Find all invoice dates of customer named Astrid.
select t2.invoicedate from customer as t1 join invoice as t2 on t1.customerid = t2.customerid where t1.firstname = "Astrid"
Hm, find them for Astrids with last name Gruber.
select t2.invoicedate from customer as t1 join invoice as t2 on t1.customerid = t2.customerid where t1.firstname = "Astrid" and lastname = "Gruber"