How can I make sure a SQL record's column doesn't refer to its own primary key?
Use a trigger:
mysql> create table node ( 
  id int auto_increment primary key, 
  parentid int, 
  foreign key (parentid) references node (id)
);

mysql> delimiter ;;

mysql> create trigger no_self_ref after insert on node for each row begin
if NEW.parentid = NEW.id then
  signal sqlstate '45000' message_text = 'no self-referencing hierarchies';
end if;
end;;

mysql> delimiter ;

Note that it must be an AFTER trigger, because the auto-increment id has not yet been generated in a BEFORE trigger.
Demo:
mysql> insert into node values (1, null);
Query OK, 1 row affected (0.00 sec)

mysql> insert into node values (2, 2);
ERROR 1644 (45000): no self-referencing hierarchies

mysql> insert into node values (2, 1);
Query OK, 1 row affected (0.01 sec)

You will also need a similar AFTER UPDATE trigger, if you want to prevent users from updating the row and setting the parentid to the same value as the id in the same row.

An alternative solution would be to make the primary key non-auto-increment. You will have to specify every id value in your INSERT statements, instead of letting them be auto-incremented. But this will allow you to use a CHECK constraint.

I have a Node table with a ParentID column, which refers to a NodeID that is its parent. I want to make sure no Node can refers to itself (i.e. a Node's ParentID cannot its own NodeID), so I tried adding a check constraint CHECK(NodeID != ParentID).
However, I got this error: Error Code: 3818. Check constraint 'node_chk_1' cannot refer to an auto-increment column.
I also couldn't add the ParentID as a foreign key of Node.
Using MySQL, How can I make sure that there are no new records where NodeID = ParentID?
