How to use SUM and MAX in select statement more than one table - oracle

Question

I have 2 Tables
table a mempur
memberno = member number
purdt = purchase date
amount = purchase amount
table b meminfo
memberno = member number
fname = first name
age = age
select a.memberno,b.fname,sum(a.amount),a.purdt,b.age from mempur a,(select max(purdt) as maxdate,memberno from mempur group by memberno) maxresult,meminfo b
where a.memberno=b.memberno
and a.purdt between '01-JAN-22' and '28-FEB-22'
and a.memberno=maxresult.memberno
and a.purdt=maxresult.maxdate
group by a.memberno,b.fname,a.purdt,b.age
order by a.memberno;
How to get my result with total purchase amount and highest date purchase from table mempur?
I use this query able to show the result but the total amount incorrect between the range.
Anyone help is much appreciated.
my sample data
MEMBERNO PURDT AMOUNT
--------------- --------------- ---------
BBMY0004580 12-AUG-21 823.65
BBMY0004580 12-AUG-21 1709.1
BBMY0004580 26-AUG-21 1015.1
BBMY0004580 28-AUG-21 1105.1
my result only show total amount 1105.1

Answer 1

You can aggregate in mempur and then join to meminfo:
SELECT i.*, p.total_amount, p.maxdate
FROM meminfo i
INNER JOIN (
SELECT memberno, SUM(amount) total_amount, MAX(purdt) maxdate
FROM mempur
WHERE purdt BETWEEN '01-JAN-22' AND '28-FEB-22'
GROUP BY memberno
) p ON p.memberno = i.memberno;
You may use a LEFT join if there are members with no purchases which you want in the results.

Answer 2

Your query gets the maximum purdt and adds up the amount for this date. It also checks whether the maximum purdt is in January or February 2022. If it is, the result gets show, if it is not, you don't show the result. This is not the query you want.
Apart from that, the query looks rather ugly. You are using an ancient join syntax that is hard to read and prone to errors. We used that in the 1980s, but in 1992 explicit joins made it into the SQL standard. You should no longer use this old comma syntax. It is strange to see it still being used. Feels like going to a museum. Then, you are using table aliases. The idea of these is to get a query more readable, but yours even lessen readability, because your alias names a and b are arbitrary. Use mnemonic names instead, e.g. mp for mempur and mi for meminfo. Then, you are comparing the date (I do hope purdt is a date!) with strings. Don't. Use date literals instead.
As to your tables: Are you really storing the age? You will have to update it daily to keep it up-to-date. Better store the date of birth and calculate the age from it in your queries.
Here is a query that gets you the maximum date and the total amount for the given date range:
select memberno, m.name, p.sum_amount, p.max_purdt, m.age
from meminfo m
left outer join
(
select memberno, sum(amount) as sum_amount, max(purdt) as max_purdt
from mempur
where purdt >= date '2022-01-01' and purdt < date '2022-03-01'
group by memberno
) p using (memberno)
order by memberno;
And here is a query that gets you the maximum overall date along with the total amount for the given date range:
select memberno, m.name, p.sum_amount, p.max_purdt, m.age
from meminfo m
left outer join
(
select
memberno,
sum(case when where purdt >= date '2022-01-01' and purdt < date '2022-03-01'
then amount
end) as sum_amount,
max(purdt) as max_purdt
from mempur
group by memberno
) p using (memberno)
order by memberno;