SQL Basic Query Structure

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Questions and Answers

What does the 'where' clause do in a SQL query?

Specifies the relations to be included in the query

Defines the conditions that the results must satisfy (correct)

Calculates the average of selected column values

Combines the results of two or more queries

What is the result of the following SQL statement? select ID, name, salary/12 from instructor

Returns only the name and department of instructors

Divides the salary by 12 for all instructors (correct)

Generates an error due to improper syntax

Returns all attributes from instructor as is

Which statement accurately describes a Cartesian product in SQL?

Eliminates duplicate tuples from the result set

Combines rows from multiple tables based on common attributes

Generates every possible combination of rows from two relations (correct)

Performs a filtered join on selected columns only

How does the natural join operate on two tables?

Matches rows with the same values for all common attributes Signup and view all the answers

Given the query 'select * from instructor, teaches where instructor.ID = teaches.ID', what is being retrieved?

Pairs of instructors with the courses they've taught Signup and view all the answers

In the query that finds the course ID and title for the Comp.Sci. department, which condition must be true?

section.course_id must equal course.course_id Signup and view all the answers

Which operator can be used to filter records that fall within multiple conditions in a SQL query?

and Signup and view all the answers

What role does the 'from' clause play in SQL queries?

Lists the tables or relations involved in the query Signup and view all the answers

What is the result of the SQL command `drop table student`?

Deletes the student table and all its contents Signup and view all the answers

What happens when you execute `delete from student`?

All the contents in the student table are deleted, but the table itself remains Signup and view all the answers

Which of the following correctly describes the `select` clause in an SQL query?

It specifies the attributes desired in the result of a query Signup and view all the answers

What does the keyword `distinct` do in an SQL query?

Forces the elimination of duplicate values from the result set Signup and view all the answers

How is the SQL command written to select all attributes from the instructor table?

select * from instructor Signup and view all the answers

Which of the following statements about SQL naming is correct?

SQL names are case-insensitive, allowing for upper- and lower-case interchangeably Signup and view all the answers

What is the consequence of using the `alter table` command with `add`?

It adds a new attribute to the table with null values for existing records Signup and view all the answers

What does the SQL command `select all dept_name from instructor` imply?

It retrieves all department names without filtering duplicates Signup and view all the answers

Which SQL query correctly retrieves the names of instructors along with the titles of the courses they teach?

select name, title from (instructor natural join teaches) join course using(course_id); Signup and view all the answers

What is a significant danger associated with using natural joins in SQL?

Natural joins may relate attributes that are unrelated by name. Signup and view all the answers

How can the rename operation be explicitly used in a SQL query?

select ID as instructor_id, name from instructor; Signup and view all the answers

What does the following SQL query accomplish? 'select distinct T.name from instructor as T, instructor as S where T.salary > S.salary and S.dept_name = ‘Comp.Sci.’'

It returns the names of instructors earning more than any instructor in 'Comp.Sci.'. Signup and view all the answers

What is the purpose of using 'using(course_id)' in an SQL join?

To ensure both tables are joined based on the 'course_id' attribute without qualification. Signup and view all the answers

Study Notes

Basic Query Structure

SQL (Structured Query Language) is used for querying, inserting, deleting, and updating data in a database.
Typical SQL query format:
- select A1, A2,..., An
- from r1, r2,..., rm
- where P
Ai represents an attribute, Ri represents a relation, and P is a predicate.
The result of a SQL query is a relation.

The Select Clause

Lists the attributes desired in the query results (projection operation).
Example: select name from instructor lists the names of all instructors.
SQL names are case insensitive.
The distinct keyword eliminates duplicates.
An asterisk (*) represents "all attributes".
The select clause can include arithmetic operations (+, -, *, /).

The Where Clause

Specifies conditions that the result must satisfy (selection operation).
Example: select name from instructor where dept_name = 'Comp.Sci.' and salary > 70000 finds instructors in the Computer Science department with a salary greater than 70000.
Logical connectives and, or, and not can be used to combine comparison results.

The From Clause

Lists the relations involved in the query (Cartesian product operation).
Example: select * from instructor, teaches generates all possible instructor-teaches pairs including all attributes from both relations.
The Cartesian product is often used with a where clause, effectively performing a selection operation.

Joins

Joins combine data from multiple relations based on matching values in common attributes.
Natural Joins match tuples with the same values for all common attributes and retain only one copy of each common column.
Example: select * from instructor natural join teaches selects all attributes from instructors and courses where the instructor ID matches the teaches ID.

Natural Join Examples

Example: select name, course_id from instructor natural join teaches lists the names of instructors along with the courses they teach.
Natural joins can be problematic with unrelated attributes having the same name, as they may be incorrectly equated.
Renaming relations and attributes using the as clause can avoid this issue.

The Rename Operation

The as clause renames relations and attributes: old-name as new-name.
Example: select ID, name, salary/12 as monthly_salary from instructor renames the salary/12 column as "monthly_salary".
Keyword as is optional.

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Description

This quiz covers the fundamental components of SQL queries, including the select and where clauses. Test your understanding of how to retrieve and manipulate data within a database using SQL syntax and operations. Ideal for beginners looking to strengthen their database skills.