Implementing Weak Entities in SQL for Dependent Data Relationships
Designing a Weak Entity for Dependent Data
Write a SQL query to design a weak entity for dependent data.
Solution:
-- Strong entity.
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
OrderDate DATE
);
-- Weak entity dependent on Orders.
CREATE TABLE OrderDetails (
OrderID INT,
ProductID INT,
Quantity INT,
PRIMARY KEY (OrderID, ProductID),
FOREIGN KEY (OrderID) REFERENCES Orders(OrderID)
);
Explanation:
- Purpose of the Query :
- The goal is to design a weak entity to represent data that depends on a strong entity.
- Key Components :
- OrderDetails: A weak entity dependent on the Orders table.
- Uses a composite primary key to include the foreign key (OrderID) from the strong entity.
- Why Use Weak Entities? :
- Weak entities represent data that cannot exist independently of a strong entity.
- They enforce referential integrity and reduce redundancy.
- Real-World Application :
- In e-commerce systems, order details depend on the existence of an order.
Notes:
- Weak entities always have a partial key (e.g., ProductID) in addition to the foreign key.
- Use them to model dependent relationships like order-line items or child records.
- Important Considerations:
- Ensure proper indexing on the composite primary key.
For more Practice: Solve these Related Problems:
- Write a SQL query to design a weak entity for a table storing invoice line items dependent on invoices.
- Write a SQL query to create a weak entity for a table managing reservations within a booking system.
- Write a SQL query to design a weak entity for a table storing comments dependent on blog posts.
- Write a SQL query to create a weak entity for a table tracking individual transactions within a bank account.
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Previous SQL Exercise: Designing a Recursive Relationship for Hierarchical Data.
Next SQL Exercise: Designing a Lookup Table for Categorical Data.
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