Update of MySQL table column to sequential digit based on another column - mysql

Question

The current table looks something like this:
[id | section | order | thing]
[1 | fruits | 0 | apple]
[2 | fruits | 0 | banana]
[3 | fruits | 0 | avocado]
[4 | veggies | 0 | tomato]
[5 | veggies | 0 | potato]
[6 | veggies | 0 | spinach]
I'm wondering how to make the table look more like this:
[id | section | order | thing]
[1 | fruits | 1 | apple]
[2 | fruits | 2 | banana]
[3 | fruits | 3 | avocado]
[4 | veggies | 1 | tomato]
[5 | veggies | 2 | potato]
[6 | veggies | 3 | spinach]
"order" column updated to a sequential number, starting at 1, based on "section" column and "id" column.

Answer 1

You can do this with an update by using a join. The second table to the join calculates the ordering, which is then used for the update:
update t join
(select t.*, #rn := if(#prev = t.section, #rn + 1, 1) as rn
from t cross join (select #rn := 0, #prev := '') const
) tsum
on t.id = tsum.id
set t.ordering = tsum.rn

Answer 2

You don't want to do this as an UPDATE, as that will be really slow.
Instead, do this on INSERT. Here's a simple one-line INSERT that will grab the next order number and inserts a record called 'kiwi' in the section 'fruits'.
INSERT INTO `table_name` (`section`, `order`, `thing`)
SELECT 'fruits', MAX(`order`) + 1, 'kiwi'
FROM `table_name`
WHERE `section` = `fruits`
EDIT: You could also do this using an insert trigger, e.g.:
DELIMITER $$
CREATE TRIGGER `trigger_name`
BEFORE INSERT ON `table_name`
FOR EACH ROW
BEGIN
SET NEW.`order` = (SELECT MAX(`order`) + 1 FROM `table_name` WHERE `section` = NEW.`section`);
END$$
DELIMITER ;
Then you could just insert your records as usual, and they will auto-update the order value.
INSERT INTO `table_name` (`section`, `thing`)
VALUES ('fruits', 'kiwi')

Answer 3

Rather than storing the ordering, you could derive it:
SELECT t.id
,t.section
,#row_num := IF (#prev_section = t.section, #row_num+1, 1) AS ordering
,t.thing
,#prev_section := t.section
FROM myTable t
,(SELECT #row_num := 1) x
,(SELECT #prev_value := '') y
ORDER BY t.section, t.id
Note that order is a keyword and is therefore not the greatest for a column name. You could quote the column name or give it a different name...