SQL : getting Comma Separating the values in single field from table

tbl_test

ColA ColB
1        x
2        y
3        z
4        m
5        n


DECLARE @STR VARCHAR(1000)
SELECT @STR = (CASE WHEN @STR IS NULL THEN ColB ELSE @STR + ', ' + ColB END)
FROM tbl_test

SELECT @STR AS ColB


ColB
x, y, z, m, n