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

[find_the_fun]

In relational algebra what does "in" mean?
For example
\(\displaystyle \sigma_{car IN car}R\) 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
\(\displaystyle \sigma_{car IN car}R\) 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.

 

[CellCoree]

In relational algebra, "in" is a keyword used to specify a condition where an attribute or value must be contained within a set or relation. This is different from the "=" operator, which is used to check for exact equality between two values or attributes.

For example, in the expression \sigma_{car IN car}R, the "car" on the left side of the "IN" keyword represents an attribute, while the "car" on the right side represents a relation. This expression would return all tuples from the relation R where the attribute "car" is contained within the relation "car".

In contrast, if we were to use the "=" operator in the expression \sigma_{car = car}R, it would return all tuples from R where the attribute "car" is exactly equal to the relation "car". This means that the two attributes must have the same value for a tuple to be included in the result.

In summary, the "in" keyword in relational algebra is used to check for containment within a set or relation, while the "=" operator is used to check for exact equality between two values or attributes. Both are important tools for specifying conditions and retrieving data in relational algebra.