What does dependency preservation mean in decomposition?

• A. Original functional dependencies can be inferred from decomposed schemas without relying on the original structure.
• B. Decomposition guarantees zero redundancy between all relations.
• C. Decomposition always yields fewer relations than the original.
• D. Integrity constraints become fully described by a single table.

Answer: (A)

Dependency preservation means you can enforce all the original functional dependencies after breaking a relation into smaller ones, using only constraints on the new pieces. In practice, you project the original set of dependencies onto each decomposed relation (for example, onto R1, R2, etc.). If the union of these projected dependencies implies the original dependencies (i.e., the closure of the projected constraints equals the original set), then the decomposition preserves dependencies. This is why the correct statement says that original functional dependencies can be inferred from the decomposed schemas without needing the original structure.

For example, splitting a relation into two parts that each enforce their own dependencies, and where those local constraints together imply the original dependencies, achieves preservation. The other ideas—zero redundancy, fewer relations, or constraints living in a single table—do not define whether dependencies remain enforceable after decomposition.