I want populate with dummy values a table.

I want produce a random produced value "RANDOM_VALUE" for every row produced.

Random value should be a string and it is range might be only 'A' or 'B'.

The script should have the ability to list a number of row with value A or B at random produced

Here a good example of how should work:

ROW   RANDOM_VALUE
1     A
2     B
3     B
4     A
...   ...

Any ideas how to get it done? Help I'm very new in SQL thanks!

Rand() is just examined once per column so would be the same for those rows to obtain for this you should use NewId() as below.

SELECT CHAR(65+ABS(CHECKSUM(NEWID()))%2), RestOfCols
FROM YourTable

If you want the output exactly according to your question just join onto a large enough table. e.g.

WITH cte AS
(
SELECT 
      ROW_NUMBER() OVER (ORDER BY (SELECT 0)) AS ROW,
      CHAR(65+ABS(CHECKSUM(NEWID()))%2) AS RANDOM_VALUE
FROM sys.objects
)
INSERT INTO DummyTable
SELECT ROW,RANDOM_VALUE 
FROM cte 
WHERE ROW<= 4

for producing test data, you will probably find it valuable to possess deterministic data. Quite simply, any time you generate data, it is the same. This way it's simpler to breed bugs.

With this, you should use hashbytes() on the deterministic seed. Quite simply:

create function dbo.fn_RandishInt(@seed nvarchar(max), @min int, @max int)
returns int
as
begin
    declare @crc bigint
    declare @p float

    set @crc = cast(cast(hashbytes('md5', @seed) as int) as bigint) & 0xffffffff
    set @p = cast(@crc as float) / 4294967296

    return round(((@max - @min) * @p) + @min, 0)
end
go


SELECT
      ROW_NUMBER() OVER (ORDER BY (SELECT 0)) AS s,
      char(dbo.fn_RandishInt(ROW_NUMBER() OVER (ORDER BY (SELECT 0)), 65, 66)) AS t
FROM
    sys.objects

this can always yield exactly the same, random-searching, results.

Make your own Random number function such as this, that can be used instead of Rand().

CREATE VIEW dbo.RandomNumberView
AS
SELECT Rand() AS RandomNumber

GO

CREATE FUNCTION dbo.RandomNumber()
RETURNS float
AS
BEGIN
    RETURN (SELECT RandomNumber FROM dbo.RandomNumberView)
END
GO