In the world of databases, data management involves not only adding and retrieving records but also deleting outdated or redundant information. When data resides across multiple tables with relationships, the need for DELETE queries with JOIN arises. In this article, we’ll explore the intricacies of crafting efficient DELETE statements using JOIN operations in SQL, illustrated with practical examples. The examples from this article are based on an Open Banking project, which is includes consents and other specific entities.

What Is a DELETE Query with JOIN?

A DELETE query with JOIN allows you to remove records from a table based on criteria that involve related data in one or more other tables. This is especially useful when working with relational databases where tables are linked via foreign keys.

Common Scenarios for DELETE with JOIN

1. Cascading Deletes: Removing records in a child table based on conditions in a parent table.
2. Cleaning Up Redundant Data: Deleting orphaned rows that no longer have a corresponding entry in a related table.
3. Targeted Data Removal: Deleting rows in one table where a condition is met in another related table.

Syntax for DELETE with JOIN

Here’s the general syntax for a DELETE query with JOIN:

DELETE o
FROM orders o
JOIN customers c ON o.customer_id = c.id
WHERE c.status = ‘inactive’;

• target_table: The table from which rows will be deleted.
• related_table: The table that provides the conditions for deletion.
• ON: Specifies the relationship between the tables.
• WHERE: Defines the criteria for deletion.

Practical Examples

1. Deleting Based on a Condition in Another Table

Suppose you have two tables: orders and customers. You want to delete orders for customers who are no longer active.

DELETE o
FROM orders o
JOIN customers c ON o.customer_id = c.id
WHERE c.status = 'inactive';

2. Removing Orphaned Records

Consider tables payments and orders. You want to delete payments for orders that no longer exist.

DELETE p
FROM payments p
LEFT JOIN orders o ON p.order_id = o.id
WHERE o.id IS NULL;

Advanced Example: Using DELETE with Multiple Joins

Here’s an example where three tables are involved. You want to delete records from access_restricted_to based on conditions in bank and consent tables.

DELETE art
FROM access_restricted_to art
JOIN consent c ON art.consent_entity_id = c.id
JOIN bank b ON c.bank_id = b.id
WHERE b.financial_institution_type = 1
AND art.access_restricted_to = 'SVGS';

Explanation:

• JOIN ensures only matching rows across access_restricted_to, consent, and bank are considered.
• The WHERE clause filters the rows to delete only those that match the specified criteria.

Best Practices for DELETE Queries with JOIN

• Test Before You Delete: Use SELECT to preview rows targeted for deletion:

SELECT art.*
FROM access_restricted_to art
JOIN consent c ON art.consent_entity_id = c.id
WHERE c.bank_id = '1';

• Backup Your Data: Always create a backup before executing DELETE queries in production.
• Use Transactions: Wrap DELETE operations in a transaction to ensure you can rollback if something goes wrong:

BEGIN TRANSACTION;
DELETE FROM access_restricted_to ...
ROLLBACK; -- Or COMMIT if everything looks good

• Optimize Performance:
• • Use indexes on columns involved in JOIN and WHERE clauses.
  • Avoid unnecessary joins by filtering early.

Conclusion

DELETE queries with JOIN are powerful tools for maintaining data integrity and cleaning up relational databases. By understanding the syntax and following best practices, you can confidently perform complex deletions while safeguarding your data. Whether you’re removing orphaned records or cleaning up related tables, the examples in this article provide a solid foundation for crafting efficient and reliable queries.

Remember: Always test and validate your queries in a non-production environment before applying them to live data. This ensures that your DELETE operations achieve the desired results without unintended consequences.