Perl Mysql DBD::mysql::st execute failed

Question

CREATE TABLE users(

  USERID BIGINT AUTO_INCREMENT,
  FNAME  CHAR(50),                        
  LNAME  CHAR(50), 
  EMAIL  CHAR (100),               
  PHONE  CHAR (50),        
  PRIMARY KEY (USER_ID),
  UNIQUE KEY EMAIL (EMAIL),
  UNIQUE KEY PHONE (PHONE)
) ENGINE=InnoDB CHARSET=utf8mb4;


CREATE TABLE items(
  ITEMS_ID       BIGINT AUTO_INCREMENT,                       
  USERID         BIGINT,    
  ITEMS_TO       BIGINT,
  ITEMS_FROM     BIGINT,                     
  ACCEPTED_ITEMS BIGINT,                     
  PRIMARY KEY (ITEMS_ID),
  FOREIGN KEY (USERID) REFERENCES users(USERID),
  FOREIGN KEY (ITEMS_TO) REFERENCES users(USERID),
  FOREIGN KEY (ITEMS_FROM) REFERENCES users(USERID),
  FOREIGN KEY (ACCEPTED_ITEMS) REFERENCES users(USERID)
) ENGINE=InnoDB CHARSET=utf8mb4;

I get this error

DBD::mysql::st execute failed: Cannot add or update a child row: a foreign key constraint fails (`db701`.`items`, CONSTRAINT `items_ibfk_2` FOREIGN KEY (`ITEMS_TO`) REFERENCES `users` (`USERID`))

when I run this:

$ID = '1';
$NULL = 'NULL';
$ID2 = '2';
$ID3 = '1';

my $update = $DBH->prepare("UPDATE items SET ITEMS_TO =?, ITEMS_FROM =? WHERE USERID =? AND ITEMS_ID = ?");
$update->execute($ID, $NULL, $ID2, $ID3);
$update->finish();

It works without ITEMS_FROM and I want to set ITEMS_FROM to null.

This works:

my $update = $DBH->prepare("UPDATE items SET ITEMS_TO =? WHERE USERID =? AND ITEMS_ID = ?");
$update->execute($ID, $ID2, $ID3);
$update->finish();

I think you've got a couple of stray `+` signs in your question, but I can't be certain. (There's one in `for +eign`, for instance, as well as several in the code which follows that.) Please edit your question to remove the ones that don't belong. — , Oct 25 '18 at 21:57

score 6 · Answer 1 · answered Oct 25 '18 at 22:04

You shouldn't put NULL in quotes. That's trying to set ITEMS_TO to the literal string 'NULL', not a null value. MySQL will then try to convert this to a BIGINT; since it doesn't look like a number it will be converted to 0. Since there's no user_id = 0 in the users table, the foreign key check fails.

You should use the Perl value undef to represent the SQL null value:

$NULL = undef;

See Which one represents null? undef or empty string

Perl Mysql DBD::mysql::st execute failed

1 Answers1