SQL MERGE

Introduction to SQL MERGE

The SQL MERGE statement is a powerful feature that allows you to perform upserts, which combine the operations of INSERT and UPDATE. This is particularly useful when you need to update existing records in a table or insert new records if they do not exist.

The basic idea behind the MERGE statement is to merge the data from a source table into a target table based on a specified condition, often a match on a primary key or unique identifier.

Basic Syntax of SQL MERGE

The syntax of the SQL MERGE statement can vary slightly between different database vendors, but a general structure is as follows:

Example: Merging Customer Data

Let's consider an example where you have a table named Customers that needs to be updated with new data from a table named NewCustomers. The goal is to update existing customer records or insert new records if they do not exist in the target table.

Vendor-Specific Considerations

While the general concept of the MERGE statement is standardized, there are vendor-specific differences you should be aware of:

• SQL Server: Supports the full MERGE statement syntax as shown in the examples above.
• Oracle: Uses a similar MERGE statement with support for additional clauses like DELETE in the WHEN MATCHED condition.
• PostgreSQL: As of version 15, PostgreSQL supports the MERGE statement. Ensure you are using the correct version.

It's crucial to refer to the documentation of the specific SQL database you are using to understand any limitations or extensions available in the MERGE implementation.