parser: ensure NOT NOT EXISTS is the same as EXISTS

pingcap / parser

A MySQL Compatible SQL Parser

Apache License 2.0

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Closed kennytm closed 3 years ago

kennytm commented 3 years ago

Fix #1251

When applying NOT to an ExistsSubqueryExpr, flip its Not flag instead of always setting it to true.

Tests

Code changes

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Need to cherry-pick to the release branch
Need to be included in the release note
- NOT NOT EXISTS (SELECT ...) is now correctly interpreted as EXISTS (SELECT ...).