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What does the selection operator (σ) do in a relational database?
What does the selection operator (σ) do in a relational database?
Which of the following is a valid syntax for the selection operator?
Which of the following is a valid syntax for the selection operator?
Which of the following conditions correctly extracts employees from Glasgow?
Which of the following conditions correctly extracts employees from Glasgow?
In the context of selection, which of the following operators can be used in the condition?
In the context of selection, which of the following operators can be used in the condition?
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What is the output of the selection operation σ city='Glasgow' (Employee)?
What is the output of the selection operation σ city='Glasgow' (Employee)?
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In the employee relation, what would be an invalid condition for selection?
In the employee relation, what would be an invalid condition for selection?
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Which of the following statements about the selection operation is correct?
Which of the following statements about the selection operation is correct?
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What is the role of boolean operators in the selection condition?
What is the role of boolean operators in the selection condition?
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What does the projection operation do in a database query?
What does the projection operation do in a database query?
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When combining projection and selection in a query, which operation is performed first?
When combining projection and selection in a query, which operation is performed first?
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In the relation produced by the projection operation, how are duplicates handled?
In the relation produced by the projection operation, how are duplicates handled?
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What is meant by 'union compatible' in the context of combining relations?
What is meant by 'union compatible' in the context of combining relations?
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Which of the following queries correctly performs a selection followed by a projection?
Which of the following queries correctly performs a selection followed by a projection?
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Which of the following describes the role of a DBMS in relation to relational algebra expressions?
Which of the following describes the role of a DBMS in relation to relational algebra expressions?
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What would be the result of applying a union operation on two incompatible relations?
What would be the result of applying a union operation on two incompatible relations?
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What is the output of the query projecting gender and salary from the Employee table?
What is the output of the query projecting gender and salary from the Employee table?
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What is the primary purpose of a join in relational algebra?
What is the primary purpose of a join in relational algebra?
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What is the result of the operation represented by the notation $\sigma NI# = ENI# ( Employee x Dependant )$?
What is the result of the operation represented by the notation $\sigma NI# = ENI# ( Employee x Dependant )$?
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What characterizes an equi-join?
What characterizes an equi-join?
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In a natural join, what happens to duplicated common attributes in the resulting relation?
In a natural join, what happens to duplicated common attributes in the resulting relation?
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What is the result of a Cartesian product between two relations?
What is the result of a Cartesian product between two relations?
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What operation is described by the notation $R1 \times B1$?
What operation is described by the notation $R1 \times B1$?
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Which statement best describes the process of joining two relations?
Which statement best describes the process of joining two relations?
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In relational algebra, what distinguishes a natural join from other joins?
In relational algebra, what distinguishes a natural join from other joins?
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What is a significant effect of performing a selection operation after a Cartesian product?
What is a significant effect of performing a selection operation after a Cartesian product?
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Which type of join directly relates the columns of two relations based on their names?
Which type of join directly relates the columns of two relations based on their names?
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Study Notes
Relational Algebra
- Relational algebra is a set of operations used to combine and manipulate relations (tables) within a database.
- These operations help determine how a database management system (DBMS) retrieves data.
- The database is built from relations, which consist of tuples(rows) and attributes(columns).
Relational Algebra Operators
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Unary Operations (applied to a single relation):
- Selection (σ): Extracts tuples that meet a specific condition based on the values of their attributes. The syntax is sigma (σ) followed by a condition and the relation name. This is often used with comparisons, logical operators, and literals.
- Projection (Π): Selects certain attributes from a relation and eliminates duplicate rows. The syntax is Pi (Π) followed by the attributes to be included in the relation.
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Binary Operations (applied to multiple relations):
- Union (∪): Combines two relations by returning all tuples that appear in either relation, removing duplicates.
- Intersection (∩): Returns tuples that appear in both of the input relations.
- Difference (-): Returns tuples that appear in the first relation but not in the second.
- Cartesian Product (×): Combines every tuple in one relation with every tuple in the other, generating all possible pairings. The result has the cartesian number of the tuples.
- Join (∞): A combination of a Cartesian Product and a Selection operation for matching tuples from two relations based on a common attribute. One of the common types of joins is an equi-join which matches attributes in two relations that are equal. A Natural Join eliminates any attribute that appears in both relations.
Data types
- Data types dictate what types of data can be stored in each column, and how that data is to be treated. Examples of this include integers, strings, real numbers, dates, and binary objects. Also consider that attributes may have domains that constrain the range of values for that attribute.
- The constraints and restrictions of the database schema affect the results of any queries.
Example Use Cases
- Operations can be composed. For example, select the customers from a city, then project only the name and address.
Foreign Keys
- Foreign keys enforce referential integrity. If a foreign key references a primary key, the value of the foreign key must exist in the relation.
More Detailed Discussion
- Relational algebra operations can be chained together to perform complex queries.
- The actual order of operations within a relational algebra statement may be significant, and DBMS may rewrite statements to improve performance.
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