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

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Discussion Overview

The discussion revolves around the meaning of "in" within the context of relational algebra, specifically in relation to the selection operator and its syntax. Participants explore how "in" is interpreted and how it differs from the equality operator "=".

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant asks for clarification on the meaning of "in" in the expression \sigma_{car IN car}R, indicating a lack of understanding of its role in relational algebra.
  • Another participant suggests that "in" typically means "is an element of," but later questions whether this interpretation aligns with the original inquiry.
  • A third participant clarifies that they are referring to the selection operator and provides context about the syntax of formulas in relational algebra, noting that the interpretation of "car IN car" may depend on the specific context.
  • There is mention of atomic formulas and how they may involve attributes and relations, indicating a complexity in understanding the expression without additional context.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the interpretation of "in," and multiple views regarding its meaning and application remain present in the discussion.

Contextual Notes

The discussion highlights the dependence on context for interpreting the syntax and semantics of relational algebra expressions, particularly regarding the roles of attributes and values in formulas.

find_the_fun
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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)
 
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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