In case of incomplete database tables, a possible world is obtained by replacing any missing value by a value from the corresponding attribute's domain that can be infinite. A possible key or possible functional dependency constraint is satisfied by an incomplete table if we can obtain a possible world that satisfies the given key or functional dependency. On the other hand, a certain key or certain functional dependency holds if all possible worlds satisfy the constraint, A strongly possible constraint is an intermediate concept between possible and certain constraints, based on the strongly possible world approach (a strongly possible world is obtained by replacing \nul's by a value from the ones appearing in the corresponding attribute of the table). A strongly possible key or functional dependency holds in an incomplete table if there exists a strongly possible world that satisfies the given constraint. In the present paper, we introduce strongly possible versions of multivalued dependencies and cross joins, and we analyse the complexity of checking the validity of a given strongly possible cross joins. We also study approximation measures of strongly possible keys (spKeys), functional dependencies (spFDs), multivalued dependencies (spMVDs) and cross joins (spCJs). We also treat complexity questions of determination of the approximation values.

翻译：在不完全数据库表的情况下，通过将任何缺失值替换为对应属性域中可能无限的值，即可获得一个可能世界。若存在一个可能世界满足给定的键或函数依赖关系，则称该不完全表满足可能键或可能函数依赖约束。另一方面，若所有可能世界均满足该约束，则称确定键或确定函数依赖成立。强可能约束是介于可能约束与确定约束之间的概念，基于强可能世界观（通过将空值替换为表中对应属性已出现值之一获得）。若存在满足给定约束的强可能世界，则称该不完全表满足强可能键或函数依赖。本文引入强可能版本的多值依赖与交叉连接算子，并分析给定强可能交叉连接有效性的判定复杂度。同时研究强可能键（spKey）、函数依赖（spFD）、多值依赖（spMVD）及交叉连接（spCJ）的近似度量，并探讨近似值确定问题的复杂度。