Q1:FIRST_VALUE/LAST_VALUE
Q2:PARTITION BY (जैसा कि रोमन पाकर ने पहले ही सुझाव दिया है)
SELECT
DISTINCT i.id AS id,
i.userid AS userid,
i.itemname AS itemname,
COALESCE(LEAD(i.id) OVER (ORDER BY i.created DESC)
,FIRST_VALUE(i.id) OVER (ORDER BY i.created DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)) AS nextitemid,
COALESCE(LAG(i.id) OVER (ORDER BY i.created DESC)
,LAST_VALUE(i.id) OVER (ORDER BY i.created DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)) AS previtemid,
COALESCE(LEAD(i.id) OVER (PARTITION BY i.userid ORDER BY i.created DESC)
,FIRST_VALUE(i.id) OVER (PARTITION BY i.userid ORDER BY i.created DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)) AS nextuseritemid,
COALESCE(LAG(i.id) OVER (PARTITION BY i.userid ORDER BY i.created DESC)
,LAST_VALUE(i.id) OVER (PARTITION BY i.userid ORDER BY i.created DESC ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)) AS prevuseritemid,
i.created AS created
FROM items i
LEFT JOIN users u
ON i.userid = u.id
ORDER BY i.created DESC;