brkyvz opened a new pull request #27677: [SPARK-30924][SQL] Add additional checks to Merge Into URL: https://github.com/apache/spark/pull/27677 ### What changes were proposed in this pull request? Merge Into is currently missing additional validation around: 1. The lack of any WHEN statements 2. The first WHEN MATCHED statement needs to have a condition if there are two WHEN MATCHED statements. 3. Single use of UPDATE/DELETE This PR introduces these validations. (1) is required, because otherwise the MERGE statement is useless. (2) is required, because otherwise the second WHEN MATCHED condition becomes dead code (3) is up for debate, but the idea there is that a single expression should be sufficient to specify when you would like to update or delete your records. We restrict it for now to reduce surface area and ambiguity. ### Why are the changes needed? To ease DataSource developers when building implementations for MERGE ### Does this PR introduce any user-facing change? Adds additional validation checks ### How was this patch tested? Unit tests
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