多个外键？

我有一张桌子，该桌子应该可以追踪从一个供应商到另一供应商的产品运输天数和成本。我们（出色地：p）在产品供应商表中存储了两个运输供应商（FedEx，UPS）和产品处理供应商（Think ... Dunder Mifflin）。因此，我的SHIPPING_DETAILS表中有三列，均引用VENDOR.no。由于某种原因，MySQL不允许我将全部三个都定义为外键。有任何想法吗？

CREATE TABLE SHIPPING_GRID(
id INT NOT NULL AUTO_INCREMENT PRIMARY KEY COMMENT 'Unique ID for each row',
shipping_vendor_no INT(6) NOT NULL COMMENT 'Foreign key to VENDOR.no for the shipping vendor (vendors_type must be 3)',
start_vendor_no INT(6) NOT NULL COMMENT 'Foreign key to VENDOR.no for the vendor being shipped from',
end_vendor_no INT(6) NOT NULL COMMENT 'Foreign key to the VENDOR.no for the vendor being shipped to',
shipment_duration INT(1) DEFAULT 1 COMMENT 'Duration in whole days shipment will take',
price FLOAT(5,5) NOT NULL COMMENT 'Price in US dollars per shipment lbs (down to 5 decimal places)',
is_flat_rate TINYINT(1) DEFAULT 0 COMMENT '1 if is flat rate regardless of weight, 0 if price is by lbs',
INDEX (shipping_vendor_no),
INDEX (start_vendor_no),
INDEX (end_vendor_no),
FOREIGN KEY (shipping_vendor_no) REFERENCES VENDOR (no),
FOREIGN KEY (start_vendor_no) REFERENCES VENDOR (no),
FOREIGN KEY (end_vendor_no) REFERENCES VENDOR (no)
) TYPE = INNODB;

问题来源于stack overflow