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Questions and Answers
Which of the following are unary relational operations?
Which of the following are unary relational operations?
What does the CARTESIAN PRODUCT operation do?
What does the CARTESIAN PRODUCT operation do?
Combines tuples from two relations in a combinatorial fashion.
What is relational algebra?
What is relational algebra?
The basic set of operations for the relational model.
What makes the relational algebra 'closed'?
What makes the relational algebra 'closed'?
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Relational algebra applies to SQL and not to NoSQL.
Relational algebra applies to SQL and not to NoSQL.
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What is the SELECT operation denoted by?
What is the SELECT operation denoted by?
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Select the EMPLOYEE tuples whose department number is 4.
Select the EMPLOYEE tuples whose department number is 4.
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Select the employee tuples whose salary is greater than $30,000.
Select the employee tuples whose salary is greater than $30,000.
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How is the SELECT operation denoted?
How is the SELECT operation denoted?
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What are the properties of the SELECT operation?
What are the properties of the SELECT operation?
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What symbol denotes the PROJECT operation?
What symbol denotes the PROJECT operation?
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How is the PROJECT operation denoted?
How is the PROJECT operation denoted?
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What does the PROJECT operation do?
What does the PROJECT operation do?
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How to list each employee's first and last name and salary?
How to list each employee's first and last name and salary?
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What does the UNION operation do?
What does the UNION operation do?
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What is type compatible in relational algebra?
What is type compatible in relational algebra?
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What does the INTERSECTION operation do?
What does the INTERSECTION operation do?
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What does the SET DIFFERENCE operation do?
What does the SET DIFFERENCE operation do?
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What is the JOIN operation denoted by?
What is the JOIN operation denoted by?
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What does the JOIN operation do?
What does the JOIN operation do?
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What are the properties of the JOIN operation?
What are the properties of the JOIN operation?
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Study Notes
Relational Algebra Operations
-
Unary Relational Operations:
- SELECT (σ) filters tuples based on a condition.
- PROJECT (π) specifies certain attributes to keep from a relation.
- RENAME (ρ) changes the name of a relation or its attributes.
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Set Theory Operations:
- UNION (U), INTERSECTION (∩), and DIFFERENCE (-) operate on relations as sets.
- CARTESIAN PRODUCT (x) combines tuples from two relations, creating a new relation with attributes from both.
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Binary Relational Operations:
- JOIN combines related tuples based on provided conditions, with several variations available.
- DIVISION is used primarily for queries that involve "for all" conditions.
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Additional Operations:
- OUTER JOINS and OUTER UNION include unmatched tuples from one or both relations respectively.
- AGGREGATE FUNCTIONS perform calculations on groups of tuples, e.g., SUM, COUNT.
CARTESIAN PRODUCT
- Denoted by x, combines all tuples from two relations, producing a relation with attributes from both.
- Resulting relation includes every possible combination of tuples from the two relations.
Relational Algebra Characteristics
- Fundamental operations allow for basic data retrievals, ensuring any operation yields a new relation (closure).
- Sequences of operations can be nested or executed individually with named intermediate results.
SELECT Operation
- Denoted by σ, the SELECT operation retains tuples meeting specific Boolean conditions.
- Results in a relation with the same attributes as the original.
- Can be cascaded or substituted with a combined selection expression.
PROJECT Operation
- Denoted by π, retains specified attributes, creating a vertical partitioning of the data.
- Removes duplicate tuples, ensuring the result is a set.
RENAME Operation
- Denoted by ρ, alters the name of a relation or its attributes.
- Allows for more manageable references in queries.
UNION and Intersections
- UNION (U) combines relations, removing duplicates.
- INTERSECTION (∩) retains tuples common to both relations.
- DIFFERENCE (-) finds tuples present in one relation but not in another.
JOIN Operation
- Denoted by ⨝, combines related tuples from two relations based on a join condition.
- The result can contain multiple combinations if they meet the specified condition (Theta-join).
Compatibility Requirements
- Relations must be type compatible to execute operations like UNION, INTERSECTION, and DIFFERENCE.
- Compatibility defined by matching attribute counts and corresponding data domain types.
Important Properties
- Closure: All relational algebra operations yield relations.
- Commutativity: Operations like SELECT can be applied in any order without changing results.
- Default Behavior: JOIN results typically reduce the number of tuples compared to a simple CARTESIAN PRODUCT.
Visualization Notes
- SELECT (σ) can be viewed as horizontal filtering of rows.
- PROJECT (π) is vertical, focusing on particular columns, differentiating attributes from rows.
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Description
Test your understanding of key relational algebra operations, including unary, set theory, and binary operations. This quiz covers essential concepts such as SELECT, PROJECT, JOIN, and the CARTESIAN PRODUCT, allowing you to assess your knowledge in this critical area of database theory.
