How to select max price using multiple query - sql

Question

i have create tow table, table and data inserting value are given below :
create table product (
pid number(10) primary key,
pname varchar2(30)
);
INSERT INTO product values(100,'Apple');
INSERT INTO product values(101,'Banana');
INSERT INTO product values(102,'Pinaple');
INSERT INTO product values(103,'Orange');
create table purchase(
invid number(10) primary key,
pid number(10),
pprice number(10)
);
alter table purchase add(constraint pid_fk FOREIGN KEY (pid) references product(pid));
INSERT INTO purchase values(10,101,30);
INSERT INTO purchase values(11,103,35);
INSERT INTO purchase values(12,103,9);
INSERT INTO purchase values(13,103,22);
INSERT INTO purchase values(14,101,12);
now i have select table purchase show give data
invid pid pprice
10 101 30
11 103 35
12 103 9
13 103 22
14 101 12
now i want to be last insert price pid=103
now need show pprice=22
i have all ready done the task using two query such as:
select max(invid) from purchase where pid=103;
result is 22 and running next statement
select max(pprice) from purchase where invid=13;
show result =22
i want to show result 22 using running one statment,

Answer 1

Probably the most direct way is to use a subquery:
SELECT MAX(pprice)
FROM purchase
WHERE invid = (SELECT MAX(invid) FROM purchase WHERE pid = 103)
Side note, you don't need to MAX() pprice based on your sample data, but I left it that way in case you need it for your real data set.

Answer 2

The aggregate FIRST/LAST function is made exactly for this kind of task. Unfortunately it seems the vast majority of developers don't use it - either because they are not aware of its existence, or because they don't understand how it works, or for who knows what other reason.
https://docs.oracle.com/cd/B19306_01/server.102/b14200/functions056.htm
The function allows for the case when the max(invid) (in your model) is not unique; in which case you must decide WHICH pprice to select. You can select the max, or min, or average pprice (or the sum for that matter), from all those that have pid = 103 and the max value in the invid column. Of course, if the max(invid) is unique (as it is in your problem), then all these functions will return just the pprice associated to that invid; but you must still use one of these aggregate functions, since that uniqueness is not known at parsing time, it is only known at execution time. min is conventionally used in this manner. So:
select min(pprice) keep (dense_rank last order by invid) as pprice
from ........
where pid = 103
;