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.find_the_fun said:In relational algebra what does "in" mean?
For example
\(\displaystyle \sigma_{car IN car}R\) where R is some relation.
"In" is a keyword used in relational algebra to check if a value is present in a set. It is used to retrieve all the rows from a relation that satisfy a given condition. It is similar to the "contains" or "belongs to" operator in other programming languages.
The "=" operator is used for exact matching of values, while the "in" operator is used for partial matching. This means that the "in" operator will return all the rows that have values that match any of the values in the set, while "=" will only return rows with an exact match.
Yes, the "in" operator can take in multiple values. This allows for more complex queries, as it can check for multiple conditions at once. For example, you can use "in" to check if a value is present in a set of 10 values, instead of using 10 separate "=" conditions.
If a value in the "in" operator set is null, the result of the condition will be null. This means that the row will not be included in the result set. This is because null values are considered unknown, so it is not possible to determine if it is present in the set or not.
Yes, the "in" operator can be combined with other operators such as "and" and "or" to create more complex conditions. For example, you can use "in" with "and" to check if a value is present in a set and if another condition is also satisfied.