How to select from a select in MySQL - mysql

Question

I have the following data:
http://i.imgur.com/e4d8M8V.png?1
The first table is labeled "Offering" and the second is labeled "Instructor."
I am trying to sum the salaries of all the professors who do not have their instructor ID appear in the offering table.
What I did first was generate a table that has the data I need in it:
select distinct i.InstructorID, i.Salary
from Instructor i
where i.InstructorID NOT IN (select o.InstructorID from Offering o);
Which gives me the desired result here:
http://i.imgur.com/rkFKseX.png?1
I then want to add these two salaries and have the result displayed in a single salary column. I've tried code like:
$MySQL:> select sum(i.Salary)
from Instructor i
where i.Salary in ( select distinct i.InstructorID, i.Salary
from Instructor i
where i.InstructorID NOT IN (select o.InstructorID from Offering o));
And get "SQLException: java.sql.SQLException: Operand should contain 1 co." However, I am not sure how to contain the result from my previous query to one column. The way I see it is if I was going to sum all of the salaries that were on the list and I just did a distinct Salary, it would only return three salaries rather than eight salaries, so it would be wrong to do that for my case too.
How would I add the two columns I have generated, and is there an easier way to complete my goal than the method I am using?

Answer 1

You can use subqueries. They are described by this article

Answer 2

You should be able to just wrap this in a sub query:
select SUM(Salary)
from (
select distinct i.InstructorID, i.Salary
from Instructor i
where i.InstructorID NOT IN (select o.InstructorID from Offering o)
)
This will work in SQL Server, I think it will work in MYSQL.

Answer 3

You could probably join the two table and then do a group By.
Something like
select sum(salary) from a, b where a.instructorid = b.instructorid and .....

Answer 4

I think this is what you're trying to do :
$MySQL:> select sum(i.Salary)
from Instructor i
where i.Salary in ( select distinct i.Salary
from Instructor i
where i.InstructorID NOT IN (select o.InstructorID from Offering o));

Answer 5

I bet InstructorID is a primary key in Instructor table and I think you don't need "distinct" in select distinct InstructorID, Salary from Instructor.
select sum(Salary)
from Instructor
where InstructorID not in (select InstructorID from Offering);

Answer 6

Please test 3 different approaches and choose the best.
NOT IN
SELECT SUM(i.Salary)
FROM Instructor i
WHERE i.InstructorID NOT IN (select o.InstructorID from Offering o);
LEFT JOIN with IS NULL
SELECT SUM(i.Salary)
FROM Instructor i LEFT JOIN Offering o USING(InstructorID)
WHERE o.InstructorID IS NULL;
NOT EXISTS
SELECT SUM(i.Salary)
FROM Instructor i
WHERE NOT EXISTS (SELECT 1 FROM Offering o WHERE o.InstructorID = i.InstructorID);