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