how to perform these queries? - sql

Question

I have these three tables:
create table albums(sernum number primary key,
Albname varchar2(30) not null,
Artist varchar2(20) not null,
Pdate number(4),
Recompany varchar2(10),
Media char(2) not null);
create table tracks(sernum number not null,
song varchar2(50) not null,
primary key(sernum, song),
foreign key(sernum) references albums(sernum));
create table performers(sernum number not null,
Artist varchar2(30) not null,
Instrument varchar2(50) not null,
primary key(sernum, Artist, Instrument),
foreign key (sernum) references albums(sernum));
I want to perform two queries in sql oracle:
list the names of the artists that used all instruments.
list the names of the albums containing the maximum number or songs.
here is my tries:
select distinct(a.Artist) from albums a where a.Artist like (select p.Artist, distinct(p.Instrument) from performers p) group by a.Artist, p.Instrument;
select a.Albname from albums a, inner join tracks t on where a.sernum in(select max(t.sernum) group by t.sernum);

Answer 1

Query 1 - get artists who have played all instruments:
SELECT
p.Artist
FROM
(
SELECT Artist, count(distinct Instrument) as InstrumentCount
FROM performers
GROUP BY artist
) p
JOIN
(
SELECT COUNT(DISTINCT Instrument) as InstrumentCount
FROM performers
) i
ON p.InstrumentCount = i.InstrumentCount
Explanation: 1st subquery gets the count of instruments played by each artist. 2nd subquery gets the count of unique instruments. The two are joined together based on this instrument count to give us only those artists whose instrument counts match the maximum.
--
Query 2 - Get albums containing the maximum number of songs:
WITH
AlbumTrackCount
(
SELECT
sernum,
COUNT(1) as TrackCount
FROM tracks
GROUP BY sernum
)
SELECT
a.Albname
FROM albums a
JOIN AlbumTrackCount atc
ON a.sernum = atc.sernum
AND atc.TrackCount =
(
SELECT MAX(TrackCount)
FROM AlbumTrackCount
)
Explanation: the WITH up top establishes a subquery we'll reuse; it gets us the track count within each album. Down below, we join the albums with this album track count, and add a filter that only those albums with a track count equal to the maximum track count of any of the albums. Note that this is different from the top query, which just got every instrument ever; here, it is important to first count up the tracks within each album, and then get the maximum of those counts.

Answer 2

Below are some of the issues with your queries:
SELECT DISTINCT (a.artist)
FROM albums a
WHERE a.artist LIKE (SELECT p.artist,
distinct(p.Instrument)
from performers p)
group by a.Artist, p.Instrument;
LIKE indicates that you're going to use a wildcard. When comparing against a sub-query in the where clause, you typically use in as the operator.
DISTINCT is not a function. It always applies to all of the columns in a SELECT statement.
DISTINCT and GROUP BY serve very similar purposes. You would rarely use both in the same statement.
You can't reference a column from a correlated sub-query (i.e. a query in the where clause), in the outer query.
SELECT a.albname
FROM albums a,
inner join tracks t
on
where a.sernum in(select max(t.sernum) group by t.sernum);
Your using both a comma and inner join to connect two tables. The comma indicates pre-SQL:1999 syntax, whereas INNER JOIN is SQL:1999. While, technically you can use both in a single FROM clause, you can't use both between a single pair of tables. Also, you shouldn't use both. Sticj to SQL:1999.
Your ON clause is empty. You should probably be joining your two tables here. If you really want to not have a join condition, change the join to CROSS JOIN (to re-iterate: you almost certainly don't actually want this).
You have a SELECT statement without a FROM clause. That is not allowed.