MySQL 如何使用外键标识另一个表中的列？

Question

I am trying to get the name of the customer who has the largest remaining principal.

In my DB loans.customer_id is a foreign key referring to customers.customer_id ;

These are the create statements:

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

USE 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;

These are the insert statements I used:

 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),

I have tried:

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

this displays ALL of the customer names next to customer_id of the MAX

What I want is to display ONE customer name, who has the customer_id of the MAX

What I'm getting is:

customer_name	customer_id
Joe	2
jane	2
Mike	2

What I want to get is:

customer_name	customer_id
jane	2

Answer 1

You did a cross join that combines every row from the customers with every row from the loans, so of course there's also a row for every customer combined with the maximum loan.

Use a proper inner join instead to just combine the loans and customers which actually belong together.

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<>fiddle

The WHERE clause can stay like it is, though it'll give you more than one customer should there be more than one customer sharing the same maximum loan. I don't know, if you want that or what the secondary criteria to chose one customer from them would be then, so I cannot address this further.

Answer 2

You can accomplish this with a couple sub-queries:

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

You can simply:

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 lets you grab that name from the customers table as an add-on to the main data which is the loans table.

Answer 4

SELECT MAX(remaining_principal),customer_name,customer_id FROM Customers

I hope this helps. Also Try to avoid principle_paid = original_principal - remaining_principal , It's remaining_principal = original_principal - principle_paid