Last Invoice using Postgres - sql

Question

I have a Postgres 9.1 database with three tables - Customer, Invoice, and Line_Items
I want to create a customer list showing the customer and last invoice date for any customer with a specific item (specifically all invoices that have the line_items.code beginning with 'L3').
First, I am trying to pull the one transaction for each customer (the last invoice with the 'L3" code) (figuring I can JOIN the customer names once this list is created).
Tables are something like this:
Customers
cust_number last_name first_name
=========== ======== ====================
1 Smith John
2 Jones Paul
3 Jackson Mary
4 Brown Phil
Transactions
trans_number date cust_number
=========== =========== ====================
1001 2014-01-01 1
1002 2014-02-01 4
1003 2014-03-02 2
1004 2014-03-06 3
Line_Items
trans_number date item_code
=========== =========== ====================
1001 2014-01-01 L3000
1001 2014-01-01 M2420
1001 2014-01-01 L3500
1002 2014-02-01 M2420
1003 2014-03-02 M2420
1004 2014-03-06 L3000
So far, I have:
Select transactions.cust_number, transactions.trans_number
from transactions
where transactions.trans_number in
( SELECT Line_Items.trans_number
FROM Line_Items
WHERE Line_Items.item_code ilike 'L3%'
ORDER BY line_items.date DESC
)
order by transactions.pt_number
This pulls all the invoices for each customer with an 'L3' code on the invoice, but I can't figure out how to just have the last invoice.

Answer 1

Use DISTINCT ON:
SELECT DISTINCT ON (t.cust_number)
t.cust_number, t.trans_number
FROM line_items l
JOIN transactions t USING (trans_number)
WHERE l.item_code ILIKE 'L3%'
ORDER BY t.cust_number, l.date DESC;
This returns at most one row per cust_number - the one with the latest trans_number. You can add more columns to the SELECT list freely.
Detailed explanation:
Select first row in each GROUP BY group?

Answer 2

you could use MIN or MAX:
SELECT Line_Items.trans_number, Max(line_items.date) As [last]
From Line_Items
Group By Line_Items.trans_number