Resolving Fan Traps in Database Modeling: Introducing Bridge Tables to Prevent Duplicate Rows

fan trap

A fan trap is a concept in database modeling and query optimization that occurs when designing a database schema that contains relationships between tables

A fan trap is a concept in database modeling and query optimization that occurs when designing a database schema that contains relationships between tables. Specifically, a fan trap arises in an entity-relationship model when you have a relationship between three entity sets where two one-to-many relationships share a common entity set.

To better understand a fan trap, let’s consider an example scenario. Let’s say we have three entity sets: “Customer,” “Order,” and “Product.” A customer can place multiple orders, and each order can involve multiple products. The relationships can be represented as follows:

– Customer to Order: One customer can have many orders. This is a one-to-many relationship.
– Order to Customer: One order can have only one customer. This is a many-to-one relationship.
– Order to Product: One order can involve multiple products. This is a one-to-many relationship.
– Product to Order: One product can be associated with multiple orders. This is a many-to-one relationship.

Now, let’s consider a query where we want to retrieve information about customers who have purchased a particular product. The query would involve joining the Customer, Order, and Product tables.

However, in our example scenario, we have a fan trap. The fan trap occurs because there are multiple possible paths to reach the Product table from the Customer table:

1. Customer -> Order -> Product
2. Customer -> Order <- Product If we simply join the tables using the first path, we might encounter an issue. This happens when a customer has placed multiple orders, each involving the same product. In that case, the join would result in duplicate rows for the customer. To avoid a fan trap, we need to consider the concept of the "bridge table" or the "association table." This table provides an indirect connection between the entities involved in a fan trap situation and prevents duplicate rows in the result set. In our example, we could introduce an intermediate table called "Order_Product" or "Order_Item" with the fields OrderID and ProductID. This bridge table resolves the fan trap issue. Now, when we want to retrieve information about customers who have purchased a particular product, we can utilize the bridge table to avoid duplicate rows. The modified query would involve joining the Customer, Order, Order_Product, and Product tables: Customer -> Order -> Order_Product -> Product

By using this path, we ensure that each customer is associated with the product they have purchased, eliminating any duplicate rows.

In conclusion, a fan trap occurs in database modeling when two one-to-many relationships share a common entity set, leading to potential duplicate rows in query results. By introducing a bridge table in such cases, we can resolve the fan trap issue and retrieve accurate and non-duplicated information from the database.

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