MySQL set value of addional field to auto_increment value

Question

I have a table with an AUTO_INCREMENT id , this table also has a parent_id field. In certain cases, when the insert data is itself a parent and has no parent, I need to set the parent_id to the same value as its own id.

Is this possible with a single sql statement, or do I have to do a LAST_INSERT_ID and a separate update statement?

Thanks

Answer 1

You have to do in separated statements.

The tree system usually assumes '0' as root or father of no-father items or father of the first level items, and '0' not exists in database.

I recommend you not to set the same id of the item as parent of this item because... this item is his own father? you really need that?

Answer 2

It is technically possible, though I advise highly against it. The only way this query could work is if you can guarantee that no records will be deleted, and even still I say don't use it. But if you must...

INSERT INTO `table` (field1, field2, parent_id)
VALUES('data1', 'data2', 
            (SELECT id FROM (SELECT max(id) AS id FROM `table`) AS a) + 1
       )

Again though, you're better off doing it in 2 queries. Reason being is the way AI works. If the last record is 20 and it gets deleted, the next entry will be 21 but the parent_id will be 20 because it will see the max(id) as 19.

Answer 3

Yes, you can. To get the next auto_increment value for your table you can do:

SELECT AUTO_INCREMENT 
FROM information_schema.TABLES 
WHERE table_name='table_name' 
AND table_schema='schema_name';

Don't use MAX , because if you remove the last row from table and calculate max, it will not give you the next auto_increment value.