Select the highest order delivered - sql

Question

Following is my output:
MONTH STAF STAFFNAME TOTAL_ORDER_DELIVERED
===== ==== ==================== =====================
JAN S009 Theresina Ertelt 1
FEB S015 Lonna Charker 1
MAR S003 Suzi Maccari 2
MAR S010 Zacharie Witty 1
MAR S020 Abbie Gosnoll 1
MAR S017 Renee Alston 1
AUG S006 Falito Ollerton 1
AUG S017 Renee Alston 1
AUG S003 Suzi Maccari 1
OCT S003 Suzi Maccari 3
OCT S020 Abbie Gosnoll 2
What I want is:
MONTH STAF STAFFNAME TOTAL_ORDER_DELIVERED
===== ==== ==================== =====================
JAN S009 Theresina Ertelt 1
FEB S015 Lonna Charker 1
MAR S003 Suzi Maccari 2
AUG S006 Falito Ollerton 1
AUG S017 Renee Alston 1
AUG S003 Suzi Maccari 1
OCT S003 Suzi Maccari 3
I want to select the highest result based on the month but can't figure what to do. Here are my query in SQL:
SELECT TO_CHAR(TO_DATE(EXTRACT(MONTH FROM receivedDate),'mm'),'MON') AS Month,
d.staffID, staffname, count(deliveryID) AS Total_Order_Delivered
FROM delivery d, deliverystaff s
WHERE (d.staffid = s.staffid)
AND (EXTRACT(YEAR FROM receivedDate) = 2020)
GROUP BY EXTRACT(MONTH FROM d.receivedDate),d.staffid, staffname
ORDER BY EXTRACT(MONTH FROM d.receivedDate),count(deliveryID) desc;

Answer 1

I would suggest using RANK here:
WITH cte AS (
SELECT TO_CHAR(TO_DATE(EXTRACT(MONTH FROM receivedDate), 'mm'), 'MON') AS Month,
EXTRACT(MONTH FROM d.receivedDate) AS month_num,
d.staffID, staffname, COUNT(deliveryID) AS Total_Order_Delivered,
RANK() OVER (PARTITION BY EXTRACT(MONTH FROM d.receivedDate), d.staffid, staffname
ORDER BY COUNT(deliveryID) DESC) rnk
FROM delivery d
INNER JOIN deliverystaff s ON d.staffid = s.staffid
WHERE EXTRACT(YEAR FROM receivedDate) = 2020
GROUP BY EXTRACT(MONTH FROM d.receivedDate), d.staffid, staffname
)
SELECT Month, staffID, staffname, Total_Order_Delivered
FROM cte
WHERE rnk = 1
ORDER BY month_num;

Answer 2

Tim's answer is fine. However, I strongly encourage you to make some changes to the query.
First, for the where clause don't use extract(). Use direct date comparisons. Second, include the year and month in the aggregation. Then, be sure that you qualify all column references.
That allows you to do:
SELECT sd.*
FROM (SELECT TO_CHAR(d.receivedDate, 'YYYY-MON') AS Month_year,
s.staffID, s.staffname, COUNT(*) AS Total_Order_Delivered,
RANK() OVER (PARTITION BY TO_CHAR(d.receivedDate, 'YYYY-MON') ORDER BY COUNT(*) DESC) as seqnum,
MIN(d.receiveddate) as min_receiveddate
FROM deliverystaff s JOIN
delivery d
ON d.staffid = s.staffid
WHERE d.receivedDate >= DATE '2020-01-01' AND
d.receivedDate < DATE '2021-01-01'
GROUP BY TO_CHAR(d.receivedDate, 'YYYY-MON') AS Month,
s.staffID, s.staffname
) sd
WHERE seqnum = 1
ORDER BY min_receiveddate;
In addition to the above, this allows you to order the results chronologically and works if you extend the time frame to more than one year.