Can somebody help me translate this into postgresql? - postgresql

Question

I am very new to SQL and I do not know much about writing code in the different DBMS. I am trying to write a report in our school's MOODLE platform, which uses postgresql, using a configurable report found here. However, the code does not work in postgresql. In particular, how do I rewrite those lines with variable assignments like #prevtime := to make the code work in postgresql?
Here is the complete code from the link.
SELECT
l.id,
l.timecreated,
DATE_FORMAT(FROM_UNIXTIME(l.timecreated),'%d-%m-%Y') AS dTime,
#prevtime := (SELECT MAX(timecreated) FROM mdl_logstore_standard_log
WHERE userid = %%USERID%% AND id < l.id ORDER BY id ASC LIMIT 1) AS prev_time,
IF (l.timecreated - #prevtime < 7200, #delta := #delta + (l.timecreated-#prevtime),0) AS sumtime,
l.timecreated-#prevtime AS delta,
"User" AS TYPE
FROM prefix_logstore_standard_log AS l,
(SELECT #delta := 0) AS s_init
# CHANGE UserID
WHERE l.userid = %%USERID%% AND l.courseid = %%COURSEID%%
%%FILTER_STARTTIME:l.timecreated:>%% %%FILTER_ENDTIME:l.timecreated:<%%
This is supposed to report the time spent by students in courses in MOODLE.

Answer 1

I assume the original query was written for MySQL. You haven't explained what the query actually does, but the #prevtime hack is usually a workaround for missing window functions, so most probably this can be done using lag() in Postgres, something along the lines:
select l.id,
l.timecreated,
to_char(to_timestamp(l.timecreated), 'dd-mm-yyyy') as dtime,
lag(timecreated) over w as prev_time,
l.timecreated - lag(timecreated) over w as delta,
'User' as type,
FROM prefix_logstore_standard_log AS l
window w as (partition by userid order by id)
WHERE l.userid = %%USERID%%
AND l.courseid = %%COURSEID%%