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

[find_the_fun]

In relational algebra what does "in" mean?
For example
[math]\sigma_{car IN car}R[/math] where R is some relation.

 

[MarkFL]

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)

 

[find_the_fun]

I mean the selection operator as described here.

 

[Evgeny.Makarov]

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.