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#### find_the_fun

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- Feb 1, 2012

- 166

For example

\(\displaystyle \sigma_{car IN car}R\) where R is some relation.

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- Thread starter
- #1

- Feb 1, 2012

- 166

For example

\(\displaystyle \sigma_{car IN car}R\) where R is some relation.

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- Feb 1, 2012

- 166

I mean the selection operator as described here.

- Jan 30, 2012

- 2,573

The subscript after σ is a formula. The exact syntax of such formulas depends on the context (software, textbook, etc.). As Wikipedia explains, a typical case is when atomic formulas have the form xRy where x is an attribute name, y is an attribute name or a constant, R is a relation such as <, and atomic formulas are joined by logical connectives. The formula "car IN car" is probably an atomic formula. Maybe the first "car" is an attribute name and the second "car" is an array of values. For a more precise answer one needs the context.

For example

\(\displaystyle \sigma_{car IN car}R\) where R is some relation.