MHB Relational algebra - What does "in" mean and how is it different than "="?

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In relational algebra, "in" signifies membership, indicating that an element belongs to a set or relation. The expression σ_{car IN car}R suggests that "car" is being evaluated against a set of values within relation R. The syntax used can vary based on the context, such as the specific software or textbook referenced. Typically, atomic formulas in relational algebra involve comparisons between attributes or constants, and "car IN car" likely represents a comparison where the first "car" is an attribute and the second is a collection of values. Understanding the precise meaning of "in" requires context regarding the specific relational algebra framework being discussed.
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In relational algebra what does "in" mean?
For example
[math]\sigma_{car IN car}R[/math] where R is some relation.
 
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Do you mean:

$\displaystyle \sigma\in\mathbb{R}$ ?

If so, the "in" means "is an element of."

edit: I just noticed this is probably not what you are referring to! (Doh)
 
I mean the selection operator as described here.
 
find_the_fun said:
In relational algebra what does "in" mean?
For example
[math]\sigma_{car IN car}R[/math] where R is some relation.
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.
 

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