Ответ 1
ALTER TABLE MyTable WITH CHECK
ADD CONSTRAINT CK_MyTable_ColumNulls CHECK (Column1 IS NOT NULL OR Column2 IS NOT NULL)
Как часть создания
CREATE TABLE MyTable (
Column1 int NULL,
Column2 int NULL,
CONSTRAINT CK_MyTable_ColumNulls CHECK (Column1 IS NOT NULL OR Column2 IS NOT NULL)
)