How to reference a column's query value vs stored value? - mysql

Question

I apologize for the title, I couldn't find a good way to word my question. I'm very new to SQL.
Essentially, I'm creating this table (the ids actually reference the students table, but I've simplified it here):
CREATE TABLE followers (student_id int not null,
followee_id int not null,
followsback boolean,
PRIMARY KEY(student_id, followee_id)
SET followsback = IF(SELECT from followers
WHERE student_id = followee_id AND
followee_id = student_id, 1, 0)
My problem lies with the IF statement. Say I ran this INSERT query:
INSERT into followers(student_id, followee_id) values(001,002)
This is supposed to store that student 001 is following student 002.
I need to select the followee (002) and check if they are following the student (001) back. To do this, I need to check the followers table for a user with student_id = followee_id (e.g student_id = 002), then check to see if that user (002) is following the original student_id (001).
The problem is that I don't know how to reference the student_id as specified within the INSERT query vs referencing the value within my SELECT query.
Then if the two students are following each other then I need to set followsback to 1.
Hopefully this makes sense, I am having a ridiculously hard time explaining this.

Answer 1

There's no syntax in MySQL's CREATE TABLE statement to do what you show. It could be done by a rare feature in the SQL specification called an "assertion"—except there is no SQL database on the market that implements this feature.
You can try to implement it as a trigger:
CREATE TRIGGER followback_ins BEFORE INSERT ON followers
FOR EACH ROW
SET NEW.followsback = EXISTS (
SELECT * from followers
WHERE student_id = NEW.followee_id AND followee_id = NEW.student_id);
But this has a problem. It only updates the followsback for the new record, not the original record.
mysql> insert into followers set student_id = 123, followee_id = 456;
mysql> insert into followers set student_id = 456, followee_id = 123;
mysql> select * from followers;
+------------+-------------+-------------+
| student_id | followee_id | followsback |
+------------+-------------+-------------+
| 123 | 456 | 0 |
| 456 | 123 | 1 |
+------------+-------------+-------------+
This is called an anomaly because when you try to store the same fact in two places, these two rows can contradict each other.
We could try to make a trigger that updates the original row too:
CREATE TRIGGER followback_ins AFTER INSERT ON followers
FOR EACH ROW
UPDATE followers AS f1 JOIN followers AS f2
ON f1.student_id=f2.followee_id AND f1.followee_id=f2.student_id
SET f1.followsback=true, f2.followsback=true;
But this is illegal. You can't update a table from a trigger on that same table (too much risk of infinite recursion).
ERROR 1442 (HY000): Can't update table 'followers' in stored function/trigger
because it is already used by statement which invoked this stored
function/trigger.
I'd suggest to forget about storing followsback at all. Instead, just store the following relationships as two rows, without a followsback column. If you want to know if they follow each other, you have to join two rows together:
SELECT COUNT(*)
FROM followers AS f1 JOIN followers AS f2
ON f1.student_id=f2.followee_id AND f1.followee_id=f2.student_id
WHERE f1.student_id = 123 AND f1.followee_id = 456.
This query will return 0 if there is no mutual following, and 2 if there is.