MySQL: Error Message Can't create table (errno: 150) - mysql

Question

I have two tables, 'po' and 'receive'
CREATE TABLE `po` (
`PO_ID` bigint(20) NOT NULL,
`SERVICE_TYPE` bit(1) DEFAULT NULL,
`ENTRY_DATE` date NOT NULL,
`RECEIPT_DATE` date DEFAULT NULL,
`TURNOVER` date DEFAULT NULL,
`MOBILIZATION` date DEFAULT NULL,
`SITE_NAME` varchar(255) NOT NULL,
`SITE_CODE` varchar(45) DEFAULT NULL,
`SITE_TIN` varchar(45) DEFAULT NULL,
`SITE_ADDRESS` varchar(255) NOT NULL,
`COST` decimal(11,2) NOT NULL,
`XML` text,
PRIMARY KEY (`PO_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8$$
CREATE TABLE `receive` (
`RECEIPT_ID` varchar(100) NOT NULL,
`RECEIPT_DATE` date NOT NULL,
`PO_NUMBER` bigint(20) NOT NULL,
`SUPPLIER_ID` int(11) NOT NULL,
PRIMARY KEY (`RECEIPT_ID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8$$
I'm trying to connect two tables by defining a foreign key 'fk_po' on the table 'receive'
ALTER TABLE `fourthwave`.`receive`
ADD CONSTRAINT `fk_po`
FOREIGN KEY (`PO_NUMBER` )
REFERENCES `fourthwave`.`po` (`PO_ID` )
ON DELETE SET NULL
ON UPDATE SET NULL
, ADD INDEX `fk_po` (`PO_NUMBER` ASC)
However, the alter query above throws an error :
Error Code: 1005. Can't create table 'fourthwave.#sql-aec_11' (errno:150)
Am i getting this error because the field's names, 'PO_ID' and 'PO_NUMBER' on both tables are different?

Answer 1

Execute SHOW ENGINE INNODB STATUS statement after ALTER TABLE, and you will see the error message - 'You have defined a SET NULL condition though some of the
columns are defined as NOT NULL'.
ALTER TABLE `receive`
ADD CONSTRAINT `fk_po`
FOREIGN KEY (`PO_NUMBER` )
REFERENCES `po` (`PO_ID` )
ON DELETE SET NULL
ON UPDATE SET NULL
, ADD INDEX `fk_po` (`PO_NUMBER` ASC);
SHOW ENGINE INNODB STATUS;

Answer 2

You need an index on PO_NUMBER in the receive table. The field you are referencing in a foreign key always should be indexed.