How to optimize this query for my school project - sql

Question

It's my assignment kindly help me to optimize below two queries.
Optimize assignment 1:
SELECT
n.node_id,
MIN(LEAST(n.date,ec.date)) date
FROM
n, ec
WHERE
(n.node_id = ec.node_id_from OR n.node_id = ec.node_id_to)
AND n.date - ec.date > 0
GROUP BY
n.node_id;
Optimize assignment 2:
SELECT
TO_CHAR(CONVERT_TIMEZONE ('UTC','America/Los_Angeles', tableA."date"), 'YYYY-MM') AS "date_month",
COUNT(DISTINCT CASE WHEN (tableB."date" IS NOT NULL) THEN tableB._id ELSE NULL END) AS "tableB.countB",
COUNT(DISTINCT CASE WHEN (tableC."date" IS NOT NULL) THEN tableC._id ELSE NULL END) AS "tableC.countC"
FROM
tableA AS tableA
LEFT JOIN
tableB AS tableB ON (DATE (CONVERT_TIMEZONE ('UTC', 'America/Los_Angeles',tableB."date"))) = (DATE (CONVERT_TIMEZONE ('UTC', 'America/Los_Angeles',tableA."date")))
LEFT JOIN
tableC AS tableC ON (DATE (CONVERT_TIMEZONE ('UTC', 'America/Los_Angeles',tableC."date"))) = (DATE (CONVERT_TIMEZONE ('UTC', 'America/Los_Angeles',tableA."date")))
WHERE
tableA."date" >= CONVERT_TIMEZONE ('America/Los_Angeles', 'UTC', DATEADD (month, -17, DATE_TRUNC('month', DATE_TRUNC('day', CONVERT_TIMEZONE ('UTC', 'America/Los_Angeles',GETDATE ()))))
GROUP BY
1
ORDER BY
1 DESC
LIMIT 500;

Answer 1

use short alias that makes sql query shorter and cleaner.
Here is the optimized version of second query
SELECT DatePart(month, a.Date-8/24) date_month,
sum(case when b.date is Not null then 1 else 0 end) countb,
sum(case when c.date is Not null then 1 else 0 end) countc,
FROM tableA a
LEFT JOIN tableB b
ON b.Date = a.Date -- Timezone offsets are not necessary,
LEFT JOIN tableC c
ON c.date = a.date -- both in same timezone
WHERE a.date >= DateAdd(hour, 8,
DATEADD (month,-17,DATE_TRUNC('month',
GETDATE () ))
GROUP BY 1
ORDER BY 1 DESC LIMIT 500;

Answer 2

Very simple solution for assignment #1
SELECT n.node_id, MIN(ec.date) as date
FROM n
JOIN ec
ON n.node_id IN (ec.node_id_from, ec.node_id_to) AND ec.date < n.date
GROUP BY n.node_id;
just using min(ec.date) instead of MIN(LEAST(n.date,ec.date)).
Because the JOIN already forces the ec.date to be lower than n.date anyway.
Also note that a where clause like
where (x >= y and x <= z)
can be changed to
where (x between y and z)