MySQL Nullable FK and the 150 error - mysql

Question

I have a problem with create table statement in MySQL.
The scenario is, I want to have a table P_CDP wchich stores some informations, and the table P_CDPFiles which stores the data of the uploaded files.
The column SelectedCDPFileID by default should be NULL unless the user selects some file. Then that column is filled by the file ID. But, I'm still getting the errno 150, why ? I think the FK names are set properly
CREATE TABLE `P_CDP` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`P_OrderID` int(11) NOT NULL DEFAULT '0',
`SelectedCDPFileID` int(11) NULL,
PRIMARY KEY (`ID`),
KEY `FK_P_CDP_P_Orders` (`P_OrderID`),
KEY `FK_P_CDP_P_CDPFiles` (`SelectedCDPFileID`),
CONSTRAINT `FK_P_CDP_P_Orders` FOREIGN KEY (`P_OrderID`) REFERENCES `P_Orders` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `FK_P_CDP_P_CDPFiles` FOREIGN KEY (`SelectedCDPFileID`) REFERENCES `P_CDPFiles` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
CREATE TABLE `P_CDPFiles` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`FileID` int(11) NOT NULL DEFAULT '0',
`P_CDPID` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`ID`),
KEY `FK_P_CDPFiles_File` (`FileID`),
KEY `FK_P_CDPFiles_P_CDP` (`P_CDPID`),
CONSTRAINT `FK_P_CDPFiles_File` FOREIGN KEY (`FileID`) REFERENCES `File` (`FileID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `FK_P_CDPFiles_P_CDP` FOREIGN KEY (`P_CDPID`) REFERENCES `P_CDP` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

Answer 1

You're attempting to reference a table that does not yet exist.
If you must have references in both directions between these two tables (which generally indicates quite poor design), you will have to:
Create the first table without any reference to the as-yet-undefined table:
CREATE TABLE `P_CDP` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`P_OrderID` int(11) NOT NULL DEFAULT '0',
`SelectedCDPFileID` int(11) NULL,
PRIMARY KEY (`ID`),
KEY `FK_P_CDP_P_Orders` (`P_OrderID`),
KEY `FK_P_CDP_P_CDPFiles` (`SelectedCDPFileID`),
CONSTRAINT `FK_P_CDP_P_Orders` FOREIGN KEY (`P_OrderID`)
REFERENCES `P_Orders` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
Create the second table, including its reference to the first table:
CREATE TABLE `P_CDPFiles` (
`ID` int(11) NOT NULL AUTO_INCREMENT,
`FileID` int(11) NOT NULL DEFAULT '0',
`P_CDPID` int(11) NOT NULL DEFAULT '0',
PRIMARY KEY (`ID`),
KEY `FK_P_CDPFiles_File` (`FileID`),
KEY `FK_P_CDPFiles_P_CDP` (`P_CDPID`),
CONSTRAINT `FK_P_CDPFiles_File` FOREIGN KEY (`FileID`)
REFERENCES `File` (`FileID`) ON DELETE NO ACTION ON UPDATE NO ACTION,
CONSTRAINT `FK_P_CDPFiles_P_CDP` FOREIGN KEY (`P_CDPID`)
REFERENCES `P_CDP` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
Alter the first table to add the intended constraint:
ALTER TABLE `P_CDP`
ADD CONSTRAINT `FK_P_CDP_P_CDPFiles` FOREIGN KEY (`SelectedCDPFileID`)
REFERENCES `P_CDPFiles` (`ID`) ON DELETE NO ACTION ON UPDATE NO ACTION;