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;


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


$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();
[download]

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