MySQL SELECT queries without LIMIT - mysql

Question

I am doing a course on Relational Databases, MySQL to be more especific. We need to create some SELECT queries for a project. The project is related to music. It has tables to represent musicians (musician), bands (band) and the musician ability to do a certain task, like singing or playing the guitar (act).
Table musician contains :
id
name
stagename
startyear
Table band contains :
code
name
type ("band" or "solo")
startyear
And finally, table act contains :
band (foreign key to code of "band" table)
musician (foreign key to id of "musician" table)
hability (guitarist, singer, like that... and a foreign key to another table)
earnings
I have doubts in two exercises, the first one asks to select musicians id and stagename who participate with more acts in bands whose type is solo.
My solution for the first one is this:
SELECT ma.id, ma.stagename
FROM musician ma, act d, band ba
WHERE ma.id = d.musician
AND ba.code = d.band
AND ba.type = "solo"
GROUP BY ma.id, ma.stagename
HAVING COUNT(ma.id) = (SELECT COUNT(d2.musician) AS count
FROM act d2, band ba2
WHERE d2.band = ba2.code
AND ba2.type = "solo"
GROUP BY d2.musician
ORDER BY count DESC
LIMIT 1);
The second one is very similar to the last one. We need to select, for every startyear, the id and stagename of a musician who can do more acts, with the corresponding number of acts and the maximum and minimum of his cachet. This is my solution:
SELECT ma.startyear, ma.id, ma.stagename, COUNT(ma.id) AS NumActs, MIN(d.earnings), MAX(d.earnings)
FROM musician ma, act d, band ba
WHERE ma.id = d.musician
AND ba.code = d.band
AND ba.type = "solo"
GROUP BY ma.year, ma.id, ma.stagename
HAVING COUNT(ma.id) = (SELECT COUNT(d2.musician) AS count
FROM act d2, band ba2
WHERE d2.band = ba2.code
AND ba2.type = "solo"
GROUP BY d2.musician
ORDER BY count DESC
LIMIT 1);
The results with my dummy data are perfect but my teacher told us we should avoid using the LIMIT option, but that's the only way we can get the highest number, at least with what we know right now.
I've seen a lot of subqueries after the FROM statement to solve this problem, however, for this project we can't use subqueries inside FROM. Is this really possible without LIMIT ?
Thanks in advance.

Answer 1

It is possible, but much worse than with sub-query in from or limit. So I'd never use it in real life :)
Well, long story short, you can do something like this:
SELECT
m.id
, m.stagename
FROM
musician m
INNER JOIN act a ON (
a.musician = m.id
)
INNER JOIN band b ON (
b.code = a.band
AND b.type = 'solo'
)
GROUP BY
m.id
, m.stagename
HAVING
NOT EXISTS (
SELECT
*
FROM
act a2
INNER JOIN band b2 ON (
b2.code = a2.band
AND b2.type = 'solo'
)
WHERE
a2.musician != a.musician
GROUP BY
a2.musician
HAVING
COUNT(a2.musician) > COUNT(a.musician)
)
;
I think you can understand the idea from the query itself as it's pretty straightforward. However, let me know if you need an explanation.
It is possible that your restriction was slightly different and you were not allowing to use subquery in your main FROM part only.
P.S. I'm also use INNER JOIN ... ON syntax as it is easier to see what are table join conditions and what are where conditions.
P.P.S. It might be mistakes in query as I do not have your data structure so cannot execute the query and check. I only checked if the idea works with my test table.

Answer 2

EDIT I just re-read the question; my initial reading missed that inline views are disallowed.
We can avoid the ORDER BY ... DESC LIMIT 1 construct by making the subquery into an inline view (or, a "derived table" in the MySQL parlance), and using a MAX() aggregate.
As a trivial demonstration, this query:
SELECT b.foo
FROM bar b
ORDER
BY b.foo DESC
LIMIT 1
can be emulated with this query:
SELECT MAX(c.foo) AS foo
FROM (
SELECT b.foo
FROM bar b
) c
An example re-write of the first query in the question
SELECT ma.id
, ma.stagename
FROM musician ma
JOIN act d
ON d.musician = ma.id
JOIN band ba
ON ba.code = d.band
WHERE ba.type = 'solo'
GROUP
BY ma.id
, ma.stagename
HAVING COUNT(ma.id)
= ( SELECT MAX(c.count)
FROM (
SELECT COUNT(d2.musician) AS count
FROM act d2
JOIN band ba2
ON ba2.code = d2.band
WHERE ba2.type = 'solo'
GROUP
BY d2.musician
) c
)
NOTE: this is a demonstration of a rewrite of the query in the question; this makes no guarantee that this query (or the query in the question) are guaranteed to return a result that satisfies any particular specification. And the specification given in the question is not at all clear.