But first a word of warning. The MERGE statement, introduced in SQL Server 2008 as an easier alternative for “delete / update / insert” logic, turned out to have issues when it was released. And now, in 2020, many of those issues still exist. So I’ll just point you to Aaron Bertrand’s excellent overview, and leave you with the recommendation to be extremely wary before using MERGE in production code.
But here, we are not going to use MERGE in production. We are merely going to set up a simple test and look at how the elements in the execution plan cooperate to produce the expected results. This is interesting even if you never use MERGE, because many of the details explained below can also occur in other execution plans.
Read the whole thing, even if you avoid
MERGE like the plague.