正如@Abra 和@Eric Brandt 指出的那样，例外情况是您正在查询多个表，并且您需要在字段前面加上表名或别名，以防止引用不明确的列（例如列在不同的表中具有相同的名称）
既然您希望改进代码，我会提出一些建议。
正如@Strawberry 所建议的，如果最终目标只是查询预订，则不需要该过程。虽然您可能打算扩展该过程的功能，但恕我直言，通常最好避免使用它们来进行简单的计算、聚合等，这些可以用纯 SQL 完成。
在使用用户输入时将字符串连接在一起形成 SQL 可能很危险。在这种情况下是安全的，因为keyboard.nextInt() 可以防止 SQL 注入攻击，但是您可以考虑使用准备好的语句来获得额外的保护，准备好的语句也可以具有其他优势。例如，某些数据库会缓存运行计划，这在更复杂的查询将运行多次时非常重要。
您可能总是希望在联接中使用“on”子句。否则你会得到一个交叉连接，这可能不是你想要的。
一旦您超越了简单的示例，在 Java 代码中嵌入 SQL 可能会变得乏味，而将其作为资源外部化会非常有用，并且可以设置为模板化 SQL，或者使用变体来处理不同数据库之间的怪癖（例如MySQL vs Oracle vs Postgres 等）。
对于诸如自然键与代理键、lowerCamel 列与低蛇形列、何时使用或避免存储过程等问题，有一大堆意见，但与其尝试详尽的“最佳实践” " 列表，无论如何这只是我的看法，这是一个替代示例，与您的原始示例相差不远（即忽略 ORM 等）
import java.nio.file.Files;
import java.nio.file.Paths;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.Scanner;
public class Hotel {
private static Connection connection;
private static ResultSet rs;
private static Scanner keyboard;
private static Statement stmt;
public static void main(String[] args) throws Exception {
if (args.length != 3) {
// Note: Don't pass passwords on the command line in real life.
System.err.println("Usage: java Hotel <dbUrl> <dbUser> <dbPwd>");
return;
}
// Initial set up
keyboard = new Scanner(System.in);
connection = DriverManager.getConnection(args[0], args[1], args[2]);
stmt = connection.createStatement();
// Do it the original way (from the post).
// orig();
// Do it a slightly different way.
anotherWay();
}
public static void orig() throws Exception {
System.out.println("(orig) Enter your ID guest:");
int gID = keyboard.nextInt();
rs = stmt.executeQuery("Select guestID from Reservation where guestId = " + gID + ";");
if (rs.next()) {
stmt.executeQuery("call calculateTotal(" + gID + ", @result);");
rs = stmt.executeQuery(
"Select firstName, lastName, Reservation.roomId as roomNumber, class, price, startDate, endDate, @result as TotalPrice From (Guest join (Reservation join Room on (Reservation.roomId = Room.id)) on Reservation.guestID = Guest.id ) where Reservation.guestID = "
+ gID + ";");
while (rs.next())
System.out.println(" Name: " + rs.getString("firstName") + " " + rs.getString("lastName")
+ "\n Room Number: " + rs.getInt("roomNumber") + "\n Class: " + rs.getString("class")
+ "\n Price: " + String.format("%.2f", rs.getFloat("price")) + "\n Start Date: "
+ rs.getDate("startDate") + "\n EndDate: " + rs.getDate("endDate") + "\n Total Price: "
+ String.format("%.2f", rs.getFloat("TotalPrice")));
}
}
public static void anotherWay() throws Exception {
System.out.println("(anotherWay) Enter your guest ID:");
int guestId = keyboard.nextInt();
// Get the SQL as a String. Normally this would be standardized into
// a utility, so it wouldn't be so messy.
String sql = new String(
Files.readAllBytes(Paths.get(Hotel.class.getResource("select_reservation.sql").toURI())));
// Prepare the statement
PreparedStatement ps = connection.prepareStatement(sql);
// Set the parameter
ps.setInt(1, guestId);
ResultSet rs = ps.executeQuery();
while (rs.next()) {
printResult(rs);
}
rs.close();
ps.close();
}
public static void printResult(ResultSet rs) throws Exception {
System.out.println(
" Name: " + rs.getString("firstName") + " " + rs.getString("lastName")
+ "\n Room Number: " + rs.getInt("roomNumber")
+ "\n Class: " + rs.getString("class")
+ "\n Price: " + String.format("%.2f", rs.getFloat("price"))
+ "\n Start Date: " + rs.getDate("startDate")
+ "\n EndDate: " + rs.getDate("endDate")
+ "\n Room Charge: " + String.format("%.2f", rs.getFloat("roomCharge"))
+ "\n Extra: " + String.format("%.2f", rs.getFloat("extra"))
+ "\n Total Price: " + String.format("%.2f", rs.getFloat("totalPrice"))
);
}
}
这样运行（现实生活中不要在命令行使用密码）...
java Hotel <url> <user> <pwd>
guest 1 的输出应如下所示...
(anotherWay) Enter your guest ID:
1
Name: Alice Smith
Room Number: 1
Class: premium
Price: 100.00
Start Date: 2020-05-15
EndDate: 2020-05-22
Room Charge: 700.00
Extra: 350.00
Total Price: 1050.00
这给客人 2...
(anotherWay) Enter your guest ID:
2
Name: Bob Jones
Room Number: 2
Class: base
Price: 75.00
Start Date: 2020-05-15
EndDate: 2020-05-22
Room Charge: 525.00
Extra: 0.00
Total Price: 525.00
这是外部化的 SQL...
select
Guest.firstName,
Guest.lastName,
Room.id as roomNumber,
Room.class,
Room.price,
Reservation.startDate,
Reservation.endDate,
(DATEDIFF(Reservation.endDate, Reservation.startDate) * Room.price) as roomCharge,
IFNULL(extra.charges,0) as extra,
((DATEDIFF(Reservation.endDate, Reservation.startDate) * Room.price)
+ IFNULL(extra.charges,0)) as totalPrice
from
Reservation
inner join Guest on Reservation.guestId = Guest.id
inner join Room on Reservation.roomId = Room.id
left join ( -- Use subquery to calculate extra charges.
select -- Could be more efficient by specifying key.
guestId,
sum(price) as charges
from
RoomService
inner join Service on RoomService.serviceId = Service.id
group by
guestId) extra on extra.guestId = Guest.id
where
Reservation.guestId = ?;
如果您想尝试一下，这里有一个完整的 MySQL 架构...
-- Make it idempotent
drop database if exists hotel;
create database hotel;
-- Create the tables (using lowerCamel cols)
CREATE TABLE hotel.Guest (
id int AUTO_INCREMENT ,
firstName varchar(40),
lastName varchar (40),
PRIMARY KEY (id)
);
CREATE TABLE hotel.Room (
id int AUTO_INCREMENT ,
class varchar(40),
price decimal(13,2),
PRIMARY KEY (id)
);
CREATE TABLE hotel.Reservation (
id int AUTO_INCREMENT ,
guestId int,
roomId int,
startDate date,
endDate date,
PRIMARY KEY (id)
);
CREATE TABLE hotel.Service (
id int AUTO_INCREMENT ,
name varchar(40),
price decimal(13,2),
PRIMARY KEY (id)
);
CREATE TABLE hotel.RoomService (
id int AUTO_INCREMENT ,
guestId int,
roomId int,
serviceId int,
PRIMARY KEY (id)
);
INSERT INTO hotel.Guest (id,firstName,lastName) VALUES (1,'Alice','Smith');
INSERT INTO hotel.Guest (id,firstName,lastName) VALUES (2,'Bob','Jones');
INSERT INTO hotel.Guest (id,firstName,lastName) VALUES (3,'Mallory','Adams');
INSERT INTO hotel.Room (id,class,price) VALUES (1,'premium',100.00);
INSERT INTO hotel.Room (id,class,price) VALUES (2,'base',75.00);
INSERT INTO hotel.Room (id,class,price) VALUES (3,'budget',50.00);
INSERT INTO hotel.Reservation (id,guestId,roomId,startDate,endDate) VALUES (1,1,1,'2020-05-15','2020-05-22');
INSERT INTO hotel.Reservation (id,guestId,roomId,startDate,endDate) VALUES (2,2,2,'2020-05-15','2020-05-22');
INSERT INTO hotel.Service (id,name,price) VALUES (1,'WIFI',100.00);
INSERT INTO hotel.Service (id,name,price) VALUES (2,'Safe',100.00);
INSERT INTO hotel.Service (id,name,price) VALUES (3,'Washing clothes',450.00);
INSERT INTO hotel.Service (id,name,price) VALUES (4,'Food delivery',250.00);
INSERT INTO hotel.RoomService (id,guestId,roomId,serviceId) VALUES (1,1,1,1);
INSERT INTO hotel.RoomService (id,guestId,roomId,serviceId) VALUES (2,1,1,4);
DELIMITER //
CREATE DEFINER=`root`@`localhost` PROCEDURE hotel.CalculateTotal(IN ID int ,OUT total float)
BEGIN
SELECT
(DATEDIFF(Reservation.endDate, Reservation.startDate) * Room.price)
INTO
total
FROM
(select * from Guest Where id = ID) As R ,
Room,
Reservation
WHERE
Room.id = Reservation.roomId
AND Reservation.guestId = R.id;
END //
DELIMITER ;