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Questions and Answers
What operation is used to filter tuples from a relation based on a selection condition?
What operation is used to filter tuples from a relation based on a selection condition?
Which operation is employed to keep specific columns and discard others in a relation?
Which operation is employed to keep specific columns and discard others in a relation?
What symbol denotes the select operation in relational algebra?
What symbol denotes the select operation in relational algebra?
In the SELECT operation, what happens to tuples that do not satisfy the selection condition?
In the SELECT operation, what happens to tuples that do not satisfy the selection condition?
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If you wanted to select all employees with a salary greater than $30,000, which of the following queries is correct?
If you wanted to select all employees with a salary greater than $30,000, which of the following queries is correct?
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What is the general form of the project operation in relational algebra?
What is the general form of the project operation in relational algebra?
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To retrieve specific attributes from employees in department number 5, which sequence of operations would you perform?
To retrieve specific attributes from employees in department number 5, which sequence of operations would you perform?
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Which operation would be represented by PHP in RDBMS for managing data directly?
Which operation would be represented by PHP in RDBMS for managing data directly?
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What does the UNION operation in relational algebra do?
What does the UNION operation in relational algebra do?
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Which conditions must be satisfied for two relations R and S to perform a UNION operation?
Which conditions must be satisfied for two relations R and S to perform a UNION operation?
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What does the INTERSECTION operation return?
What does the INTERSECTION operation return?
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In relational algebra, what is a JOIN operation primarily used for?
In relational algebra, what is a JOIN operation primarily used for?
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If R(A1, A2) and S(B1, B2) are two relations, how is a JOIN operation typically expressed?
If R(A1, A2) and S(B1, B2) are two relations, how is a JOIN operation typically expressed?
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What is the output of the expression RESULT ← RESULT1 ∪ RESULT2?
What is the output of the expression RESULT ← RESULT1 ∪ RESULT2?
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What is a primary requirement for performing an INTERSECTION operation?
What is a primary requirement for performing an INTERSECTION operation?
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The result of which operation will include all related tuples from two relations based on a condition?
The result of which operation will include all related tuples from two relations based on a condition?
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Study Notes
Querying Database Lecture Notes
- Relational algebra is a fundamental set of operations for the relational model.
- Operators in relational algebra enable users to specify retrieval requests, or queries.
- Relational algebra operations create new relations.
- A sequence of relational algebra operations forms a relational algebra expression.
Unary Relational Operations: SELECT
- The SELECT operation, denoted by σ (sigma), selects a subset of tuples from a relation based on a selection condition.
- The selection condition acts as a filter.
- Only tuples satisfying the condition are kept; others are discarded.
- The general form is σ
(R) where R is the relation.
Unary Relational Operations: PROJECT
- The PROJECT operation, denoted by π (pi), keeps certain columns (attributes) from a relation and discards the others.
- It creates a vertical partitioning of the relation.
- The general form is π
(R).
Single Expression versus Sequence of Relational Operations
- To retrieve employees who work in department 5, apply a select and a project operation.
- This can be written as a single expression: πFNAME, LNAME, SALARY(σDNO=5(EMPLOYEE))
Relational Algebra Operations from Set Theory: UNION
- UNION, denoted by ∪, is a binary operation.
- The result includes all tuples present in either relation R or S, or both. (Duplicates are eliminated)
- Relations R and S must be type compatible (same number of attributes with compatible types).
Relational Algebra Operations from Set Theory: INTERSECTION
- INTERSECTION, denoted by ∩, is a binary operation.
- The result includes tuples present in both relations R and S.
- Relations R and S must be type compatible.
Binary Relational Operations: JOIN
- JOIN, denoted by ⋈, combines the sequence of CARTESIAN PRODUCT followed by SELECT into a single operation.
- This allows combining related tuples from various relations.
- Important for relational databases with more than one relation.
- The general form is R⋈condition> S, where R and S are relations resulting from general relational algebra expressions.
DML Commands in SQL
- INSERT command: Adds data to a table by inserting rows.
- SELECT command: Retrieves data from a table. (Can use JOIN)
- UPDATE command: Modifies data in a table.
- DELETE command: Removes data from a table.
Data Control Language (DCL)
- DCL commands (GRANT and REVOKE) control database system access rights and permissions.
- GRANT command: Allows new access privileges to database objects, actions, or functions.
- REVOKE command: Removes access privileges previously granted to a user account.
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