Syntax:
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<pre>
<i>operand</i> [NOT ]EXISTS (<i>subquery</i>)
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The EXISTS operator returns TRUE if the subquery returns one or more rows and FALSE otherwise. <br \>
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The NOT EXISTS operator returns TRUE if the subquery returns 0 rows and FALSE otherwise.<br \>
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Finding all articles which have readers:
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<pre>
FROM Article
  WHERE EXISTS (FROM ReaderLog(id)
                WHERE ReaderLog.article_id = Article.id)
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Finding all articles which don't have readers:
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<pre>
FROM Article
  WHERE NOT EXISTS (FROM ReaderLog(id)
                WHERE ReaderLog.article_id = Article.id)
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