Relational Algebra Operators-
Before you go through this article, make sure that you have gone through the previous article on Introduction to Relational Algebra.
The operators in relational algebra are classified as-
In this article, we will discuss about Selection Operator.
- Selection Operator (σ) is a unary operator in relational algebra that performs a selection operation.
- It selects those rows or tuples from the relation that satisfies the selection condition.
- Select tuples from a relation “Books” where subject is “database”
σsubject = “database” (Books)
- Select tuples from a relation “Books” where subject is “database” and price is “450”
σsubject = “database” ∧ price = “450” (Books)
- Select tuples from a relation “Books” where subject is “database” and price is “450” or have a publication year after 2010
σsubject = “database” ∧ price = “450” ∨ year >”2010″ (Books)
- We may use logical operators like ∧ , ∨ , ! and relational operators like = , ≠ , > , < , <= , >= with the selection condition.
- Selection operator only selects the required tuples according to the selection condition.
- It does not display the selected tuples.
- To display the selected tuples, projection operator is used.
- Selection operator always selects the entire tuple. It can not select a section or part of a tuple.
- Selection operator is commutative in nature i.e.
σ A ∧ B (R) = σ B ∧ A (R)
σ B (σ A(R)) = σ A (σ B(R))
- Degree of the relation from a selection operation is same as degree of the input relation.
- The number of rows returned by a selection operation is obviously less than or equal to the number of rows in the original table.
- Minimum Cardinality = 0
- Maximum Cardinality = |R|
Next Article- Projection Operation in Relational Algebra
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