radgrid header as a dynamic data from the database how can i declare it ?

Suppose table1 is having colmn_name with value A B C.
Table 2 is having column A B C with some values.

First fire query on table1 to get columns in variabl.
Use comma separated login for that.

Declare @SelectColum  varchar(max)

SELECT @SelectColum = SUBSTRING(
(SELECT ',' + s.colmn_name
FROM table1 s
ORDER BY s.colmn_name
FOR XML PATH('')),2,200000) AS CSV

-- @SelectColum variable having your required column
Select @SelectColum from table2

You dont need to do any coding in C#. Just use above logic in sql to get data.
If still having doubt then explore your database design. (What is relation in table?
column name? What output you want?