Updating a column with the results of a query in PostgreSQL

Question

I have the following table in PostgreSQL 9.2 which contains time stamps:

gid [PK] (bigserial), timestamp_mes (timestamp without time zone), time_diff (interval)
1, 2012-01-23 11:03:40, empty
2, 2012-01-23 11:03:42, empty
3, 2012-01-23 11:03:44, empty

I have added a interval column (time_diff) and would like to fill it with time difference values resulting from this query:

SELECT timestamp_mes - lag(timestamp_mes, 1) 
over (order by timestamp_mes) as diff
from gc_entretien.trace order by timestamp_mes

I have tried the following query to update the time_diff column, with no success:

UPDATE gc_entretien.trace set time_diff = 
(SELECT trace.timestamp_mes - lag(trace.timestamp_mes, 1) 
over (order by trace.timestamp_mes) 
from gc_entretien.trace order by timestamp_mes);

This results in an error:

ERROR: more than one row returned by a subquery used as an expression

How should I proceed to update the time_diff column with the values resulting from the time difference query?

Answer 1

Something like this:

with new_values as (
   SELECT gid, 
          timestamp_mes - lag(timestamp_mes, 1) over (order by timestamp_mes) as diff
   from gc_entretien.trace 
)
update gc_entretien.trace as tr
  set time_diff = nv.diff
from new_values nv
where nv.gid = tr.gid;

Answer 2

You can't directly use a window function in an UPDATE, so you instead need to use it in a sub-SELECT - which you have done. However, the way you've tried to use that sub-SELECT in your UPDATE is not valid syntax. You need to put the sub-SELECT in the FROM clause of your update, as explained by the Postgres docs here:

http://www.postgresql.org/docs/9.2/static/sql-update.html

The correct syntax for what you want to do is:

UPDATE gc_entretien.trace t
SET time_diff = subquery.diff
FROM (SELECT {{SomeUniqueId}}, 
             timestamp_mes - lag(timestamp_mes, 1) over (order by timestamp_mes) as diff
      FROM gc_entretien.trace order by timestamp_mes) AS subquery
WHERE t.{{SomeUniqueId}} = subquery.{{SomeUniqueId}}

Obviously, you'll need to substitute in the column name of some unique id that your rows have where I've written {{SomeUniqueId}}

Answer 3

Actually you are getting this error because your subquery returns multiple result,

I am not able to understand your query so,

I will give you an example to solve it,

update table t1 set time_diff= select *your_operation* from table t2 where t1.id=t2.id

Here :-your_operation means the logic of finding time difference,