Trouble adding foreign key - mysql

Question

I'm trying to add a foreign key constraint to the table ag by using:
alter table ag
add foreign key fk_ag_protein1 (protein_PID) references protein (PID);
But I get the following error message:
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails
(`mux_new`.`#sql-884_3`, CONSTRAINT `#sql-884_3_ibfk_1` FOREIGN KEY (`protein_PID`)
REFERENCES `protein` (`PID`))"
To get more details I checked output from:
SHOW ENGINE INNODB STATUS\G
Which was:
LATEST FOREIGN KEY ERROR
------------------------
TRANSACTION 193923, ACTIVE 0 sec inserting, thread declared inside InnoDB 4999
mysql tables in use 2, locked 2
5 lock struct(s), heap size 1248, 2 row lock(s), undo log entries 1
MySQL thread id 3, OS thread handle 0x1714, query id 143 localhost ::1 root copy
to tmp table
alter table ag
add foreign key fk_ag_protein1 (protein_PID) references protein (PID)
Foreign key constraint fails for table `mux_new`.`#sql-884_3`:
,
CONSTRAINT `#sql-884_3_ibfk_1` FOREIGN KEY (`protein_PID`) REFERENCES `protein
` (`PID`)
Trying to add in child table, in index `fk_ag_protein1` tuple:
DATA TUPLE: 2 fields;
0: len 4; hex 80000000; asc ;;
1: len 3; hex 002711; asc ' ;;
But in parent table `mux_new`.`protein`, in index `PRIMARY`,
the closest match we can find is record:
PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 696e66696d756d00; asc infimum ;;
But I do not understand this at all. The table ag currently contains some data but protein does not.
Any ideas of what my problem could be or things I could check?
Protein table:
describe protein output:
+---------------------------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------------------+---------+------+-----+---------+-------+
| PID | int(11) | NO | PRI | NULL | |
| uniprot_UniprotAC | char(6) | YES | MUL | NULL | |
| pubmedhits_id | int(11) | YES | MUL | NULL | |
| internallyDefinedNames_id | int(11) | YES | MUL | NULL | |
| comment | int(11) | YES | MUL | NULL | |
+---------------------------+---------+------+-----+---------+-------+
show create table protein output:
CREATE TABLE `protein` (
`PID` int(11) NOT NULL,
`uniprot_UniprotAC` char(6) COLLATE utf8_unicode_ci DEFAULT NULL,
`pubmedhits_id` int(11) DEFAULT NULL,
`internallyDefinedNames_id` int(11) DEFAULT NULL,
`comment` int(11) DEFAULT NULL,
PRIMARY KEY (`PID`),
KEY `fk_protein_uniprot1_idx` (`uniprot_UniprotAC`),
KEY `fk_protein_pubmedhits1_idx` (`pubmedhits_id`),
KEY `fk_protein_internallyDefinedNames1_idx` (`internallyDefinedNames_id`),
KEY `fk_protein_comments1_idx` (`comment`),
CONSTRAINT `protein_ibfk_4` FOREIGN KEY (`uniprot_UniprotAC`) REFERENCES `uniprot` (`UniprotAC`),
CONSTRAINT `protein_ibfk_1` FOREIGN KEY (`comment`) REFERENCES `comments` (`idcomments`),
CONSTRAINT `protein_ibfk_2` FOREIGN KEY (`internallyDefinedNames_id`) REFERENCES `internallydefinednames` (`id`),
CONSTRAINT `protein_ibfk_3` FOREIGN KEY (`pubmedhits_id`) REFERENCES `pubmedhits` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
ag table:
describe ag:
+-------------+--------------------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------------+--------------------------------+------+-----+---------+-------+
| Article_AID | mediumint(5) unsigned zerofill | NO | PRI | NULL | |
| Name | varchar(200) | YES | | NULL | |
| Form | varchar(150) | YES | | NULL | |
| Mw | varchar(40) | YES | | NULL | |
| Source | varchar(260) | YES | | NULL | |
| protein_PID | int(11) | YES | | NULL | |
+-------------+--------------------------------+------+-----+---------+-------+
show create table ag:
`ag` (
`Article_AID` mediumint(5) unsigned zerofill NOT NULL,
`Name` varchar(200) COLLATE utf8_unicode_ci DEFAULT NULL,
`Form` varchar(150) COLLATE utf8_unicode_ci DEFAULT NULL,
`Mw` varchar(40) COLLATE utf8_unicode_ci DEFAULT NULL,
`Source` varchar(260) COLLATE utf8_unicode_ci DEFAULT NULL,
`protein_PID` int(11) DEFAULT NULL,
PRIMARY KEY (`Article_AID`),
KEY `fk_ag_Article1_idx` (`Article_AID`),
CONSTRAINT `fk_ag_Article1` FOREIGN KEY (`Article_AID`) REFERENCES `article` (`AID`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

Answer 1

I figured out what the problem was. In the ag table the protein_PID column was added when there was already data in the table and when it was first created it was set to be not null. Then the rows where all set to 0 for protein_PID as default and since there was no data in protein I could not add the foreign key.

Answer 2

Make PID and protein_PID both to not NULL
May this will solve your issue