Firebird Query- Return first row each group

Question

In a firebird database with a table \"Sales\", I need to select the first sale of all customers. See below a sample that show the table and desired result of query.

Answer 1

Search for the sales with no earlier sales:

SELECT S1.*
FROM SALES S1
LEFT JOIN SALES S2 ON S2.CUSTOMERID = S1.CUSTOMERID AND S2.DTHRSALE < S1.DTHRSALE
WHERE S2.ID IS NULL

Define an index over (customerid, dthrsale) to make it fast.