SQL，同一表中的外键

Question

Can I have a column in a table be a foreign key for another column in the table? For example, if one column is OrderId and another column is RelatedOrderId , could OrderId be a "foreign key" for RelatedOrderId ? The idea behind such a relationship would be that it would tell whoever is looking at the table that those two columns can have overlapping values.

Answer 1

This may be as close as you can get:

CREATE TABLE `a9` (
  `orderId` int(11) NOT NULL,
  `relatedOrderId` int(11) DEFAULT NULL,
  PRIMARY KEY (`orderId`),
  KEY `abcd3` (`relatedOrderId`),
  CONSTRAINT `abcd3` FOREIGN KEY (`relatedOrderId`) REFERENCES `a9` (`orderId`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

insert a9(orderId,relatedOrderId) values (1,null); -- success
insert a9(orderId,relatedOrderId) values (2,3); -- Error 1452
insert a9(orderId,relatedOrderId) values (2,1); -- success

And the referential integrity is not terribly strong with a NULL in the column relatedOrderId . But at least it suggests that if not NULL then it is valid.

To achieve a circular reference, one would need to do the below which seems fishy.

insert a9(orderId,relatedOrderId) values (1,null); -- success
insert a9(orderId,relatedOrderId) values (2,1); -- success
update a9 set relatedOrderId=2 where orderId=1; -- success

select * from a9;
+---------+----------------+
| orderId | relatedOrderId |
+---------+----------------+
|       2 |              1 |
|       1 |              2 |
+---------+----------------+

Forgive the datatypes chosen. It is only an example.