Just getting confused on basic stuff -
could someone explain me this -
select s.name from students s
inner join friends f on f.id = s.id
inner join packages p on p.id = s.id
where p.salary < (select pp.salary from packages pp where pp.id = f.friend_id)
order by (select pp.salary from packages pp where pp.id = f.friend_id) ASC;
the salary comparison part - i.e select pp.salary from packages pp where pp.id = f.friend_id should not yield the same salary result? - so how can we compare.
for references, use the below sample tables
table 1- students
columns - id, name
table 2 - friends (here each id is linked with one friend_id (his best friend))
columns - id , friend_id
table3 - packages
columns - id , salary
Trying to find out the name of the friend whose best friend's salary is more than his salary.
I am confused at understanding this solution.
That where subquery part is wrong cause the subquery will return multiple record and which can't be used with < operator since it's accepts scalar value. Rather change that to a JOIN as well like
JOIN packages pp ON pp.id = f.friend_id
AND p.salary < pp.salary
Change your query to be
select s.name from students s
inner join friends f on f.id = s.id
inner join packages p on p.id = s.id
JOIN packages pp ON pp.id = f.friend_id
AND p.salary < pp.salary
order by pp.salary;
Related
I have a database with the tables:
Student(SID,Name,Surname,Age)
Registration(StudentID,CourseID)
Course(CID,Name,Cost)
I would like to extract only the name of the courses with students younger than 20. Will the query below do just that?
SELECT C.NAME
FROM Course C
INNER JOIN Registration
INNER JOIN Student S
WHERE CID = CourseID
AND SID = StudentID
AND Age < 20
GROUP BY C.NAME
I would also like to extract the number of students in each course having students younger than 20. Is it correct to do it as below?
SELECT count(S.NAME)
,C.NAME
FROM Student S
INNER JOIN Course C
INNER JOIN Registration
WHERE Age < 20
AND CID = CourseID
AND SID = StudentID
GROUP BY C.NAME
You are missing the ON part for the join otherwise it would just be a CROSS JOIN.
Your first query should look like this if you want just a distinct list of student names:
SELECT DISTINCT C.NAME
FROM Course C
INNER JOIN Registration R ON C.CID = R.CourseID
INNER JOIN Student S ON R.StudentID = S.SID
WHERE Age < 20
Your second query shouldn't really have the C.Name in the select if you want to get just a count unless you want a count of how many students have that name.
SELECT count(*)
FROM Student S
INNER JOIN Registration R ON s.SID = R.StudentID
INNER JOIN Course C ON c.CID = R.CourseID
WHERE Age < 20
GROUP BY C.NAME
First join these tables, then group by Course's PK(CID), Add the HAVING condition to filter the course which has students younger than 20.
Then use Course table to join the result to get the course name and count of students in the course.
SELECT
T1.Name,
T2.StudentCount
FROM
Course T1
INNER JOIN (
SELECT
c.CID,
COUNT(s.SID) AS StudentCount
FROM
Course c
LEFT JOIN Registration r ON c.CID = r.CourseID
LEFT JOIN Student s ON s.SID = r.StudentID
GROUP BY c.CID
HAVING COUNT(IF(s.Age < 20, 1, NULL)) > 0
) T2 ON T1.CID = T2.CID
More correctly, you should move the conditions of the join, to the join statements themselves by including them in the on clause instead of the where. While the results may not change in this instance, if you were to start including outer joins you would encounter difficulties.
SELECT count(S.NAME)
,C.NAME
FROM Student S
INNER JOIN Registration R
ON s.SID = R.StudentID
INNER JOIN Course C
ON c.CID = R.CourseID
WHERE Age < 20
GROUP BY C.NAME
There's a fiddle here showing it in action: http://sqlfiddle.com/#!9/c3b8f/1
Your first query will also produce the results you want, but again, you should move the join predicates to the join itself. Also, you don't need to perform the grouping just to get distinct values, mysql has an expression for that called distinct. So rewritten, the first query would look like:
SELECT DISTINCT C.NAME
FROM Student S
INNER JOIN Registration R
ON s.SID = R.StudentID
INNER JOIN Course C
ON c.CID = R.CourseID
WHERE Age < 20.
Again, the results are the same as what you have already but it is easier to 'read' and will put you in good stead when you move on to other queries. As it stands you have mixed implicit and explicit join syntax.
This fiddle demonstrates both queries: http://sqlfiddle.com/#!9/c3b8f/4
edit
I may have misinterpreted your original question - if you want the total number of students enrolled in a course with at least one student under 19, you can use a query like this:
select name, count(*)
from course c
inner join registration r
on c.cid = r.courseid
where exists (
select 1
from course cc
inner join registration r
on cc.cid = r.courseid
inner join student s
on s.sid = r.studentid
where cc.cid = c.cid
group by cc.cid
having min(s.age) < 20
)
group by name;
Again with the updated fiddle here: http://sqlfiddle.com/#!9/c3b8f/17
I am a newbie to SQL working on an assignment to find the actor or actress with the most appearances. A diagram of the database I'm working with is here:
Here was the query I was trying to use:
SELECT DISTINCT n.name, count(n.name)
FROM cast_info c
INNER JOIN name n
ON (n.id = c.person_id)
INNER JOIN title t
ON (c.movie_id = t.id)
CROSS JOIN role_type r
WHERE (r.role = 'actor' OR r.role = 'actress')
GROUP BY n.name
This is intended to get a count of how many times different actors showed up, which I can then sort and select the top one. But it doesn't work. Something else I did was:
SELECT n.name, count(n.name) AS amount
FROM cast_info c
INNER JOIN name n
ON (n.id = c.person_id)
INNER JOIN title t
ON (c.movie_id = t.id)
LEFT JOIN role_type r
ON c.role_id = r.id
AND (r.role = 'actor' OR r.role = 'actress')
GROUP BY amount
ORDER BY amount DESC
LIMIT 1
But that gives the error
aggregate functions are not allowed in GROUP BY
LINE 1: SELECT COUNT(*) AS total FROM (SELECT n.name, count(n.name) ...
Tips?
I am going to take a stab at each of these questions for you, because this assignment is obviously causing you some trouble.
You can find everything you need in your cast_info table and your role_type table, unless you need to display the actors/actresses actual name.
I would start by selecting all rows that represent an actor or actress in a movie. This should be a unique combination, as a person can't be an actor in the same movie twice. Once you've done that, group by the persons id and get the count() of rows, which should effectively be the number of movies. I think the error you're getting is exactly for the reason it sounds, you can't use an aggregate column in your order by. A workaround for that would be to use this as a subquery, and use MAX() to get most appearances.
Try this:
SELECT c.personid, MAX(numMovies) AS mostApperances
FROM(SELECT c.personid, COUNT(*) AS numMovies
FROM cast_info c
JOIN role_type r ON r.id = c.role_id
WHERE r.role = 'actor' OR r.role = 'actress'
GROUP BY c.personid) t
Try this
SELECT DISTINCT n.name, count(n.name)
FROM cast_info c
INNER JOIN name n
ON n.id = c.person_id
INNER JOIN title t
ON c.movie_id = t.id
LEFT JOIN role_type r
ON c.role_id = r.id
AND (r.role = 'actor' OR r.role = 'actress')
GROUP BY n.name
I have three tables, libraryitems, copies and loans.
A libraryitem hasMany copies, and a copy hasMany loans.
I'm trying to get the latest loan entry for a copy only; The query below returns all loans for a given copy.
SELECT
libraryitems.title,
copies.id,
copies.qruuid,
loans.id AS loanid,
loans.status,
loans.byname,
loans.byemail,
loans.createdAt
FROM copies
INNER JOIN libraryitems ON copies.libraryitemid = libraryitems.id AND libraryitems.deletedAt IS NULL
LEFT OUTER JOIN loans ON copies.id = loans.copyid
WHERE copies.libraryitemid = 1
ORDER BY copies.id ASC, loans.createdAt DESC
I know there needs to be a sub select of some description in here, but struggling to get the correct syntax. How do I only return the latest, i.e MAX(loans.createdAt) row for each distinct copy? Just using group by copies.id returns the earliest, rather than latest entry.
Image example below:
in the subquery , getting maximum created time for a loan i.e. latest entry and joining back with loans to get other details.
SELECT
T.title,
T.id,
T.qruuid,
loans.id AS loanid,
loans.status,
loans.byname,
loans.byemail,
loans.createdAt
FROM
(
SELECT C.id, C.qruuid, L.title, MAX(LN.createdAt) as maxCreatedTime
FROM Copies C
INNER JOIN libraryitems L ON C.libraryitemid = L.id
AND L.deletedAt IS NULL
LEFT OUTER JOIN loans LN ON C.id = LN.copyid
GROUP BY C.id, C.qruuid, L.title) T
JOIN loans ON T.id = loans.copyid
AND T.maxCreatedTime = loans.createdAt
A self left join on loans table will give you latest loan of a copy, you may join the query to the other tables to fetch the desired output.
select * from loans A
left outer join loans B
on A.copyid = B.copyid and A.createdAt < B.createdAt
where B.createdAt is null;
This is your query with one simple modification -- table aliases to make it clearer.
SELECT li.title, c.id, c.qruuid,
l.id AS loanid, l.status, l.byname, l.byemail, l.createdAt
FROM copies c INNER JOIN
libraryitems li
ON c.libraryitemid = li.id AND
li.deletedAt IS NULL LEFT JOIN
loans l
ON c.id = l.copyid
WHERE c.libraryitemid = 1
ORDER BY c.id ASC, l.createdAt DESC ;
With this as a beginning let's think about what you need. You want the load with the latest createdAt date for each c.id. You can get this information with a subquery:
select l.copyid, max(createdAt)
from loans
group by l.copyId
Now, you just need to join this information back in:
SELECT li.title, c.id, c.qruuid,
l.id AS loanid, l.status, l.byname, l.byemail, l.createdAt
FROM copies c INNER JOIN
libraryitems li
ON c.libraryitemid = li.id AND
li.deletedAt IS NULL LEFT JOIN
loans l
ON c.id = l.copyid LEFT JOIN
(SELECT l.copyid, max(l.createdAt) as maxca
FROM loans
GROUP BY l.copyid
) lmax
ON l.copyId = lmax.copyId and l.createdAt = lmax.maxca
WHERE c.libraryitemid = 1
ORDER BY c.id ASC, l.createdAt DESC ;
This should give you the most recent record. And, the use of left join should keep all copies, even those that have never been leant.
Using MySQL I am trying to pull ALL people who's company_id matches 3 and also compare whether that person exists in the favourites table.
What I have noticed is that if a person has no job_id then they are not output by the below query.
The below query outputs 1 row, but in the people table there are 3 rows that match the company_id of 3
SELECT *,
j.company_id as companyid,
f.id IS NOT NULL AS jid,
p.id as pid,
f.id AS fave_id,
f.id IS NOT NULL AS fave
FROM people p
INNER JOIN job j
ON p.job_id = j.id
LEFT JOIN favourites f
ON f.people_id=p.id
AND f.user_id = 12
WHERE p.company_id = 3
ORDER BY p.id ASC
SQLFiddle
Any help is appreciated to get the query to output all rows that match and also check the favourites table.
If you put a left join on job, it will show people that have no job_id. I have updated the fiddle here.
SELECT
p.*,
j.company_id as companyid,
f.id IS NOT NULL AS jid,
p.id as pid,
f.id AS fave_id,
f.id IS NOT NULL AS fave
FROM people p
LEFT JOIN job j
ON j.id = p.job_id
LEFT JOIN favourites f
ON f.people_id=p.id
AND f.user_id = 12
WHERE p.company_id = 3
ORDER BY p.id ASC
You should change your INNER JOIN for the job table to a LEFT OUTER JOIN so they're selected regardless of whether or not they exist in the job table.
Is it possible to INNER JOIN a MySQL query to achieve this result?
I have a table with Strategies and a table with Members. The Strategy table holds the ID of the author that corresponds to their ID in the Member table and the ID of an author that updated the existing author's work. Is it possible to grab a reference to both of these people at the same time? Something like the following, which returns no errors, but also no results...
SELECT * FROM Strategies
INNER JOIN Members AS a
INNER JOIN Members AS b
WHERE Strategies.ID='2'
AND Strategies.AuthorID = a.ID
AND Strategies.UpdateAuthorID = b.ID
Use a LEFT JOIN:
SELECT
s.*,
a.Name AS MemberName,
b.Name AS UpdatedMemberName
FROM Strategies AS s
LEFT JOIN Members AS a ON s.AuthorID = a.ID AND s.ID = 2
LEFT JOIN Members AS b ON s.UpdateAuthorID = b.ID AND s.ID = 2 ;
If you want them in one column use COALESCE:
SELECT
s.*,
COALESCE(a.Name, b.Name) AS MemberName
FROM Strategies AS s
LEFT JOIN Members AS a ON s.AuthorID = a.ID AND s.ID = 2
LEFT JOIN Members AS b ON s.UpdateAuthorID = b.ID AND s.ID = 2
SELECT toD.dom_url AS ToURL,
fromD.dom_url AS FromUrl,
rvw.*
FROM reviews AS rvw
LEFT JOIN domain AS toD
ON toD.Dom_ID = rvw.rev_dom_for
LEFT JOIN domain AS fromD
ON fromD.Dom_ID = rvw.rev_dom_from
if domain is table name