Summary on Unnesting Queries

• DISTINCT at top level: Can ignore duplicates.

  • Can sometimes infer DISTINCT at top level! (e.g. subquery clause matches at most one tuple)

• DISTINCT in subquery w/o DISTINCT at top: Hard to convert.

• Subqueries inside OR: Hard to convert.

• ALL subqueries: Hard to convert.

  • EXISTS and ANY are just like IN.

• Aggregates in subqueries: Tricky.

• Good news: Some systems now rewrite under the covers (e.g. DB2).

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