Beginner MySQL query with COUNT-"filter" - mysql

Question

I post this question here to get more clarity over my query while learning SQL
(The example is simplfied)
I have the following tables:
BookTable(bookID, isbn, title) // Holds every book, not the ammuont, just the writing
CopyTable(copyID, bookID) // Represent a physical copy
AuthorTable(authorID, fName, lName) // Represents an author
WriteTable(authorID, bookID) // Represents who wrote what
I want to select every author (Preferably like {authordID, fname, lname} ), if that author has a book written, which has more than 5 copies.
I am trying something like this:
SELECT DISTINCT authorID, fname, lname // My final "output table"
FROM T_Author
WHERE authorID IN
SELECT authorID, bookID
FROM T_Write
WHERE bookID IN
SELECT bookID, COUNT(*) AS count
FROM T_Copy
GROUP BY bookID // This part I doubt the most
WHERE count > 5
So my idea is:
Select every BookID that appears more than 5 times in CopyTable
Select every author that wrote any of those books from WriteTable
Write out the name of the author with data from AuthorTable
I am not able to test this if it acutally works, but is this the "Right" way to think in this problem?
Thanks in advance for any guidance.

Answer 1

You are pretty close. Try this:
SELECT a.authorID, a.fname, a.lname // My final "output table"
FROM T_Author a
WHERE a.authorID IN (SELECT w.authorID
FROM T_Write w
WHERE w.bookID IN (SELECT c.bookID
FROM T_Copy c
GROUP BY c.bookID // This part I doubt the most
HAVING COUNT(*) > 5
)
);
Notes:
Subqueries need their own parentheses.
For IN, the returned value has to exactly match what is being compared. In general, you cannot return two columns.
Use HAVING to filter after aggregation.
SELECT DISTINCT is not needed in the outer query. It just adds processing overhead.
Use table aliases and qualified column names in any query that has more than one table reference.