I don't know how to explain the problem in a generic way so i'll post the specific case:
i have 3 tables:
Sailors:
S(ids, names, rating, age)
Boats:
B(idb, nameb, color)
Bookings:
Bo(ids, idb, date)
i have to write a query that finds all the sailors who have booked EVERY boat.
Even if i posted a specific case i'd like a generic answare that can be applied to every problem of tha same kind.
thank you in advance.
You can get the sailors's ids who have booked every boat with this query:
select ids
from bookings
group by ids
having count(distinct idb) = (select count(*) from boats)
So use it either with the operator IN:
select * from sailors
where ids in (
select ids
from bookings
group by ids
having count(distinct idb) = (select count(*) from boats)
)
or join it to sailors:
select s.*
from sailors s
inner join (
select ids
from bookings
group by ids
having count(distinct idb) = (select count(*) from boats)
) t on t.ids = s.ids
You can use sum with in:
select * from sailors s1 group by ids having (select sum(idb in (select b2.idb from bookings b2 where b2.ids = s1.id)) from boats) = (select count(*) from boats)
Related
I have the following tables:
Students (id, name, surname, study_year, department_id)
Courses(id, name)
Course_Signup(id, student_id, course_id, year)
I want to display the courses to which at least 10 students have signed up for, only using subqueries (no group-by, join or set operations). This could be easily implemented using data aggregation and join:
SELECT c.name, COUNT(csn.course_id)
FROM Course_Signup csn
JOIN Courses c
ON csn.course_id = c.id
GROUP BY c.name
HAVING COUNT(csn.course_id) >= 10
But how would I do this only using subqueries? Is there any other way, other than COUNT, to get the number of courses? Thank you, in advance!
You can use a correlated sub-query to retrieve the name:
SELECT (SELECT c.name FROM Courses c WHERE csn.course_id = c.id) AS name,
COUNT(*)
FROM Course_Signup csn
GROUP BY
course_id
HAVING COUNT(*) >= 10
Note: you should also GROUP BY the primary key the uniquely identifies the course as there may be two courses with the same name.
If you also don't want to use GROUP BY then:
SELECT name
FROM Courses c
WHERE 10 <= ( SELECT COUNT(*)
FROM Course_Signup csn
WHERE csn.course_id = c.id )
or, to also get the number of sign-ups:
SELECT *
FROM (
SELECT name,
( SELECT COUNT(*)
FROM Course_Signup csn
WHERE csn.course_id = c.id ) AS num_signups
FROM Courses c
)
WHERE num_signups >= 10;
You could do:
SELECT c.name
FROM Courses c
WHERE (
SELECT COUNT(*)
FROM Course_Signup csn
WHERE csn.course_id = c.id
) >= 10
which only uses a subquery and has no group-by, join or set operations.
fiddle
If you wanted the actual count in the result set then you would need to repeat the subquery in the select list.
You might also need to do COUNT(DISTINCT cs.student_id) if there might be duplicates; particularly if the same student can sign up in multiple years - but then you might want to restrict to a single year anyway.
I have a users table and each user has flights in a flights table. Each flight has a departure and an arrival airport relationship within an airports table. What I need to do is count up the unique airports across both departure and arrival columns (flights.departure_airport_id and flights.arrival_airport_id) for each user, and then assign them a rank via dense_rank and then retrieve the rank for a given user id.
Basically, I need to order all users according to how many unique airports they have flown to or from and then get the rank for a certain user.
Here's what I have so far:
SELECT u.rank FROM (
SELECT
users.id,
dense_rank () OVER (ORDER BY count(DISTINCT (flights.departure_airport_id, flights.arrival_airport_id)) DESC) AS rank
FROM users
LEFT JOIN flights ON users.id = flights.user_id
GROUP BY users.id
) AS u WHERE u.id = 'uuid';
This works, but does not actually return the desired result as count(DISTINCT (flights.departure_airport_id, flights.arrival_airport_id)) counts the combined airport ids and not each unique airport id separately. That's how I understand it works, anyway... I'm guessing that I somehow need to use a UNION join on the airport id columns but can't figure out how to do that.
I'm on Postgres 13.0.
I would recommend a lateral join to unpivot, then aggregation and ranking:
select *
from (
select f.user_id,
dense_rank() over(order by count(distinct a.airport_id) desc) rn
from flights f
cross join lateral (values
(f.departure_airport_id), (f.arrival_airport_id)
) a(airport_id)
group by f.user_id
) t
where user_id = 'uuid'
You don't really need the users table for what you want, unless you do want to allow users without any flight (they would all have the same, highest rank). If so:
select *
from (
select u.id,
dense_rank() over(order by count(distinct a.airport_id) desc) rn
from users u
left join flights f on f.user_id = u.id
left join lateral (values
(f.departure_airport_id), (f.arrival_airport_id)
) a(airport_id) on true
group by u.id
) t
where id = 'uuid'
You're counting the distinct pairs of (departure_airport_id, arrival_airpot_id). As you suggested, you could use union to get a single column of airport IDs (regardless of whether they are departure or arrival airports), and then apply a count on them:
SELECT user_id, DENSE_RANK() OVER (ORDER BY cnt DESC) AS user_rank
FROM (SELECT u.id AS user_id, COALESCE(cnt, 0) AS cnt
FROM users u
LEFT JOIN (SELECT user_id, COUNT DISTINCT(airport_id) AS cnt
FROM (SELECT user_id, departure_airport_id AS airport_id
FROM flights
UNION
SELECT user_id, arrival_airport_id AS airport_id
FROM flights) x
GROUP BY u.id) f ON u.id = f.user_id) t
Problem in general words: I need to select value from one table referenced to the most repeated values in another table.
Tables have this structure:
screenshot
screenshot2
The question is to find country which has the most results from sportsmen related to it.
First, INNER JOIN tables to have relation between result and country
SELECT competition_id, country FROM result
INNER JOIN sportsman USING (sportsman_id);
Then, I count how much time each country appear
SELECT country, COUNT(country) AS highest_participation
FROM (SELECT competition_id, country FROM result
INNER JOIN sportsman USING (sportsman_id))
GROUP BY country
;
And got this screenshot3
Now it feels like I'm one step away from solution ))
I guess it's possible with one more SELECT FROM (SELECT ...) and MAX() but I can't wrap it up?
ps:
I did it with doubling the query like this but I feel like it's so inefficient if there are millions of rows.
SELECT country
FROM (SELECT country, COUNT(country) AS highest_participation
FROM (SELECT competition_id, country FROM result
INNER JOIN sportsman USING (sportsman_id)
) GROUP BY country
)
WHERE highest_participation = (SELECT MAX(highest_participation)
FROM (SELECT country, COUNT(country) AS highest_participation
FROM (SELECT competition_id, country FROM result
INNER JOIN sportsman USING (sportsman_id)
) GROUP BY country
))
Also I did it with a view
CREATE VIEW temp AS
SELECT country as country_with_most_participations, COUNT(country) as country_participate_in_#_comp
FROM(
SELECT country, competition_id FROM result
INNER JOIN sportsman USING(sportsman_id)
)
GROUP BY country;
SELECT country_with_most_participations FROM temp
WHERE country_participate_in_#_comp = (SELECT MAX(country_participate_in_#_comp) FROM temp);
But not sure if it's easiest way.
If I understand this correctly you want to rank the countries per competition count and show the highest ranking country (or countries) with their count. I suggest you use RANK for the ranking.
select country, competition_count
from
(
select
s.country,
count(*) as competition_count,
rank() over (order by count(*) desc) as rn
from sportsman s
inner join result r using (sportsman_id)
group by s.country
) ranked_by_count
where rn = 1
order by country;
If the order of the result rows doesn't matter, you can shorten this to:
select s.country, count(*) as competition_count
from sportsman s
inner join result r using (sportsman_id)
group by s.country
order by count(*) desc
fetch first rows with ties;
You seem to be overcomplicating this. Starting from your existing join query, you can aggregate, order the results and keep the top row(s) only.
select s.country, count(*) cnt
from sportsman s
inner join result r using (sportsman_id)
group by s.country
order by cnt desc
fetch first 1 row with ties
Note that this allows top ties, if any.
SELECT country
FROM (SELECT country, COUNT(country) AS highest_participation
FROM (SELECT competition_id, country FROM result
INNER JOIN sportsman USING (sportsman_id)
) GROUP BY country
order by 2 desc
)
where rownum=1
I have tables:
Student(sID, firstName, lastName, email, cgpa)
Course(cNum, name, dept, credit)
Offering(oID, cNum, dept, year, term, instructor)
Took(sID, oID, grade)
I'm trying to complete the question:
Find all courses for the term 2017F and the current enrolment
I currently have this query to grab the number of students enrolled in each course:
SELECT Took.oID, COUNT(*) AS enrolment
FROM Took
GROUP BY Took.oID
HAVING COUNT(*) > 0
Nested inside of this statement to grab the correct courses that I want the enrolment counts for:
SELECT oID
FROM Offering
WHERE Offering.year = 2017
AND Offering.term = 'F'
Both of which are nested inside of this query to tie everything together:
SELECT DISTINCT Offering.cNum, Course.name, (I WOULD LIKE COUNT(*) AS enrolment HERE)
FROM Offering NATURAL JOIN Course
WHERE Offering.oID IN (
SELECT oID
FROM Offering
WHERE Offering.year = 2017
AND Offering.term = 'F'
AND oID IN (
SELECT Took.oID, COUNT(*) AS enrolment
FROM Took
GROUP BY Took.oID
HAVING COUNT(*) > 0))
GROUP BY Offering.cNum, Course.name;
My question is, how can I pass the resulting COUNT(*) AS enrolment from the furthest nested query to the initial query so that it can be displayed in the resulting projection? (This is homework)
If I understand correctly you can try to use a subquery in from with JOIN instead of where subquery.
Then you can get count column from the subquery.
SELECT DISTINCT Offering.cNum, Course.name,t1.enrolment
FROM Offering
JOIN (
SELECT Took.oID,
COUNT(*) AS enrolment
FROM Took
GROUP BY Took.oID
HAVING COUNT(*) > 0
) t1 on t1.oID = Offering.oID
NATURAL JOIN Course
WHERE Offering.year = 2017 AND Offering.term = 'F'
Try this
SELECT c.*
, (
SELECT COUNT(*)
FROM Took
WHERE oID = o.oID
) AS theCount
FROM Course c
JOIN Offering o ON o.cNum = c.cNum
WHERE o.year = 2017 AND o.term = 'F'
May be this
SELECT Course.name, Course.cNum, count(*) as enrolment
FROM Course
JOIN Offering ON Course.cNum = Offering.cNum
JOIN Took ON Offering.oID = Took.oID
WHERE Offering.year = 2017
AND Offering.term = 'F'
GROUP BY Course.name, Course.cNum
HAVING count(*) > 0;
I need a help constructung an sql query for mysql database. 2 Table as follows:
tblcities (id,name)
tblmembers(id,name,city_id)
Now I want to retrieve the 'city' details that has maximum number of 'members'.
Regards
SELECT tblcities.id, tblcities.name, COUNT(tblmembers.id) AS member_count
FROM tblcities
LEFT JOIN tblmembers ON tblcities.id = tblmembers.city_id
GROUP BY tblcities.id
ORDER BY member_count DESC
LIMIT 1
Basically: retrieve all cities and count how many members each has, sort by that member count in descending order, making the highest count first - then show only that first city.
Terrible, but that's a way of doing it:
SELECT * FROM tblcities WHERE id IN (
SELECT city_id
FROM tblMembers
GROUP BY city_id
HAVING COUNT(*) = (
SELECT MAX(TOTAL)
FROM (
SELECT COUNT(*) AS TOTAL
FROM tblMembers
GROUP BY city_id
) AS AUX
)
)
That way, if there is a tie, still you'll get all cities with the maximum number of members...
Select ...
From tblCities As C
Join (
Select city_id, Count(*) As MemberCount
From tblMembers
Order By Count(*) Desc
Limit 1
) As MostMembers
On MostMembers.city_id = C.id
select top 1 c.id, c.name, count(*)
from tblcities c, tblmembers m
where c.id = m.city_id
group by c.id, c.name
order by count(*) desc