Propagate Parent Key Changes via Cascade Trigger

Cascade Update Trigger using Stored Procedure

Write a PostgreSQL query to create a trigger that calls a stored procedure to cascade update a foreign key field in a child table when a parent table's primary key is changed.

Solution:

-- Create or replace a function named cascade_update that returns a trigger
CREATE OR REPLACE FUNCTION cascade_update() RETURNS TRIGGER AS $$
BEGIN
    -- Update the parent_id in child_table to match the new ID in the parent table
    UPDATE child_table
    SET parent_id = NEW.id
    WHERE parent_id = OLD.id;
    -- Return the updated row
    RETURN NEW;
END;
-- Specify the language used in the function as PL/pgSQL
$$ LANGUAGE plpgsql;
-- Create a trigger named trg_cascade_update on parent_table to cascade update child_table on primary key change
CREATE TRIGGER trg_cascade_update
-- Trigger after an update of the id column in parent_table
AFTER UPDATE OF id ON parent_table
-- Execute the cascade_update function for each affected row
FOR EACH ROW EXECUTE FUNCTION cascade_update();

Explanation:

• Purpose of the Query:
• The goal is to maintain referential integrity by updating related foreign key values in a child table when the parent table's primary key changes.
• This demonstrates using a stored procedure to perform a cascade update via a trigger.
• Key Components:
• The trigger listens for changes to the id column in the parent_table.
• The stored procedure updates the corresponding records in the child_table.
• Real-World Application:
• Useful in systems where changes in a master record must be propagated to dependent records automatically.

Notes:

• Verify that proper foreign key relationships exist between the tables.
• Ensure that recursive triggers are prevented if necessary.

For more Practice: Solve these Related Problems:

• Write a PostgreSQL query to create a trigger that cascades primary key changes to multiple child tables with composite keys.
• Write a PostgreSQL query to create a trigger that cascades updates only if the new primary key satisfies a specific validation rule.
• Write a PostgreSQL query to create a trigger that performs a cascade update and logs each update into a separate log table.
• Write a PostgreSQL query to create a trigger that conditionally cascades updates based on a check against a status flag.

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Previous PostgreSQL Exercise: Timestamp Update Trigger using Stored Procedure.