SQL语句可以限制字段不为空

SELECT DISTINCT p.CompanyId, dbo.ProductBrand.Id, dbo.ProductBrand.BrandName
FROM         dbo.ProductBDetail AS p LEFT OUTER JOIN
                      dbo.ProductBase AS pb ON p.PrtId = pb.Id LEFT OUTER JOIN
                      dbo.ProductBrand ON pb.BrandId = dbo.ProductBrand.Id
WHERE     (dbo.ProductBrand.Id IS NOT NULL)