SQL更新语句Oracle

Question

I need to update a quantity in to my database. I tried with a update statement, (trigger oracle, don't work because definitely something wrong)

My sql string is:

String sql1 = "update book set qty = qty - 1 where isbn='" + txt_carr.getText() + "'";  

txt_carr is a txtField where I write a isbn (for add a cart), but the quantity don't change for that precise ISBN .

Also in the cart when you close the program, should update the quantity (even at a fixed, for example 5)

I tried this:

String sql3= "UPDATE book SET quantity= 5 (select isbn cart where isbn=) ";  

Where isbn = to all isbn in your cart. So it should only update the quantity (in table book) of the books in the shopping cart and not all.

I know that a trigger would work much better, but do not know how to write it.

Answer 1

SQL语法无效：

String sql3= "UPDATE book SET quantity= '5' where book.isbn = 'YOUR_ISBN'";  

Answer 2

Try this one:

UPDATE book b   SET b.quantity =
      (SELECT COUNT (*)
         FROM cards c
        WHERE c.isbn = b.isbn) WHERE b.isbn = 'YOUR_ISBN';

So your code will be as:

String sql1= "UPDATE book b SET b.quantity =(SELECT COUNT (*) FROM card c WHERE c.isbn = b.isbn) WHERE b.isbn = '" + txt_carr.getText() + "' "; 

Not: I'm not sure about table names.