Explain the following SQL MERGE statement.

Question

Assisting AI · Accepted Answer

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.

Explain the following SQL MERGE statement.

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