JOIN
[dbo].[TestPersons] [T] ON ([T].[TestPersonID] = (1 + ABS(CRYPT_GEN_RANDOM(8)%5000)))
This query works as it selects a random row from the entire set of data in the table.  However there are cases where we need to specify a restricted subset to randomize from.  For example, we may need to randomize data only for Persons with an AreaID
of A0001 or A0008.  So in that case, and using the sample data above, we would want the  randomization to only select from rows in TestPersons that have an AreaID of A0001 or A0008.  How would I go about accomplishing this?  I've tried
adding a WHERE clause but it seems it's ignored because of the INNER LOOP JOIN.  I've also tried including [P].[AreaID] = [T].[AreaID] in the join hint but to no avail.
I also realize having sample data with only the set that we need would solve this problem but for our needs we need a large test set as our randomization requirements depend on the situation.
Any assistance is greatly appreciated!
Best Regards
Brad

Answer 1

DECLARE @TestPersons TABLE (TestPersonID int NOT NULL IDENTITY(1,1), FirstName varchar(50), LastName varchar(50), AreaID varchar(50))
INSERT INTO @TestPersons (FirstName, LastName, AreaID)
VALUES ('Carlos', 'Matlock', 'A0009'), ('William', 'Rivas', 'A0001'), ('Kathryn', 'Rice', 'A0008'), ('John', 'Ball', 'A0009'), ('Robert', 'Barnhill', 'A0009'), ('Timothy', 'Stein', 'A0009'),
('Christopher', 'Smith', 'A0001'), ('Brian', 'Speakman', 'A0001'), ('Harold', 'Clark', 'A0009'), ('Tim', 'Henson', 'A0009'), ('Victor', 'Chilson', 'A0009')
;WITH subset AS (
SELECT ROW_NUMBER() OVER (ORDER BY TestPersonID) AS sID, *
FROM @TestPersons
WHERE FirstName LIKE '%e%'
SELECT *
FROM subset
WHERE sID = round((((SELECT COUNT(*) FROM subset) - 1 - 1)*rand())+1,0)
This will grab a random row from a subset (defined in the CTE).