A specific set algebra question, and how to solve it in Excel, has been bugging me for years. Literally. I’ve asked a bunch of people, and the more advanced Excel warriors have given partial answers.
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ADDED- And if you do not want any empty cells in between, try this.
=IFERROR(INDEX($A$1:$A$10, SMALL(IF(NOT (IFERROR(MATCH ($A$1:$A$10,$B$1:$B$10,0),0)), ROW($A$1:$A$10)),ROW())),””)