**Inner**: Matches only. **Left/Right**: Preserve one side; null for no match. **Full outer**: All from both. **Cross**: Cartesian product. **Left semi**: Rows in left with match in right (EXISTS). **Left anti**: Rows in left with no match (NOT EXISTS). **Why each**: Inner for fact-dim; left for optional dimension; semi/anti for filtering without bringing right columns. **Scalability**: Broadcast for small side (<10MB); sort-merge for large-large; shuffle hash default....
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