FirebirdSQL Unique & Max (or MaxValue) - sql

Question

I want to run a report from sales of any customer that has ordered in the last two years.
I can run a report of all invoices dated within two years then remove duplicates in excel, but I would rather do it directly within (Firebird) SQL
I can use a WHERE date < 1 Jan 2015 (2 years or thereabours), but how do I get it to only show the customer once? I thought if I used MAX(Date) therefore showing the most recent date in that two year period. Where am I going wrong? I believe I need to use a UNIQUE() function like UNIQUE(ORDERCUSTOMER) within the SELECT clause.
SELECT
FINANCIALSALESINVOICES.TRANSACTIONDATE,
FINANCIALSALESINVOICES.INVOICECUSTOMER,
FINANCIALSALESINVOICES.ORDERCUSTOMER,
FINANCIALSALESINVOICES.INVOICENUMBER,
FINANCIALSALESINVOICES.SOURCENUMBER,
MAX(FINANCIALSALESINVOICES.TRANSACTIONDATE)
FROM FINANCIALSALESINVOICES
WHERE (FINANCIALSALESINVOICES.TRANSACTIONDATE>={d '2015-01-01'})
ORDER BY FINANCIALSALESINVOICES.INVOICECUSTOMER, FINANCIALSALESINVOICES.TRANSACTIONDATE
I did having it showing the max date for each instance of invoice in the past two years, but now can't fine that file or replicate it.

Answer 1

One approach is to use a subquery in the WHERE clause which checks for the most recent invoice:
SELECT
t.TRANSACTIONDATE,
t.INVOICECUSTOMER,
t.ORDERCUSTOMER,
t.INVOICENUMBER,
t.SOURCENUMBER
FROM FINANCIALSALESINVOICES t
WHERE t.TRANSACTIONDATE >= date '2015-01-01' AND
t.TRANSACTIONDATE = (SELECT MAX(f.TRANSACTIONDATE)
FROM FINANCIALSALESINVOICES f
WHERE t.ORDERCUSTOMER = f.ORDERCUSTOMER AND
f.TRANSACTIONDATE >= date '2015-01-01')
ORDER BY t.INVOICECUSTOMER,
t.TRANSACTIONDATE

Answer 2

With Firebird 3 you can use row_number() to assign a unique value to each row within a group (partition), that value can then be filtered on:
select
a.TRANSACTIONDATE,
a.INVOICECUSTOMER,
a.ORDERCUSTOMER,
a.INVOICENUMBER,
a.SOURCENUMBER
from (
select
TRANSACTIONDATE,
INVOICECUSTOMER,
ORDERCUSTOMER,
INVOICENUMBER,
SOURCENUMBER,
row_number() over (partition by INVOICECUSTOMER, order by TRANSACTIONDATE desc) as rownr
from FINANCIALSALESINVOICES
where TRANSACTIONDATE >= date '2015-01-01'
) a
where a.rownr = 1
order by a.INVOICECUSTOMER, a.TRANSACTIONDATE
See also Window (Analytical) Functions in the Firebird 3 release notes.