Dependency Preservation in DBMS: Definition and Examples

In database design, dependency preservation means that when a database is split into smaller tables, all original functional dependencies can still be enforced without needing to join tables. It ensures that constraints remain easy to check after decomposition.

How It Works

Dependency preservation is about keeping the rules that data must follow, even after breaking a big table into smaller ones. Imagine you have a big recipe book, and you split it into smaller booklets. Dependency preservation means you can still check each recipe's steps without needing to look at other booklets.

In databases, these rules are called functional dependencies. When you split a table, you want to make sure these dependencies are still easy to check in the smaller tables. If you lose this, you might have to join tables often, which slows down the system and makes it harder to keep data correct.

Example

This example shows a table with functional dependencies and how dependency preservation works after decomposition.

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Original Table: R(A, B, C)
Functional Dependencies: A -> B, B -> C

Decompose into:
R1(A, B)
R2(B, C)

Both dependencies A -> B and B -> C are preserved in R1 and R2 respectively.

Output

R1 enforces A -> B
R2 enforces B -> C
No need to join tables to check dependencies

When to Use

Dependency preservation is important when designing a database to keep data consistent without complex checks. Use it when decomposing tables during normalization to avoid expensive joins for enforcing rules.

For example, in a sales database, preserving dependencies helps quickly verify that each product code matches its price without joining multiple tables, improving performance and reliability.

Key Points

• Dependency preservation keeps functional dependencies enforceable after splitting tables.
• It avoids costly joins when checking data rules.
• Essential for efficient and reliable database normalization.
• Helps maintain data integrity easily.

Key Takeaways

Dependency preservation ensures functional dependencies remain enforceable after table decomposition.

It helps maintain data integrity without expensive table joins.

Use dependency preservation to improve database performance and simplify constraint checks.

It is a key goal during database normalization.