SQL copying data with new foreign keys

3 min read 05-10-2024

Copying Data and Maintaining Foreign Key Integrity in SQL

Moving data between tables or databases while maintaining the relationships defined by foreign keys can be tricky. This article will break down the common challenges and provide practical solutions for copying data while preserving foreign key integrity.

The Scenario:

Imagine you have a database with two tables: Customers and Orders. Orders has a foreign key referencing Customers, meaning each order must be associated with a valid customer.

-- Customers table
CREATE TABLE Customers (
    CustomerID INT PRIMARY KEY,
    CustomerName VARCHAR(255)
);

-- Orders table
CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATE,
    FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
);

Now, let's say you need to create a copy of the Orders table called ArchivedOrders. You want to move the existing orders into this new table but maintain the link to the Customers table.

The Challenges:

Direct Copying: A simple INSERT INTO ... SELECT ... statement won't work because the foreign key constraint in ArchivedOrders will point to the original Customers table.
New Foreign Key: Creating a new foreign key constraint in ArchivedOrders that references the original Customers table will result in errors if the copied CustomerID values don't exist in the Customers table.

The Solutions:

Here are two common approaches to solve this problem:

1. Creating a New Customer Table:

This approach involves creating a new Customers table, copying the relevant customer data, and then referencing this new table in the foreign key constraint of ArchivedOrders.

-- Create a new Customers table for the ArchivedOrders
CREATE TABLE ArchivedCustomers (
    CustomerID INT PRIMARY KEY,
    CustomerName VARCHAR(255)
);

-- Copy Customer data
INSERT INTO ArchivedCustomers (CustomerID, CustomerName)
SELECT CustomerID, CustomerName
FROM Customers;

-- Create ArchivedOrders table with foreign key referencing ArchivedCustomers
CREATE TABLE ArchivedOrders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATE,
    FOREIGN KEY (CustomerID) REFERENCES ArchivedCustomers(CustomerID)
);

-- Copy Order data
INSERT INTO ArchivedOrders (OrderID, CustomerID, OrderDate)
SELECT OrderID, CustomerID, OrderDate
FROM Orders;

2. Maintaining the Original Customer Table:

This solution uses a technique called mapping. You create a mapping table that links the old and new CustomerID values. This allows you to maintain the existing foreign key relationship in Orders while using a new CustomerID in ArchivedOrders.

-- Create a mapping table
CREATE TABLE CustomerMapping (
    OriginalCustomerID INT,
    NewCustomerID INT,
    PRIMARY KEY (OriginalCustomerID, NewCustomerID)
);

-- Generate a new CustomerID for each record
-- Note: You can replace 'generate_new_id()' with your preferred method for generating unique IDs
INSERT INTO CustomerMapping (OriginalCustomerID, NewCustomerID)
SELECT CustomerID, generate_new_id() 
FROM Customers;

-- Create ArchivedOrders table with foreign key referencing Customers
CREATE TABLE ArchivedOrders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATE,
    FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)
);

-- Copy Order data using the mapping table
INSERT INTO ArchivedOrders (OrderID, CustomerID, OrderDate)
SELECT OrderID, cm.NewCustomerID, OrderDate
FROM Orders o
JOIN CustomerMapping cm ON o.CustomerID = cm.OriginalCustomerID;

Choosing the Best Solution:

The best approach depends on your specific needs:

If you need a separate, independent copy of both customers and orders, creating a new Customers table is the most straightforward solution.
If you want to maintain the original Customers table and link orders to it, using a mapping table is the recommended option.

Conclusion:

Copying data while respecting foreign key constraints can be complex. By understanding the potential challenges and using the appropriate techniques, you can ensure your data remains consistent and relationships are preserved. Whether you choose to create a new table or use a mapping approach, carefully consider your requirements and the best solution for your specific scenario.