Dependency rules define the many-to-one relationships between attributes (fields/columns) in a single entity. They consist of a left-hand side (LHS) which is composed of one or more attributes, and a right-hand side (RHS) composed of a single attribute.
In a dependency rule, the "LEFT-HAND side" or LHS attributes determines the "RIGHT-HAND side" or RHS attribute, represented as LHS g RHS.
For all dependency rules, the value in the LHS attribute(s) always determines the value in the RHS attribute. Certain dependency rules may be bilateral (LHS g RHS AND LHS f RHS), but only if the data supports this. The majority of dependency rules are LHS g RHS only.
- Example
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Say you have the dependency CUSTOMER_ID g CUSTOMER_NAME.
For the dependency to be 100% TRUE, the distinct value in CUSTOMER_ID would always result in the same value for CUSTOMER_NAME.
It is acceptable for different values of CUSTOMER_ID to identify the same values in CUSTOMER_NAME.
However, the dependency CUSTOMER_ID g CUSTOMER_NAME does not mean that the rule CUSTOMER_NAME g CUSTOMER_ID is also TRUE.
While it is possible for a dependency to be bilateral, that is the value in the RHS attribute likewise determining the LHS attribute, the data must support this.