Final - Relational Algebra Flashcards
What is relational algebra?
A basic set of operations for the relational model
Central to how RDBMSs process queries for execution
Why is it called an algebra?
Can specify expressions, manipulate operations and operands
Operands are relations
What is a relational algebra expression?
Sequence of relational algebra operations
What is the closure of relational algebra?
Each operation takes one or more relations as input and returns a single relation as output
What are unary relational operators?
Operators that only take in 1 relation
Examples: Select, Project, Assignment, Rename
What does the Select operation do?
A subset of the tuples from a relation that satisfies a selection condition
What is the syntax for the Select operation?
σ<select>(R)</select>
What does the Project operation do?
Selects columns from the table and discards the other columns
What is the syntax for the Project operation?
π<attribute>(R)</attribute>
What is a key feature of expressions in relational algebra?
Can nest operations to form more complex expressions
What does the Assignment operation do?
Creates names for intermediate result relations
What is the syntax for the Assignment operation?
<variable> ← <rel>
</rel></variable>
What does the Rename operation do?
Gives a new schema to a relation
What is the syntax for the Rename operation?
ρ_{R(A1, A2, … An)}(E)
What are binary relational operators?
Operators that take two relations as input
Examples: Union, Difference, Intersection, Cartesian Product, Join, Left Outer Join
What does the Union operation do?
Includes all tuples that are either in R or in S or in both R and S
What is the syntax for the Union operation?
R ∪ S
What does the Difference operation do?
Includes all tuples that are in R but not in S
What is the syntax for the Difference operation?
R – S
What does the Intersection operation do?
Includes all tuples that are in both R and S
What is the syntax for the Intersection operation?
R ∩ S
What does the Cartesian Product operation do?
Includes all permutations of tuples in R with tuples in S
What is the syntax for the Cartesian Product operation?
R x S
What is the Inner Join operation equivalent to?
Equivalent to a Cartesian product and a select
What is the syntax for the Inner Join operation?
R ⋈<join> S</join>
What does the Left Outer Join operation do?
Not specified in the provided content
What is the syntax for the Left Outer Join operation?
R ⟕<join> S</join>
What does the Aggregation operation do?
Performs operations like count, sum, max, min, avg
What is the syntax for the Aggregation operation?
ℱ_{Opr(attributes)}(R)
What is the result of the aggregation operation?
A scalar, not a relation
