如何检查输入的值是否等于sql数据库中的值

Question

edit2: I've wrote this method to solve the problem:

public boolean codeExists(int code) {
        final String SELECT_QUERY = "SELECT * FROM productcatalogue WHERE code = ?";
        try {

            PreparedStatement statement = conn.prepareStatement(SELECT_QUERY); // Create query statement
            statement.setInt(1, code);

            ResultSet queryResult = statement.executeQuery();
            if (!queryResult.next()) { // Check if code entered exists
                return false; // Tells that the author doesn't exist
            }
        } catch (SQLException sqlException) {

            sqlException.printStackTrace();

        }
        return true; //Otherwise the item exists
    }

edit: I now know that ResultSet never returns null. It will return an empty ResultSet.

I have a database with 7 rows, and 4 columns in each. I am prompting a user to enter a code, and I need to cross reference the database to see if the entered code equals one of the codes in the database (first column in each row).

At first, I was checking if the result set came up as null, but that was not working. I was trying if(rs.wasNull()) but it never seemed to execute.

Does anybody have any advice on what to look at to solve my issue? Here's a code snippet if helps at all.

    if(!rs.wasNull()) {
        while(rs.next()) {
            Products temp = new Products(rs.getString(1), rs.getString(2), rs.getDouble(3), rs.getBoolean(4));
            productsList.add(temp);
            // For testing purposes
            //out.println(temp.toString());
            request.setAttribute("productsList", productsList);
            request.getRequestDispatcher("Scan.jsp").forward(request, response);
        }
    }

    else if(rs.wasNull()) {
        Products wrongCode = null;
        productsList.add(wrongCode);    
        request.setAttribute("productsList", productsList);
        request.getRequestDispatcher("Scan.jsp").forward(request, response);
    }

Answer 1

If the result is empty it will never go into while loop. This is a condition which is used to separate code logic. Calling additional code on the result set is useless.

You should check the jdbc api for ResultSet#wasNull() :

wasNull boolean wasNull() throws SQLException Reports whether the last column read had a value of SQL NULL. Note that you must first call one of the getter methods on a column to try to read its value and then call the method wasNull to see if the value read was SQL NULL. Returns: true if the last column value read was SQL NULL and false otherwise Throws: SQLException - if a database access error occurs or this method is called on a closed result set