SQL join: selecting the last records in a one-to-many relationship

Question

Suppose I have a table of customers and a table of purchases. Each purchase belongs to one customer. I want to get a list of all customers along with their last purchase in

Answer 1

Tested on SQLite:

SELECT c.*, p.*, max(p.date)
FROM customer c
LEFT OUTER JOIN purchase p
ON c.id = p.customer_id
GROUP BY c.id

The max() aggregate function will make sure that the latest purchase is selected from each group (but assumes that the date column is in a format whereby max() gives the latest - which is normally the case). If you want to handle purchases with the same date then you can use max(p.date, p.id).

In terms of indexes, I would use an index on purchase with (customer_id, date, [any other purchase columns you want to return in your select]).

The LEFT OUTER JOIN (as opposed to INNER JOIN) will make sure that customers that have never made a purchase are also included.