Understanding Inner Joins in SQL

What does an INNER JOIN do in SQL?

• Returns all rows from both tables, even if there is no match

• Returns only rows where there is a match in both tables

• Returns all rows from the left table and matched rows from the right table

• Returns all rows from the right table and matched rows from the left table

Which keyword is used to perform an inner join in SQL?

• OUTER JOIN

• INNER JOIN

• LEFT JOIN

• RIGHT JOIN

Given two tables, Students and Classes, which SQL statement correctly performs an inner join on the column ClassID?

• SELECT * FROM Students LEFT JOIN Classes ON Students.ClassID = Classes.ClassID;

• SELECT * FROM Students INNER JOIN Classes ON Students.ClassID = Classes.ClassID;

• SELECT * FROM Students RIGHT JOIN Classes ON Students.ClassID = Classes.ClassID;

• SELECT * FROM Students FULL JOIN Classes ON Students.ClassID = Classes.ClassID;

If a student is not enrolled in any class, will they appear in the result of an inner join between Students and Classes?

• Yes, always

• No, only if there is a matching class

• Yes, but only if the class is empty

• Yes, if the student’s name is not NULL

Which of the following best describes the result of an inner join?

• The intersection of two tables

• The union of two tables

• The difference between two tables

• The sum of two tables

Suppose Table A has 10 rows and Table B has 8 rows. If 5 rows match on the join condition, how many rows will the inner join return?

• 8

• 10

• 5

• 18

Which clause is used to specify the condition for an inner join?

• WHERE

• ON

• HAVING

• GROUP BY

What will happen if you forget to specify the join condition in an inner join?

• SQL will return an error

• SQL will return a cross join (cartesian product)

• SQL will return no rows

• SQL will return only the first row

Which of the following is a correct way to write an inner join using the WHERE clause (old style)?

• SELECT * FROM A, B WHERE A.id = B.id;

• SELECT * FROM A INNER JOIN B;

• SELECT * FROM A LEFT JOIN B ON A.id = B.id;

• SELECT * FROM A RIGHT JOIN B ON A.id = B.id;

If you want to select only the columns Name from Students and Subject from Classes using an inner join, which is correct?

• SELECT Name, Subject FROM Students INNER JOIN Classes ON Students.ClassID = Classes.ClassID;

• SELECT * FROM Students INNER JOIN Classes;

• SELECT Name, Subject FROM Students, Classes;

• SELECT Name, Subject FROM Students LEFT JOIN Classes ON Students.ClassID = Classes.ClassID;