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Questions and Answers
Which of the following relational algebraic expressions is equivalent to the SQL statement SELECT * FROM Orders WHERE ord_num = '1002'?
Which of the following relational algebraic expressions is equivalent to the SQL statement SELECT * FROM Orders WHERE ord_num = '1002'?
- σord_num = '1001'(Orders)
- σord_num = '1003'(Orders)
- σord_num = '1002'(Orders) (correct)
- πord_num(Orders)
What type of relational algebraic operation should Ken use to find out which customers have placed orders?
What type of relational algebraic operation should Ken use to find out which customers have placed orders?
Intersection
Which of the following SQL statements creates a natural join?
Which of the following SQL statements creates a natural join?
- SELECT Customer.c_id, Orders.ord_no FROM Customer CROSS JOIN Orders;
- SELECT * FROM Customer, Orders;
- SELECT * FROM Customer INNER JOIN Orders ON Customer.c_id = Orders.c_id;
- SELECT Customer.c_id, last_name, address, city, state, zip, Orders.ord_no FROM Customer, Orders WHERE Customer.c_id = Orders.c_id; (correct)
Which type of join keeps all rows in both relations, entering nulls in the columns with no matching values?
Which type of join keeps all rows in both relations, entering nulls in the columns with no matching values?
Which relational algebraic operations will return specific columns from one single relation?
Which relational algebraic operations will return specific columns from one single relation?
Which SQL statement is equivalent to the algebraic operation σEmployee.e_id = Customer.e_id(Employee X Customer)?
Which SQL statement is equivalent to the algebraic operation σEmployee.e_id = Customer.e_id(Employee X Customer)?
Which relational algebraic operations will return one or more entire rows from one single relation?
Which relational algebraic operations will return one or more entire rows from one single relation?
Which relational algebraic operation multiplies two relations to create a new relation containing every possible pair of rows from the two original relations?
Which relational algebraic operation multiplies two relations to create a new relation containing every possible pair of rows from the two original relations?
Which SQL statement is equivalent to the algebraic operation Employee X Customer?
Which SQL statement is equivalent to the algebraic operation Employee X Customer?
Which SQL statement is equivalent to the algebraic operation Employee1 ∪ Employee2?
Which SQL statement is equivalent to the algebraic operation Employee1 ∪ Employee2?
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Study Notes
Relational Algebra Concepts
- Relational Algebra Expression:
σord_num = '1002'(Orders)is equivalent to selecting orders withord_numof '1002'. - Intersection: Used to find common customers who have placed orders from two distinct tables: Customers and Orders.
- Natural Join: Achieved via SQL statement that combines Customer and Orders tables on the matching
c_idfield. - Full Outer Join: Retains all rows from both relations, inserting nulls where there are no matching values.
Relational Algebra Operations
- Projection: Retrieves specific columns from a single relation, focusing on particular attributes.
- Selection: Returns one or more complete rows from a single relation based on specified criteria.
- Cartesian Product: Combines two relations to produce a new relation with all possible pairs of rows from the original tables.
SQL Equivalents of Algebraic Operations
- Join Operation:
σEmployee.e_id = Customer.e_id(Employee X Customer)corresponds to selecting rows in SQL wheree_idmatches in both Employee and Customer tables. - Equivalent SQL for Cartesian Product:
SELECT Employee.*, Customer.* FROM Employee, Customer;represents the Cartesian product of Employee and Customer relation. - Union Operation:
Employee1 ∪ Employee2indicates a union of two different employee lists, providing a combined set of results from both.
These points summarize essential relational algebra operations and their SQL counterparts, facilitating a clearer understanding of database queries and their respective algebraic representations.
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