I have a table Student in SQL Server with these columns:
[ID], [Age], [Level]
I want the query that returns each age value that appears in Students, and finds the level value that appears most often. For example, if there are more 'a' level students aged 18 than 'b' or 'c' it should print the pair (18, a).
I am new to SQL Server and I want a simple answer with nested query.
You can do this using window functions:
select t.*
from (select age, level, count(*) as cnt,
row_number() over (partition by age order by count(*) desc) as seqnum
from student s
group by age, level
) t
where seqnum = 1;
The inner query aggregates the data to count the number of levels for each age. The row_number() enumerates these for each age (the partition by with the largest first). The where clause then chooses the highest values.
In the case of ties, this returns just one of the values. If you want all of them, use rank() instead of row_number().
One more option with ROW_NUMBER ranking function in the ORDER BY clause. WITH TIES used when you want to return two or more rows that tie for last place in the limited results set.
SELECT TOP 1 WITH TIES age, level
FROM dbo.Student
GROUP BY age, level
ORDER BY ROW_NUMBER() OVER(PARTITION BY age ORDER BY COUNT(*) DESC)
Or the second version of the query using amount each pair of age and level, and max values of count pair age and level per age.
SELECT *
FROM (
SELECT age, level, COUNT(*) AS cnt,
MAX(COUNT(*)) OVER(PARTITION BY age) AS mCnt
FROM dbo.Student
GROUP BY age, level
)x
WHERE x.cnt = x.mCnt
Demo on SQLFiddle
Another option but will require later version of sql-server:
;WITH x AS
(
SELECT age,
level,
occurrences = COUNT(*)
FROM Student
GROUP BY age,
level
)
SELECT *
FROM x x
WHERE EXISTS (
SELECT *
FROM x y
WHERE x.occurrences > y.occurrences
)
I realise it doesn't quite answer the question as it only returns the age/level combinations where there are more than one level for the age.
Maybe someone can help to amend it so it includes the single level ages aswell in the result set: http://sqlfiddle.com/#!3/d597b/9
with combinations as (
select age, level, count(*) occurrences
from Student
group by age, level
)
select age, level
from combinations c
where occurrences = (select max(occurrences)
from combinations
where age = c.age)
This finds every age and level combination in the Students table and counts the number of occurrences of each level.
Then, for each age/level combination, find the one whose occurrences are the highest for that age/level combination. Return the age and level for that row.
This has the advantage of not being tied to SQL Server - it's vanilla SQL. However, a window function like Gordon pointed out may perform better on SQL Server.
Related
I am trying to write an sql that will allow me select each student’s 3rd best assignment mark in each subject. I have tried with the query below but it isn't working for me. I will be grateful to get some answers. I am getting an error [Code: 0, SQL State: 21000] ERROR: more than one row returned by a subquery used as an expression.
This is the table structure Students , Courses(Id) , bridging table called StudentsCourses(ID, StudentID,CourseID) and then assignment table which has StudentsCourse(FK) and Grade
select max(Assignments.Grade)
from Assignments
where grade < (select max(Assignments.Grade)
from Assignments
where grade < (select max(Assignments.Grade)
from Assignments
group by Assignments.StudentCourseID))
You can use window functions:
select *
from (
select a.*, row_number() over(partition by student_id, subject_id order by grade desc)
from assignments a
) a
where rn = 3
Your question is a bit unclear about the structure of table assignments. This assumes that a student is identified by student_id and a subject by subject_id - you many need to ajust that to your actual column names.
Use row_number():
select a.*
from (select a.*,
row_number() over (partition by student_id, StudentCourseID order by grade desc) as seqnum
from assignments a
) a
where seqnum = 3;
Note: If all the assignments have the same value, this will return the highest value.
If you want the third highest distinct score, then use dense_rank() instead of row_number().
I am practicing SQL in Google Cloud Platform and have a table with the most popular names in USA.
I need to write a query that returns the most popular name in every year.
The Table has 6 columns: id, state, gender, year, name, Number of occurrences of the name
So far I have:
SELECT DISTINCT year
FROM 'table'
But I don't now what next...
What you are looking for is the called the mode in statistics. One way to get this uses aggregation and window fucntions:
select year, name
from (select year, name, count(*) as cnt,
row_number() over (partition by year order by count(*) desc) as seqnum
from t
group by year, name
) yn
where seqnum = 1;
The above version returns one arbitrary name if there are ties for the most frequent. If you do want ties, use rank() instead of row_number().
SELECT MAX(Score)
FROM Students
WHERE Score < (SELECT MAX(Score) FROM Students);
the above query works perfectly and fetches the record that has 2nd highest score, whereas the query mentioned below does not fetch anything
SELECT *
FROM Students
WHERE Score < (SELECT MAX(Score) FROM Students);
here Students is the table from which I want to fetch all the details of that record which has 2nd highest score in the entire table.
I want that 2nd query should get executed, thanks in advance for helping me out.
I have not used any database, I'm simply trying out these queries in w3schools.
With standard SQL, this is typically solved using window functions:
select *
from (
select *, dense_rank() over (order by score desc) as rnk
from students
) t
where rnk = 2;
The above is ANSI standard SQL and works on all modern DBMS.
How about ordering and getting the first record returned using LIMIT:
SELECT *
FROM Students
WHERE Score < (SELECT MAX(Score) FROM Students)
ORDER BY Score DESC
LIMIT 1;
I need to get the Top 10 results for each Region, Market and Name along with those with highest counts (Gaps). There are 4 Regions with 1 to N Markets. I can get the Top 10 but cannot figure out how to do this without using a Union for every Market. Any ideas on how do this?
SELECT DISTINCT TOP 10
Region, Market, Name, Gaps
FROM
TableName
ORDER BY
Region, Market, Gaps DESC
One approach would be to use a CTE (Common Table Expression) if you're on SQL Server 2005 and newer (you aren't specific enough in that regard).
With this CTE, you can partition your data by some criteria - i.e. your Region, Market, Name - and have SQL Server number all your rows starting at 1 for each of those "partitions", ordered by some criteria.
So try something like this:
;WITH RegionsMarkets AS
(
SELECT
Region, Market, Name, Gaps,
RN = ROW_NUMBER() OVER(PARTITION BY Region, Market, Name ORDER BY Gaps DESC)
FROM
dbo.TableName
)
SELECT
Region, Market, Name, Gaps
FROM
RegionsMarkets
WHERE
RN <= 10
Here, I am selecting only the "first" entry for each "partition" (i.e. for each Region, Market, Name tuple) - ordered by Gaps in a descending fashion.
With this, you get the top 10 rows for each (Region, Market, Name) tuple - does that approach what you're looking for??
I think you want row_number():
select t.*
from (select t.*,
row_number() over (partition by region, market order by gaps desc) as seqnum
from tablename t
) t
where seqnum <= 10;
I am not sure if you want name in the partition by clause. If you have more than one name within a market, that may be what you are looking for. (Hint: Sample data and desired results can really help clarify a question.)
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).