25. A method of restructuring a relational database having at least one relation, each said relation having a plurality of tuples, a set of attributes, a set of candidate keys and a set of non-key attributes; wherein each tuple is managed as a two-dimensional relation for each attribute and each relation has a candidate key serving as a set of attributes which can uniquely identify one of said tuples, comprising the steps of: a) checking, with reference to values of tuples which are present in the relations, whether a first set of attributes is functionally dependent on a second set of attributes; b) dividing a first relation into a second and a third relation by a projecting operation for a designated attribute set; c) performing said steps a) and b) in order to determine from attribute sets A and B which are included in said relation and are mutually primary, when said attribute set A is a proper subset of said set of candidate keys and when said attribute set B is a subset of a non-key attribute set serving as a subset of the candidate key; and d) performing said steps a) and b) when said attribute sets A and B are proper subsets of said non-key attribute set in said attribute sets A and B which are mutually primary and included in a relation.
25. A method of restructuring a relational database having at least one relation, each said relation having a plurality of tuples, a set of attributes, a set of candidate keys and a set of non-key attributes; wherein each tuple is managed as a two-dimensional relation for each attribute and each relation has a candidate key serving as a set of attributes which can uniquely identify one of said tuples, comprising the steps of: a) checking, with reference to values of tuples which are present in the relations, whether a first set of attributes is functionally dependent on a second set of attributes; b) dividing a first relation into a second and a third relation by a projecting operation for a designated attribute set; c) performing said steps a) and b) in order to determine from attribute sets A and B which are included in said relation and are mutually primary, when said attribute set A is a proper subset of said set of candidate keys and when said attribute set B is a subset of a non-key attribute set serving as a subset of the candidate key; and d) performing said steps a) and b) when said attribute sets A and B are proper subsets of said non-key attribute set in said attribute sets A and B which are mutually primary and included in a relation. 29. A method according to claim 25, wherein step b) further comprises the step of informing an operator of a result determined in step a), inputting an enable/disable division instruction from the operator, and controlling division processing in accordance with the instruction.