I have two tables as follows:
I want to find the StudentId, FirstName, StudentLoginInfoId, LoginDate. I am expecting only one entry per student with higher LoginDate.
Expected result:
You could use ROW_NUMBER to number output of the result-set for each partition (here each student) in a subquery and achieve your desired output by applying a condition of the number assigned for each student to be 1 which will equal one row.
select studentid, firstname, studentlogininfoid, logindate
from (
select
s.studentid, s.firstname, sl.studentlogininfoid, sl.logindate,
row_number() over (partition by sl.studentid order by sl.logindate desc) as rn
from student s
inner join studentlogininfoid sl on s.studentid = sl.studentid
) t
where rn = 1
Explaining arguments for row_number:
PARTITION BY specifies what are your groups to enumerate separately (start from 1 for each group)
ORDER BY specifies how should rows be enumerated (based on which order)
If we enumerate rows for each student and sort them from latest date descending, then the first row for each student (the row with rn = 1) will contain highest login date value for that student.
You can use "CROSS APPLY" to find what you want:
SELECT S.StudentId
, S.FirstName
, SLI.StudentLoginInfoId
, SLI.LoginDate
FROM Student S
CROSS APPLY (SELECT TOP 1 * FROM StudentLoginInfo SLI WHERE S.StudentId = SLI.StudentId ORDER BY LoginDate DESC) SLI
Related
I am having trouble displaying data from two tables, using what I think should be a group method.
I currently have a table containing pupils, and another containing the grades achieved (points) each year and term. See below:
PupilID, FirstName, Surname, DOB
GradeID, PupilID, SchoolYear, Term, Points
I want to query both tables and display all pupils with their latest grade, this should look for the maximum SchoolYear, then the maximum Term, and display the Points alongside the PupilID, FirstName and Surname.
I would appreciate any help anyone can offer with this
This will select the latest grade per pupil based on SchoolYear and Term
select * from (
select p.*, g.schoolyear, g.term,
row_number() over (partition by PupilID order by SchoolYear desc, Term desc) rn
from pupils p
join grades g on g.PupilID = p.PupilID
) t1 where rn = 1
try this
declare varSchoolYear int
declare vartrem int
set varSchoolYear=(select max (SchoolYear) from Grade)
set vartrem=(select max(term) from Pupil where SchoolYear=varSchoolYear)
select a.firstname,b.idgrade
from pupil a
inner join grade b
on a.pupilid = b.pupilid
where b.term=vartrem and b.SchoolYear=varSchoolYear
I have a table named grades. A column named Students, Practical, Written. I am trying to figure out the top 5 students by total score on the test. Here are the queries that I have not sure how to join them correctly. I am using oracle 11g.
This get's me the total sums from each student:
SELECT Student, Practical, Written, (Practical+Written) AS SumColumn
FROM Grades;
This gets the top 5 students:
SELECT Student
FROM ( SELECT Student,
, DENSE_RANK() OVER (ORDER BY Score DESC) as Score_dr
FROM Grades )
WHERE Student_dr <= 5
order by Student_dr;
The approach I prefer is data-centric, rather than row-position centric:
SELECT g.Student, g.Practical, g.Written, (g.Practical+g.Written) AS SumColumn
FROM Grades g
LEFT JOIN Grades g2 on g2.Practical+g2.Written > g.Practical+g.Written
GROUP BY g.Student, g.Practical, g.Written, (g.Practical+g.Written) AS SumColumn
HAVING COUNT(*) < 5
ORDER BY g.Practical+g.Written DESC
This works by joining with all students that have greater scores, then using a HAVING clause to filter out those that have less than 5 with a greater score - giving you the top 5.
The left join is needed to return the top scorer(s), which have no other students with greater scores to join to.
Ties are all returned, leading to more than 5 rows in the case of a tie for 5th.
By not using row position logic, which varies from darabase to database, this query is also completely portable.
Note that the ORDER BY is optional.
With Oracle's PLSQL you can do:
SELECT score.Student, Practical, Written, (Practical+Written) as SumColumn
FROM ( SELECT Student, DENSE_RANK() OVER (ORDER BY Score DESC) as Score_dr
FROM VOTES ) as score, students
WHERE score.score_dr <= 5
and score.Student = students.Student
order by score.Score_dr;
You can easily include the projection of the first query in the sub-query of the second.
SELECT Student
, Practical
, Written
, tot_score
FROM (
SELECT Student
, Practical
, Written
, (Practical+Written) AS tot_score
, DENSE_RANK() OVER (ORDER BY (Practical+Written) DESC) as Score_dr
FROM Grades
)
WHERE Student_dr <= 5
order by Student_dr;
One virtue of analytic functions is that we can just use them in any query. This distinguishes them from aggregate functions, where we need to include all non-aggregate columns in the GROUP BY clause (at least with Oracle).
I was told to Find out which occupation has the greatest number of patients with conditionID=MC8
I dk how to do the greatest part.....
Here my code right now
SELECT occupation
FROM Patient
WHERE EXISTS
(SELECT PatientID FROM PatientMedcon
Where conditionID=’MC8’)
GROUP BY occupation
HAVNG count(occupation) = (Select max(occupation)
From Patient
You should approach these types of queries using regular joins and then add additional factors. The following gets the count of patients for each occupation with that condition:
SELECT occupation, COUNT(*)
FROM Patient p JOIN
PatentMedcon pm
ON p.PatientId = pm.PatientId and
pm.conditionId = 'MC8'
GROUP BY occupation
ORDER BY COUNT(*) DESC;
If you want the top row, that depends on the database. It might be select top 1, limit 1 at the end, fetch first 1 rows only at the end, or even something else.
Let's say I have a bases with a table:
-courses (key: name [ofthecourse], other attributes: year in which the course takes place)
I want to complete a query looking for an answer to the question:
On which year of study there is a maximum number of courses?
Normally, the query would be:
SELECT TOP 1 STUDYEAR
FROM COURSES
GROUP BY STUDYEAR
ORDER BY COUNT(CNO) DESC;
But my question is, which query could complete this without using the TOP 1 phrase?
You can use an inner query to get the maximum count. The only difference is though that it can return more than one record if they have the same count.
SELECT STUDYEAR
FROM COURSES
GROUP BY STUDYEAR
HAVING COUNT(CNO) = (SELECT MAX(CNOCount) FROM
(SELECT COUNT(CNO) CNOCount
FROM COURSES
GROUP BY STUDYEAR) X)
Another version with only one inner query:
SELECT STUDYEAR
FROM
(SELECT STUDYEAR, ROW_NUMBER() OVER (ORDER BY COUNT(CNO) DESC) RowNumber
FROM COURSES
GROUP BY STUDYEAR) X
WHERE RowNumber = 1
Not sure how to ask a followup on SO, but this is in reference to an earlier question:
Fetch one row per account id from list
The query I'm working with is:
SELECT *
FROM scores s1
WHERE accountid NOT IN (SELECT accountid FROM scores s2 WHERE s1.score < s2.score)
ORDER BY score DESC
This selects the top scores, and limits results to one row per accountid; their top score.
The last hurdle is that this query is returning multiple rows for accountids that have multiple occurrences of their top score. So if accountid 17 has scores of 40, 75, 30, 75 the query returns both rows with scores of 75.
Can anyone modify this query (or provide a better one) to fix this case, and truly limit it to one row per account id?
Thanks again!
If you're only interested in the accountid and the score, then you can use the simple GROUP BY query given by Paul above.
SELECT accountid, MAX(score)
FROM scores
GROUP BY accountid;
If you need other attributes from the scores table, then you can get other attributes from the row with a query like the following:
SELECT s1.*
FROM scores AS s1
LEFT OUTER JOIN scores AS s2 ON (s1.accountid = s2.accountid
AND s1.score < s2.score)
WHERE s2.accountid IS NULL;
But this still gives multiple rows, in your example where a given accountid has two scores matching its maximum value. To further reduce the result set to a single row, for example the row with the latest gamedate, try this:
SELECT s1.*
FROM scores AS s1
LEFT OUTER JOIN scores AS s2 ON (s1.accountid = s2.accountid
AND s1.score < s2.score)
LEFT OUTER JOIN scores AS s3 ON (s1.accountid = s3.accountid
AND s1.score = s3.score AND s1.gamedate < s3.gamedate)
WHERE s2.accountid IS NULL
AND s3.accountid IS NULL;
select accountid, max(score) from scores group by accountid;
If your RDBMS supports them, then an analytic function would be a good approach particularly if you need all the columns of the row.
select ...
from (
select accountid,
score,
...
row_number() over
(partition by accountid
order by score desc) score_rank
from scores)
where score_rank = 1;
The row returned is indeterminate in the case you describe, but you can easily modify the analytic function, for example by ordering on (score desc, test_date desc) to get the more recent of two matching high scores.
Other analytic functions based on rank will achieve a similar purpose.
If you don't mind duplicates then the following would probably me more efficient than your current method:
select ...
from (
select accountid,
score,
...
max(score) over (partition by accountid) max_score
from scores)
where score = max_score;
If you are selecting a subset of columns then you can use the DISTINCT keyword to filter results.
SELECT DISTINCT UserID, score
FROM scores s1
WHERE accountid NOT IN (SELECT accountid FROM scores s2 WHERE s1.score < s2.score)
ORDER BY score DESC
Does your database support distinct? As in select distinct x from y?
This solutions works in MS SQL, giving you the whole row.
SELECT *
FROM scores
WHERE scoreid in
(
SELECT max(scoreid)
FROM scores as s2
JOIN
(
SELECT max(score) as maxscore, accountid
FROM scores s1
GROUP BY accountid
) sub ON s2.score = sub.maxscore AND s2.accountid = s1.accountid
GROUP BY s2.score, s2.accountid
)