The existential conditions of reflectivity and transitivity under strong or weak holding are discussed. It is proved by graph algorithm that they are complete to NSD strong holding.

• 本文给出了空值环境下子集依赖的概念（强、弱保持），讨论了强、弱保持下的自反律、传递律的存在条件，在此基础上用图算法证明了它们关于NSD强保持是完备的。