ERROR 3780: Referencing column 'PO' and referenced column 'Order Number' in foreign key constraint

[Paul Kagan]

I get the following error when trying to add a foreign key...

I don't know why it does not work...


First I post the details of the error then I will post the structure of the tables.



ERROR 3780: Referencing column 'PO' and referenced column 'Order Number' in foreign key constraint

Operation failed: There was an error while applying the SQL script to the database.
Executing:
ALTER TABLE `awldm`.`services`
ADD CONSTRAINT `Purchase_Order`
FOREIGN KEY (`PO`)
REFERENCES `awldm`.`purchase orders` (`Order Number`)
ON DELETE NO ACTION
ON UPDATE NO ACTION;

ERROR 3780: Referencing column 'PO' and referenced column 'Order Number' in foreign key constraint 'Purchase_Order' are incompatible.
SQL Statement:
ALTER TABLE `awldm`.`services`
ADD CONSTRAINT `Purchase_Order`
FOREIGN KEY (`PO`)
REFERENCES `awldm`.`purchase orders` (`Order Number`)
ON DELETE NO ACTION
ON UPDATE NO ACTION


Tables are as follows:

CREATE TABLE `services` (
`Service` int NOT NULL,
`PO` varchar(35) NOT NULL,
PRIMARY KEY (`Service`,`PO`),
UNIQUE KEY `PO_UNIQUE` (`PO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;


CREATE TABLE `purchase orders` (
`Order Number` varchar(35) CHARACTER SET latin1 NOT NULL,
`Supplier` varchar(77) CHARACTER SET latin1 NOT NULL,
`Date` date NOT NULL,
PRIMARY KEY (`Order Number`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;