Coalesce in sql server to concatenate many rows into a single string

Evaluates the arguments in order and returns the current value of the first expression that initially does not evaluate to NULL.

DECLARE @str NVARCHAR(256)

       SELECT @str = COALESCE(@str + ', ', '') + cast(CatID AS NVARCHAR)

       FROM TableName

       WHERE ID = 1

       SELECT @str

END