Relational Database Standardization Theory & amp; function dependency and Paradigm Theory

Relational Database Standardization Theory & functional dependency and paradigm theory the relational model can be formally represented as R (U, D, dom, F); U: Attribute Set, D: indicates the attribute domain, dom: ing set of attributes like domains. F: indicates the dependency set of data between attributes. 1. function dependencies are defined as follows: assume that the relational mode R (U), X, and Y are the subsets of U. If any possible relational R in r (U, if two rows have the same value on X but different values on Y, the attribute subset X function determines the attribute subset Y, or the Y function depends on X, x-> Y. (1) dependencies between ordinary functions and non-ordinary functions: in the relational mode R (U), for the subset X and Y of U, if X --> Y, however, if Y is not a subset of X, X-> Y is called a non-trivial function dependency. If Y is a subset of X, X-> Y is called a trivial function dependency. Tip: For any relational mode, the dependency of ordinary functions is inevitable. (2) full function dependency and partial function dependency: in the relational mode R (U), if X-> Y, and for any real subset of x xx, if XX does not determine Y, all functions depend on X and are recorded as X-(F) Y. If X-> Y, but Y is not fully dependent on X, some functions of Y depend on X as X (P) Y. Tip: If Y is dependent on some functions of X, the "part" in X can determine the association with Y. From the perspective of data dependency, X has the "redundancy" attribute. In fact, some dependencies and transmission dependencies are two important reasons for redundancy and exceptions. (3) the transfer function dependency is in the relational mode R (U). If X-> Y, Y-> Z, and Y does not decide X, Z does not belong to X, the Z transfer function depends on X and is recorded as X-> (T) Z. Tip: If condition Y is required to pass the function dependency definition, it is not determined because if Y-> X is used, X <-> Y is used. In fact, Z is directly dependent on X, that is, direct function dependency, rather than passing function dependencies. 2. Paradigm Theory: 1NF: requires that attributes cannot be divided. The first paradigm is the minimum requirement for designing database tables. Such as contact: Electric ring: XXX; mailbox: liu@qq.com. This does not conform to 1NF. 2NF of the second paradigm: Based on 1NF, each non-primary attribute relies entirely on the candidate key. 3NF: Based on 2NF, all non-primary attributes in the relational mode R (U, F) do not have a transfer dependency on any candidate key. The relational R is the third paradigm.