Lec-14: Foreign Key🔑 with On Delete Cascade with Execution
Understanding the Purpose of ON DELETE CASCADE in Foreign Key Constraints
Introduction to ON DELETE CASCADE
- The session introduces a critical question regarding the purpose of "ON DELETE CASCADE" in foreign key constraints, emphasizing its popularity and importance in database management.
Referential Integrity Explained
- The concept of referential integrity is highlighted as a primary goal when using foreign keys, ensuring that relationships between tables remain consistent.
- An example is provided with an employee table acting as a parent table, referencing multiple child tables such as salary and dependent tables.
Deletion Scenarios
- The discussion shifts to deletion scenarios where deleting an entry from the parent table (employee table) can lead to issues if not handled properly.
- It explains that deleting an employee (e.g., "Van") necessitates removing related entries from child tables like salary and dependents to maintain data integrity.
Functionality of ON DELETE CASCADE
- "ON DELETE CASCADE" automatically deletes corresponding records in child tables when a record in the parent table is deleted, preserving referential integrity.
Practical Demonstration
- A practical demonstration begins with creating a salary detail table that references the employee table using "ON DELETE CASCADE."
- Data insertion into this new table illustrates how values must correspond correctly to avoid integrity constraint violations.
Error Handling During Insertion
- An error occurs when attempting to insert invalid data (e.g., ID 10 which does not exist in the parent table), showcasing how referential integrity constraints work.
Creating Dependent Table
- A dependent table is created similarly, also referencing the employee table with "ON DELETE CASCADE," reinforcing consistency across all related tables.
Final Setup Verification
- After setting up all three tables (employee, salary, and dependent), verification through selection queries confirms their correct setup and relationships.
Deletion Impact Analysis
- The focus returns to deletion operations; deleting from the employee table demonstrates cascading effects on both salary and dependent tables due to established foreign key constraints.
Conclusion on Referential Integrity Maintenance
- The session concludes by reiterating that "ON DELETE CASCADE" effectively maintains referential integrity by ensuring related records are automatically removed when necessary.
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