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Relvar
Definition
A relvar, short for relation variable, is a variable whose value is a relation. The term distinguishes a named, changeable database variable from the particular relation value it holds at one moment. C. J. Date and Hugh Darwen use this distinction in relational theory. It helps explain database updates without treating a mathematical relation itself as something that changes internally.
What it means
A numerical variable can hold five now and seven later, while the number five remains the same value. Similarly, a relvar can hold one relation value before an update and another afterward. A relation consists of tuples described by a heading of named attributes and types. The relvar has a declared type and applicable constraints. This theoretical distinction is useful even when a practical database product uses the word table for several related concepts.
How it works
A relvar declaration establishes the kind of relation it can hold. Assignments and updates replace its current value with another valid relation value, subject to constraints. In relational terminology, a base relvar stores a value directly, while a virtual relvar is defined through an expression over other data. Keys and other constraints restrict permitted values. The actual syntax and update capabilities depend on the relational language or implementation; the term does not imply that every SQL table matches the mathematical model exactly.
Three practical examples
- A database theory lesson distinguishes the named collection Suppliers from the set of supplier tuples it currently contains.
- A designer states a key constraint that must hold for every permitted value of an inventory relvar.
- An AI data-preparation team distinguishes a reusable dataset definition from a particular extracted snapshot, using the relvar idea as conceptual background.
Think of it this way
Think of a labeled whiteboard whose contents can be replaced. The board's label identifies the variable, while a photograph captures one particular set of contents. The analogy is limited because a relation is a mathematical value with set-based properties, not an arbitrary visual arrangement. Tuple order on the board is not part of the relation's meaning.
Worked example
Suppose a relvar Stock has attributes Item and Quantity and initially holds the tuples (A, 3) and (B, 5). An update changes A's quantity to four. The relvar now holds the relation {(A, 4), (B, 5)}. The earlier relation {(A, 3), (B, 5)} remains a distinct value; it is simply no longer the current value of Stock. If Item is a key, assigning {(A, 4), (A, 7)} would violate that key constraint. This informal set notation illustrates values and constraints, not a specific executable database language.
Illustrative case study
In a fictional training project, Hazel Data reviews confusing documentation that uses table to mean a variable, a query result and a printed display. The team introduces relvar when discussing changeable database variables and relation value when discussing results. It keeps familiar product terms in operational instructions but explains the mapping. Reviewers can now ask whether a statement concerns a schema, current data or presentation. The case illustrates clearer terminology rather than a claim that renaming concepts changes the database's behavior.
Common misconceptions and mistakes
A relvar is not a single row and is not merely a variable that stores a table's name. A relation value is not ordered by row position. Mathematical relations do not contain duplicate tuples, while SQL often uses bag-like query semantics unless duplicates are removed. A key is a constraint on possible values, not just a pattern observed in one current sample. The term also does not imply physical storage: a virtual relvar can represent a derived view.
Business use
The concept helps teams reason about constraints, updates and snapshots precisely. When reviewing a data contract, distinguish what the variable may contain from what it happens to contain today. For AI evaluation, preserve an actual dataset snapshot or reproducible version reference rather than citing only a mutable collection name. Database designers can use relvar terminology to explain why a rule must remain true across updates. Operational documentation should still identify the exact product behavior instead of assuming a theoretical term settles implementation details.
People also ask
Is a SQL table always a relvar?
It can be a useful approximation in discussion, but SQL features such as duplicate rows and nulls mean the correspondence needs care.
Does a relvar's name change after every update?
No. The variable can retain its identity while holding a different relation value.
Can a query produce a relation without creating a relvar?
Yes. A relation-valued expression can produce a value without introducing a new named variable.
Check your understanding
A named inventory collection changes from two tuples to three. What changes in relvar terminology?
Answer: Its current relation value changes. The relvar itself remains the named variable, subject to its declared type and constraints.
Related terms
Learn next
Compare an integer variable with an integer value, then make the same distinction for a relation. Write two valid values and one invalid value for a relvar with a key constraint. Finally, compare the theoretical set semantics with the behavior of a SQL query you use.
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Sources
Sources: Hugh Darwen, “What Is a Key?” https://www.dcs.warwick.ac.uk/~hugh/M359/What-Is-a-Key.html; C. J. Date, “To Be Is to Be a Value of a Variable,” 2006, https://www.dcs.warwick.ac.uk/~hugh/TTM/relvars.pdf; Date and Darwen, “Tutorial D,” https://www.dcs.warwick.ac.uk/~hugh/CHAP05.pdf. Primary author-hosted search excerpts consulted; full formal and editorial review remain pending. The entry explains relational-theory terminology and does not equate every SQL construct with a relvar without qualification.
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