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While creating merges, I was told of something called a merge log. I have not heard of this before. What is a merge log in SQL? How is it structured, and what are its uses? I appreciate any responses. Thank you in advance.

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As mentioned, we don't use the term "merge log" in the SQL Server world. We would have to see the context in where you heard that term. Without that, we are only guessing.

One possibility is that whoever referred to this meant the OUTPUT clause. This allow you to have SQL Server "spit out" the modifications performed by a modification statement (MERGE, INSERT, UPDATE, DELETE). You can for instance use the OUTPUT clause to log the modifications to some table.

Documentation for OUTPUT and a blog post.

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There is no such thing as a "merge log" in Microsoft SQL Server but there is the MERGE statement (which is what your link in the comments refers to). The MERGE statement automatically consolidates one dataset from another by doing the appropriate INSERT, UPDATE, and DELETE statements.

Perhaps you're thinking of the general Transaction Log which logs all DML statements. It's used to ensure transactional consistency which is one of the key factors of the ACID principles. Furthermore maybe you read how to minimally log data changes with the MERGE statement as this article discusses?

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