Updates which include auto increment column - sql

Question

I am using PostgreSQL and I would like to update a table which would include an auto number column id. I know it might something very simple but I have done something like this;
CREATE SEQUENCE SEQ_ID
MINVALUE 1
START WITH 1
INCREMENT BY 1
CACHE 10
update trk2
set (id, track_id, track_point) =
(select nextval('seq_id'), trk1.track_fid, trk1.wkb_geometry
from track_points_1 as trk1
where track_fid = 0)
The thing is that it is giving me the following error (code reformatted):
ERROR: syntax error at or near "select"
LINE 3: set (id, track_id, track_point)=(select nextval('seq_id'), t...
Can anybody help please?

Answer 1

Your query is broken in several places. It could look something like this:
UPDATE trk2
SET (id, track_id, track_point) =
(next_id, x.track_fid, x.wkb_geometry)
FROM (
SELECT id
,nextval('seq_id') AS next_id
,track_fid
,wkb_geometry
FROM track_points_1
WHERE track_fid = 0
) x
WHERE trk2.id = x.id; -- adapt to your case
Or simpler (preferable syntax):
UPDATE trk2
SET (id, track_id, track_point) =
(nextval('seq_id'), x.track_fid, x.wkb_geometry)
FROM track_points_1 x
WHERE x.track_fid = 0
AND trk2.id = x.id; -- adapt to your case
Major points:
An UPDATE without a WHERE clause only makes sense if you really need to change each and every row in the table. Else it is wrong or at least sub-optimal.
When you retrieve values from another table, you get a CROSS JOIN between target and source if you don't add a WHERE clause connecting target with source - meaning that every row of the target table will be updated with every row in the source table. This can take a very long time and lead to arbitrary results. The last UPDATE wins. In short: this is almost always complete nonsense and extremely expensive at that.
In my example I link target and source by the id column. You have to replace that with whatever fits in your case.
You can assign multiple values in one SET clause in an UPDATE, but you can only return a single value from a correlated subselect expression. Therefore, it is strictly not possible to have a subselect in a SET clause with multiple values.
Your initial syntax error comes from a missing pair of parenthesis around your subselect. But adding that only reveals the error mentioned above.
Depending on what you are after, you would include nextval('seq_id') in the subquery or in the SET clause directly. This can lead to very different results, especially when you have rows in the subquery that are not used in the UPDATE.
I placed it in the SET clause because I suspect, that's what you want. The sequence of rows is still arbitrary. If you want more control over which numbers are assigned, you need to define what you want and then take a different route.

Answer 2

Tyler Eaves is correct, this action is not advisable.
However, if you insist, this may help:
Once you have a sequence on a column you don't have to call nextval() in the SET statement. Just leave it out and the column will auto-increment.
UPDATE
trk2
SET
(
track_id = [track_id Value],
track_point = [track_point Value]
)
or, include it and set it to default or null
UPDATE
trk2
SET
(
id = default,
track_id = [track_id Value],
track_point = [track_point Value]
)
Using your example:
UPDATE
trk2
SET
(
track_id,
track_point
) = (
SELECT
trk1.track_fid,
trk1.wkb_geometry
FROM
track_points_1 AS trk1
WHERE
track_fid = 0
)

Answer 3

I think you need a second set of parentheses on the RHS of the SET clause:
UPDATE trk2
SET (id, track_id, track_point) =
((SELECT nextval('seq_id'), trk1.track_fid, trk1.wkb_geometry
FROM track_points_1 as trk1
WHERE track_fid = 0));
The first set of parentheses matches the parentheses on the LHS of the SET clause. The second set wraps up the sub-query for re-use.