MySQL 如何使用外键标识另一个表中的列？

Question

我正在尝试获取拥有最大剩余本金的客户的名称。

在我的数据库loans.customer_id是一个外键，指的是customers.customer_id ；

这些是创建语句：

    CREATE SCHEMA IF NOT EXISTS `mortgages` DEFAULT CHARACTER SET utf8 ;

使用mortgages ；

    CREATE TABLE IF NOT EXISTS `mortgages`.`customers` (
      `customer_id` INT NOT NULL AUTO_INCREMENT,
      `customer_name` VARCHAR(100) NULL,
      `customer_address` VARCHAR(200) NULL,
       `customer_email` VARCHAR(100) NULL,
      PRIMARY KEY (`customer_id`))
     ENGINE = InnoDB;


    CREATE TABLE IF NOT EXISTS `mortgages`.`loans` (
      `loan_id` INT NOT NULL auto_increment,
      `original_principal` DOUBLE NULL,
      `remaining_principal` DOUBLE NULL,
      `princiapl_paid` DOUBLE AS (original_principal - remaining_principal),
       `interest_paid` DOUBLE NULL,
      `customer_id` INT NOT NULL,
      PRIMARY KEY (`loan_id`),
      INDEX `fk_loans_customers_idx` (`customer_id` ASC) VISIBLE,
      CONSTRAINT `fk_loans_customers`
        FOREIGN KEY (`customer_id`)
        REFERENCES `mortgages`.`customers` (`customer_id`)
        ON DELETE NO ACTION
        ON UPDATE NO ACTION)
    ENGINE = InnoDB;

这些是我使用的插入语句：

 INSERT INTO customers VALUES
    (default, 'Joe', '1313 Mockingbird Ln, Transylvania, PA 19666', 
    'jsmith@munsters.com'),
    (default, 'Jane', '123 Heaven Blvd, Cloud City, MA 00911', 'doejd@aol.com'),
    (default, 'Mike', '230860, Club Rd., New York City, NY 10108', 'mikejones@nightclub.com'), 

     INSERT INTO loans VALUES
    (default, 245000.00, 198232.21, default, 21833.25, 1),
    (default, 708921.50, 551123.80, default, 880082.99, 2),
    (default, 99000.00, 1001.12, default, 4591.12, 3),

我努力了：

SELECT customers.customer_name, loans.customer_id
from customers, loans
where (loans.remaining_principal=(SELECT MAX(loans.remaining_principal) FROM loans))

这将显示 MAX 的customer_id旁边的所有客户名称

我想要的是显示一个客户名称，其customer_id ID 为 MAX

我得到的是：

顾客姓名	客户ID
乔	2
简	2
麦克风	2

我想要得到的是：

顾客姓名	客户ID
简	2

Answer 1

你做了一个交叉连接，将客户的每一行与贷款的每一行结合起来，所以当然每个客户也有一行与最大贷款相结合。

而是使用适当的内部连接来组合实际属于一起的贷款和客户。

SELECT co.customer_name,
       lo.customer_id
       FROM customers co
            INNER JOIN loans lo
                       ON lo.customer_id = co.customer_id
       WHERE lo.remaining_principal = (SELECT max(li.remaining_principal)
                                              FROM loans li);

db<>小提琴

WHERE子句可以保持原样，但如果有多个客户共享相同的最大贷款，它会为您提供多个客户。 我不知道，如果你想要这样，或者从他们那里选择一个客户的次要标准是什么，所以我无法进一步解决这个问题。

Answer 2

您可以通过几个子查询来完成此操作：

SELECT customer_name, customer_id
FROM customers
where customer_id = (
    SELECT customer_id FROM loans WHERE remaining_principal = (
        SELECT MAX(remaining_principal) FROM loans))

Answer 3

您可以简单地：

SELECT loans.customer_id, customers.customer_name
FROM loans
JOIN customers 
ON loans.customer_id = customers.customer_id
WHERE (loans.remaining_principal = (SELECT MAX(loans.remaining_principal) FROM loans));

JOIN 允许您从客户表中获取该名称，作为主要数据（即贷款表）的附加组件。

Answer 4

SELECT MAX(remaining_principal),customer_name,customer_id FROM Customers

我希望这有帮助。 还要尽量避免principle_paid = original_principal - remaining_principal ，它是remaining_principal = original_principal - principle_paid