Outer Join in Database System Concepts - 7th Edition
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Questions and Answers

What does a join condition define?

  • The type of join used
  • How tuples in two relations match (correct)
  • The process of combining tables
  • How to treat unmatched tuples in each relation
  • In a natural join, how are matching columns determined?

  • By using a wildcard character
  • By explicitly specifying them
  • By looking for common column names (correct)
  • By sorting the tables alphabetically
  • What does a full outer join return?

  • Only matched rows
  • Unmatched rows from both tables (correct)
  • Rows common to both tables
  • Randomly selected rows
  • How does an inner join differ from a left outer join?

    <p>An inner join includes only common rows</p> Signup and view all the answers

    When using a right outer join, what happens to unmatched rows from the right table?

    <p>They are included in the result</p> Signup and view all the answers

    In database views, what is the purpose of hiding parts of the logical model?

    <p>To limit access to sensitive data</p> Signup and view all the answers

    What does an outer join operation in database systems aim to avoid?

    <p>Loss of information</p> Signup and view all the answers

    Which type of join adds tuples from one relation that do not match tuples in the other relation to the result of the join?

    <p>Outer join</p> Signup and view all the answers

    In a left outer join, what does the 'left' refer to?

    <p>The first relation specified in the operation</p> Signup and view all the answers

    What kind of values are used in an outer join to indicate missing information?

    <p>Null values</p> Signup and view all the answers

    Which type of join computes the join and then adds tuples from both relations even if they do not match?

    <p>Full outer join</p> Signup and view all the answers

    What is a key advantage of using outer joins compared to inner joins in database operations?

    <p>Better handling of missing data</p> Signup and view all the answers

    What type of join operation in SQL matches tuples with the same values for all common attributes?

    <p>Natural join</p> Signup and view all the answers

    In SQL, what type of operation takes two relations and returns another relation as a result?

    <p>Join operation</p> Signup and view all the answers

    Which clause in SQL is typically used to incorporate join operations as subquery expressions?

    <p>FROM clause</p> Signup and view all the answers

    What does an inner join in SQL require for tuples in the two relations to be included in the result?

    <p>Matching under some condition</p> Signup and view all the answers

    In SQL, what does an outer join do that differentiates it from an inner join?

    <p>Includes unmatched tuples from one relation</p> Signup and view all the answers

    Which SQL join operation is essentially a Cartesian product with specified attribute conditions?

    <p>Cross join</p> Signup and view all the answers

    In SQL, what is the purpose of the 'natural join' construct?

    <p>To join multiple relations based on a common attribute</p> Signup and view all the answers

    What is the danger associated with 'natural join' in SQL?

    <p>Wrongly equating unrelated attributes with the same name</p> Signup and view all the answers

    Which version of the SQL query correctly lists names of students and instructors along with the titles of courses taken?

    <p>select name, title from student natural join takes, course where takes.course_id = course.course_id;</p> Signup and view all the answers

    What does the 'natural join' in SQL help avoid?

    <p>Incorrect attribute equating</p> Signup and view all the answers

    How does the 'natural join' construct differ from an 'inner join'?

    <p>Natural join does not require a specified condition for matching rows</p> Signup and view all the answers

    Why is it important to use caution when using 'natural join' in SQL?

    <p>'Natural join' may exclude relevant data based on common attributes</p> Signup and view all the answers

    Which of the following SQL queries creates a view named 'faculty'?

    <p>create view faculty as select ID, name, dept_name from instructor</p> Signup and view all the answers

    In SQL, which query retrieves the names of all instructors in the Biology department?

    <p>select name from faculty where dept_name = 'Biology'</p> Signup and view all the answers

    What does it mean for a view relation to be recursive in the context of database views?

    <p>It depends on itself in its definition</p> Signup and view all the answers

    Which statement accurately describes the relationship between two view relations, v1 and v2?

    <p>v1 depends on v2 if there is a direct path from v1 to v2 in their definitions</p> Signup and view all the answers

    What information does the 'departments_total_salary' view provide?

    <p>Total salary of all instructors in each department</p> Signup and view all the answers

    Study Notes

    Joined Relations

    • Join operations take two relations and return as a result another relation.
    • A join operation is a Cartesian product that requires tuples in the two relations to match under some condition.
    • It also specifies the attributes that are present in the result of the join.
    • Three types of joins: Natural join, Inner join, Outer join.

    Natural Join

    • Natural join matches tuples with the same values for all common attributes, and retains only one copy of each common column.
    • Example: List the names of instructors along with the course ID of the courses that they taught.
    • SQL construct: select name, course_id from student natural join takes;

    Outer Join

    • An extension of the join operation that avoids loss of information.
    • Uses null values.
    • Computes the join and then adds tuples from one relation that do not match tuples in the other relation to the result of the join.
    • Three forms of outer join: Left outer join, Right outer join, Full outer join.

    Left Outer Join

    • course natural left outer join prereq
    • In relational algebra: course ⟕ prereq

    Right Outer Join

    • course natural right outer join prereq
    • In relational algebra: course ⟖ prereq

    Full Outer Join

    • course natural full outer join prereq
    • In relational algebra: course ⟗ prereq

    Views

    • In some cases, it is not desirable for all users to see the entire logical model (that is, all the actual relations stored in the database).
    • Create a view that restricts access to specific data, e.g., an instructor's name and department, but not their salary.

    View Definition and Use

    • Create a view of instructors without their salary: create view faculty as select ID, name, dept_name from instructor
    • Use the view to find all instructors in the Biology department: select name from faculty where dept_name = 'Biology'

    Views Defined Using Other Views

    • One view may be used in the expression defining another view.
    • A view relation v1 is said to depend on view relation v2 if either v1 depends directly on v2 or there is a path of dependencies from v1 to v2.
    • A view relation v is said to be recursive if it depends on itself.

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    Description

    Explore the concept of outer join in the context of database systems as explained in Database System Concepts - 7th Edition by Silberschatz, Korth and Sudarshan. Learn how outer join differs from traditional join operations and understand its three forms: left outer join, right outer join, and full outer join.

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