Set insert trigger - value to highest value in column + 1 - sql

Question

I have a lists table with the following columns
lists:
- id
- category_id
- name
- sort_order
When adding a new row to the table I want to set the value of the sort order to the highest sort order value + 1 where the category_id is the same as the new row's category_id. Is there a way to achieve this?
UPDATE
So with the recommnendation to create an insert trigger I tried this
CREATE FUNCTION set_list_sort_order()
RETURNS trigger AS $$
BEGIN
NEW.sort_order := select (select sort_order from lists where category_id = NEW.category_id order by sort_order DESC limit 1) + 1
RETURN NEW;
END
$$ LANGUAGE 'plpgsql';
CREATE TRIGGER set_list_sort_order_trigger
BEFORE INSERT ON lists
FOR EACH ROW
EXECUTE PROCEDURE set_list_sort_order()
But I get this error:
ERROR: syntax error at or near "select"
LINE 4: NEW.sort_order := select (select sort_order from lists w...

Answer 1

You can set up a trigger. Or you can set up the logic directly in the insert:
insert into lists (category_id, name, sort_order)
select category_id, name,
coalesce(max(sort_order) + 1, 1)
from (values (?, ?)) v(category_id, name) left join
lists l
using (category_id)
group by v.category_id, v.name;
Note that a query like this could have race conditions. So, you might want to go the route of using a trigger.

Answer 2

you can try to create stored procedure with something like this:
DEClARE #category_id INT = 1
;WITH NextNum AS
(
SELECT
CASE
WHEN MAX(Table1.sort_order) IS NULL THEN 1
ELSE MAX(Table1.sort_order) + 1
END AS NextValue
FROM Table1 AS Table1
WHERE Table1.category_id = #category_id
)
INSERT INTO Table1
(
id
category_id
name
sort_order
)
SELECT #id,
#category_id
#name
NextNum.NextValue
FROM NextNum