I’ve blogged about generic methods of getting prime 1 or prime n per class queries earlier than on this weblog.
An Oracle particular model in that submit used the arcane KEEP
syntax:
SELECT
max(actor_id) KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id),
max(first_name) KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id),
max(last_name) KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id),
max(c) KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id)
FROM (
SELECT actor_id, first_name, last_name, depend(film_id) c
FROM actor
LEFT JOIN film_actor USING (actor_id)
GROUP BY actor_id, first_name, last_name
) t;
It is a bit tough to learn while you see it for the primary time. Consider it as a sophisticated approach to say you wish to get the primary worth per group. This hypothetical syntax could be a lot nicer:
SELECT
FIRST(actor_id ORDER BY c DESC, actor_id),
FIRST(first_name ORDER BY c DESC, actor_id),
FIRST(last_name ORDER BY c DESC, actor_id),
FIRST(c ORDER BY c DESC, actor_id)
FROM (...) t;
So, we’re getting the FIRST
worth of an expression per group once we order the group contents by the ORDER BY
clause.
Oracle’s syntax takes into consideration that ordering could also be non-deterministic, resulting in ties when you don’t embrace a singular worth within the
ORDER BY
clause. In that case, you may combination all of the ties, e.g. to get anAVG()
if that is sensible in your corporation case. If you happen to don’t care about ties, or guarantee there aren’t any ties,MAX()
is an OK workaround, or since 21c,ANY_VALUE()
Now, there’s fairly a little bit of repetition while you’re projecting a number of columns per group like that. Window capabilities have a WINDOW
clause, the place frequent window specs will be named for repeated use. However GROUP BY
doesn’t have such a function, most likely as a result of solely few circumstances come up the place this could be helpful.
However fortunately, Oracle has:
OBJECT
varieties, that are simply nominally typed row worth expressionsANY_VALUE
, an combination perform that generates any worth per group, which has been added in Oracle 21c
With these two utilities, we will do that:
CREATE TYPE o AS OBJECT (
actor_id NUMBER(18),
first_name VARCHAR2(50),
last_name VARCHAR2(50),
c NUMBER(18)
);
And now:
SELECT
ANY_VALUE(o(actor_id, first_name, last_name, c))
KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id)
FROM (...) t;
Be aware, it will be potential to make use of MAX()
in older Oracle variations, when you work round this error message as nicely:
ORA-22950: can not order objects with out MAP or ORDER technique
That is only a workaround, after all. It’s tedious to handle named OBJECT
varieties like that for each case of aggregation. If you happen to don’t want the sort security, you may all the time additionally simply use JSON as an alternative:
SELECT
ANY_VALUE(JSON_OBJECT(actor_id, first_name, last_name, c))
KEEP (DENSE_RANK FIRST ORDER BY c DESC, actor_id)
FROM (...) t;