Subract and Add two columns from different table - sql

Question

I want to subract and add two columns from different table.
table book:
BookID | BookName | Author | Edition | PublishingYear | copies| Shelf | Row
1 | SQL | Robert | 3 | 2005 | 3 | A |third
table Issue: (in this I have created join with tblPerson to show the PersonName instead of PersonID)
BookID | BookName | DateIssue | ReturnDate | PersonName | copies
1 | SQL | 2015-10-12 | 2015-10-12 | john | 1
table Return:
BookID | BookName | DateIssue | ReturnDate | PersonName | copies
1 | SQL | 2015-10-12 | 2015-10-12 | john | 1
Sql Query:
Select (tblBook.copies) - (tblIssue.copies)
FROm tblBook
FULL join tblIssue
ON tblBook.copies = tblIssue.copies
This query doesn't subract these two columns(copies).
I want to minus the column (copies) tblIssue from tblbook column copies(original value) when I issue the book.
And when I return the book from tbl Return, it gives me the original value in the column(copies) in tblBook.

Answer 1

When you are doing joins, you need to join tables on they keys that binds the tables togheter.
Joining two tables on copies makes no logical sense, so instead join it on the primary key foreign key references BookID.
Select (tblBook.copies) - (tblIssue.copies)
FROm tblBook
FULL join tblIssue
ON tblBook.BookId = tblIssue.BookId
This will produce a following result, if no negations whas made.
BookID | BookName | Author | Edition | PublishingYear | copies| Shelf | Row | BookID | BookName | DateIssue | ReturnDate | PersonName | copies
1 | SQL | Robert | 3 | 2005 | 3 | A |third| 1 | SQL | 2015-10-12 | 2015-10-12 | john | 1
Thereby subtracting 3-1 which should equal 2. But it is always important to consider what the join result should be and then plan out your join strategy.
EDIT 1
An example query of how many books are availeble at given time
Select tblBook.BookId, MAX(tblBook.copies) - SUM(tblIssue.copies) as countOfAvailebleBooks
FROm tblBook
FULL join tblIssue
ON tblBook.BookId = tblIssue.BookId
WHERE tblIssue.ReturnDate >= '2015-10-12'AND tblIssue.IssueDate <= '2015-10-12'
This will produce an aggregate result for a given date, which is 2015-10-12 for this case, of how many books you are availeble, it is not optimal but what you have given, it seems like it is the best solution.