Explain the following SQL MERGE statement.

The MERGE statement synchronizes two tables by performing INSERT, UPDATE, or DELETE operations in a single statement. In the provided code:

• MERGE INTO customer_target USING customer_source establishes the target and source tables.
• ON customer_target.customer_id = customer_source.customer_id defines the join condition.
• WHEN MATCHED THEN UPDATE SET ... updates rows in the target when a matching key exists, copying the name and city from the source.
• WHEN NOT MATCHED THEN INSERT (customer_id, customer_name, city) VALUES (...) inserts new rows into the target when the key is absent in the target.
• WHEN NOT MATCHED BY SOURCE THEN DELETE removes rows from the target that no longer exist in the source.

This pattern is common in ETL pipelines to keep a target staging or reporting table in sync with a source system. It ensures atomicity and reduces round‑trips compared to separate SELECT/INSERT/UPDATE/DELETE statements.