sql HAVING & averages [closed] - sql

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Learning the having clause, and averages (did AVG before), and I have to admit, I'm confused.
I'll post a problem here, one of the simpler ones on this weeks' assignment, and ask someone to explain it to me in 'real world' terminology (not the textbook language, which is sparse at best).I kinda see what I need to do, but averaging multiple values within same column has me somewhat confused, as does using 'Having' over 'Where'.
SQL> desc inventory
Name
----------------------------
BOOK_CODE
BRANCH_NUM
ON_HAND
SQL> SELECT * FROM INVENTORY;
BOOK BRANCH_NUM ON_HAND
---- ---------- ----------
0180 1 2
0189 2 2
0200 1 1
0200 2 3
0378 3 2
079X 2 1
079X 3 2
079X 4 3
0808 2 1
1351 2 4
1351 3 2
1382 2 1
138X 2 3
2226 1 3
2226 3 2
2226 4 1
2281 4 3
2766 3 2
2908 1 3
2908 4 1
3350 1 2
3743 2 1
3906 2 1
3906 3 2
5163 1 1
5790 4 2
6128 2 4
6128 3 3
6328 2 2
669X 1 1
6908 2 2
7405 3 2
7443 4 1
7559 2 2
8092 3 1
8720 1 3
9611 1 2
9627 3 5
9627 4 2
9701 1 2
9701 2 1
9701 3 3
9701 4 2
9882 3 3
9883 2 3
9883 4 2
9931 1 2

Answer 1

Maybe this helps:
You know how SELECT determines which COLUMNS to return? And you use WHERE in order to choose which ROWS will be returned? Obvious so far, right. But when you aggregate values from multiple rows together into one returned value, then you have to use HAVING if you want to perform some filtering on those.
For example:
SELECT author, title
FROM books
WHERE author = 'Twain, Mark'
would give you the list of all the books Mark Twain wrote.
And
SELECT author, count(*)
FROM books
GROUP BY author
would give you the count of books written by each author.
Finally, if you wanted only those authors who had written 3 or more books:
SELECT author, count(*)
FROM books
GROUP BY author
HAVING count(*) >= 3
I intentionally simplified the data here, but you can probably see how it applies. Another example could be to show all the authors whose books average less than $10:
SELECT author, average(price)
FROM books
GROUP BY author
HAVING average(price) < 10
If you tried to use WHERE here, all you could do is filter out rows in which the price of a single book was less:
SELECT author, average(price)
FROM books
GROUP BY author
WHERE price < 10
That would be the wrong answer of course, mostly because it would fail. But even if it returned a result, it would be the average price of all books less than $10 -- not the same at all.