I've got a student table and an enrollment table; a student could have multiple enrollment records that can be active or inactive.
I want to get a select that has a single student record and an indicator as to whether that student has active enrollments.
I thought about doing this in an inline UDF that uses the student ID in a join to the enrollment table, but I wonder if there's a better way to do it in a single select statement.
The UDF call might look something like:
Select Student_Name,Student_Email,isEnrolled(Student_ID) from Student
What might the alternative - with one SQL statement - look like?
select Student_Name,
Student_Email,
(select count(*)
from Enrollment e
where e.student_id = s.student_id
) Number_Of_Enrollments
from Student e
will get the number of enrollments, which should help.
Why not join to a secondary select? Unlike other solutions this isn't firing a subquery for every row returned, but gathers the enrollment data for everyone all at once. The syntax may not be quite correct, but you should get the idea.
SELECT
s.student_name,
s.student_email,
IsNull( e.enrollment_count, 0 )
FROM
Students s
LEFT OUTER JOIN (
SELECT
student_id,
count(*) as enrollment_count
FROM
enrollments
WHERE
active = 1
GROUP BY
student_id
) e
ON s.student_id = e.student_id
The select from enrollments could also be redone as a function which returns a table for you to join on.
CREATE FUNCTION getAllEnrollmentsGroupedByStudent()
RETURNS #enrollments TABLE
(
student_id int,
enrollment_count int
) AS BEGIN
INSERT INTO
#enrollments
(
student_id,
enrollment_count
) SELECT
student_id,
count(*) as enrollment_count
FROM
enrollments
WHERE
active = 1
GROUP BY
student_id
RETURN
END
SELECT
s.student_name,
s.student_email,
e.enrollment_count
FROM
Students s
JOIN
dbo.getAllEnrollmentsGroupedByStudent() e
ON s.student_id = e.student_id
Edit:
Renze de Waal corrected my bad SQL!
Try someting like this:
SELECT Student_Name, Student_Email, CAST((SELECT TOP 1 1 FROM Enrollments e WHERE e.student_id=s.student_id) as bit) as enrolled FROM Student s
I think you can also use the exists statement in the select but not positive
try to avoid using udfs or subqueries, they are performance killers. banjolity seems to havea good solution otherwise because it uses a derivd table instead of a UDF or subselect.
select students.name,
decode(count(1), 0, "no enrollments", "has enrollments")
from students, enrollments
where
students.id = enrollments.sutdent_id and
enrollments.is_active = 1 group by students.name
Of course, replace the decode with a function your database uses (or, a case statement).
Related
Relational Schema:
Students (**sid**, name, age, major)
Courses (**cid**, name)
Enrollment (**sid**, **cid**, year, term, grade)
Write a SQL query that returns the name of the students who took all courses.I'm not sure how I capture the concept of 'ALL' in a SQL query.
EDIT:
I want to be able write it without aggregation as I want to use the same logic for writing the query in relational algebra as well.
Thanks for the help!
One way of writing such queries is to count the number of course and number of courses each student took, and compare them:
SELECT s.*
FROM students s
JOIN (SELECT sid, COUNT(DISTINCT cid) AS student_courses
FROM enrollment
GROUP BY sid) e ON s.sid = e.sid
JOIN (SELECT COUNT(*) AS cnt
FROM courses) c ON cnt = student_cursed
This gives course combinations that are possible but haven't been taken...
SELECT s.sid, c.cid FROM students CROSS JOIN courses
EXCEPT
SELECT sid, cid FROM enrollment
So, you can then do the same with the student list...
SELECT sid FROM students
EXCEPT
(
SELECT DISTINCT
sid
FROM
(
SELECT s.sid, c.cid FROM students CROSS JOIN courses
EXCEPT
SELECT sid, cid FROM enrollment
)
AS not_enrolled
)
AS slacker_students
I don't like it, but it avoids aggregation...
SELECT *
FROM Students
WHERE NOT EXISTS (
SELECT 1 FROM Courses
LEFT OUTER JOIN Enrollment ON Courses.cid = Enrollment.cid
AND Enrollment.sid = Students.sid
WHERE Enrollment.sid IS NULL
)
btw. names of tables should be in singular form, not plural
I have 4 tables, I would like to select one column from each table, but only if the department has both 'Mick' and 'Dave working in it (must have both names, not one or the other). But it does not seem to be working properly:
SELECT SCHOOL_NAME, TOWN, COUNTY
FROM STUDENTS
NATURAL JOIN SCHOOLS NATURAL JOIN TOWNS NATURAL JOIN
COUNTIES
WHERE FIRST_NAME IN ('Mick','Dave)
/
I'm going wrong somewhere (probably lots of places :( ). Any help would be great
Don't use NATURAL JOIN. It is an abomination, because it does not take properly declared foreign key relationships into account. It only looks at the names of columns. This can introduce really hard to find errors.
Second, what you want is aggregation:
select sc.SCHOOL_NAME, t.TOWN, c.COUNTY
from STUDENTS st join
SCHOOLS sc
on st.? = sc.? join
TOWNS t
on t.? = ? join
COUNTIES c
on c.? = t.?
where FIRST_NAME in ('Mick', 'Dave')
group by sc.SCHOOL_NAME, t.TOWN, c.COUNTY
having count(distinct st.first_name) = 2;
The ? are placeholders for table and column names. If you are learning SQL, it is all the more important that you understand how columns line up for joins in different tables.
A where clause can only check the values in a single row. There is a separate row for each student, so there is no way -- with just a where -- to find both students. That is where the aggregation comes in.
You need at least three Join conditions, and properly end the string Dave with quote :
SELECT SCHOOL_NAME, TOWN, COUNTY
FROM SCHOOLS h
JOIN TOWNS t ON (t.id=h.town_id)
JOIN COUNTIES c ON (t.county_id=c.id)
WHERE EXISTS ( SELECT school_id
FROM STUDENTS s
WHERE s.first_name in ('Mick','Dave')
AND school_id = h.id
GROUP BY school_id
HAVING count(1)>1
);
SQL Fiddle Demo
You can use an analytic function in a sub-query to count the students who have the name Mick or Dave for each school_id (assuming that is your identifier for a school):
SELECT SCHOOL_NAME, TOWN, COUNTY
FROM ( SELECT *
FROM (
SELECT d.*,
COUNT(
DISTINCT
CASE WHEN FIRST_NAME IN ( 'Mick', 'Dave' ) THEN FIRST_NAME END
) OVER( PARTITION BY school_id )
AS num_matched
FROM STUDENTS d
)
WHERE num_matched = 2
)
NATURAL JOIN SCHOOLS
NATURAL JOIN TOWNS
NATURAL JOIN COUNTIES;
SQLFiddle
You would also be better to use an INNER JOIN and explicitly specify the join condition rather than relying on NATURAL JOIN.
Now there are two tables student(sid, name) and course(cid, name), which relation is many-to-many. So there is another table student_course(sid, cid) that stores the information about what courses have been chosen by whom.
How to write the sql that can get the courses that have been chosen by all the students?
Standard solution: use the NOT EXISTS ( ... NOT EXISTS (...)) construct:
find all courses in which all students take part
==>> there must not exist a student that does not take part in this course
SELECT * FROM course c
WHERE NOT EXISTS (
SELECT * from student s
WHERE NOT EXISTS (
SELECT * from student_course cs
WHERE cs.sid = s.sid
AND cs.cid = c.cid
)
)
;
This query is often faster (given appropiate indexes) than the count() == count() variant. The reason for this: you do not have to count all the (distinct) records; once you found one student that does not take this course you can omit this course from your list of suspects. Also: ANTI-joins often can make use of indexes [so can a count(), but that still has to count all the (distinct) keyvalues in the index]
Select c.cid, c.name
From course c where
(select count(1) from student) = (select count(1) from student_course sc where sc.cid = c.cid);
See SQL Fiddle
It finds all courses where the count of entries for that course in the student_course table matches the number of students
CID NAME
1 Test Course1
4 Test Course4
I am having a hard time understand how the create view, TRANSCRIPTVIEW, manages to set the grade of 0 for those who did not take a course. An explanation would help, the solution and question is below. Thanks.
Student(Id,Name)
Transcript(StudId,CourseName,Semester,Grade)
Formulate the following query in SQL:
Create a list of all students (Id, Name) and, for each student, list the average grade for the courses taken in the S2002 semester.
Note that there can be students who did not take any courses in S2002. For these, the average grade should be listed as 0.
Solution:
We first create a view which augments TRANSCRIPT with rows that enroll every student into a NULL course with the grade of 0. Therefore, students who did not take anything in semester ’S2002’ will have the average grade of 0 for that semester.
Below is what confuses me, how does this work and why does it work?
CREATE VIEW TRANSCRIPTVIEW AS (
( SELECT * FROM Transcipt)
UNION
(
SELECT S.Id,NULL,’S2002’,0
FROM Student S)
WHERE S.Id NOT IN (
SELECT T.StudId
FROM Transcript T
WHERE T.Semester = ’S2002’) )
)
Remaining solution:
SELECT S.Id, S.Name, AVG(T.Grade)
FROM Student S, TRANSCRIPTVIEW T
WHERE S.Id = T.StudId AND T.Semester = ’S2002’ GROUP BY S.Id
how the create view, TRANSCRIPTVIEW, manages to set the grade of 0 for those who did not take a course
The set of students who did not take a course in semester S2002 have no record in the transcript table for that semester. Those who did take a course in that semester do have a record in the table for that semester. The query supplies values NULL, 'S2002',0 for students if they are not in the Transcript table for semester S2002:
SELECT S.Id,NULL,’S2002’,0 FROM Student S) -- this parenthesis is wrong
-- this following where conditions looks for students NOT IN the 2002 subset:
WHERE S.Id NOT IN
-- this next part gets a list of studentids for semester 2002
(
SELECT T.StudId FROM Transcript T
WHERE T.Semester = ’S2002’
)
The solution in your qusetion is ridiculous. The better solution is:
SELECT S.Id, S.Name, AVG(case when T.Semester = ’S2002’ then T.Grade end) as AvgS2002Grade
FROM Student S left outer join
TRANSCRIPTVIEW T
on S.Id = T.StudId AND T.Semester = ’S2002’
GROUP BY S.Id
The query in your question is overly complicated. It is using a union (which should really be union all for performance reasons) to be sure all students are included. Gosh, this is what left outer join is for. It is doing filtering in the where clause, when a conditional aggregation is more suitable. It uses archaic join syntax, instead of the ANSI standard.
I hope you are not learning SQL with those shortcomings.
When you use mysql, you can do something like this:
SELECT * FROM student WHERE teacher_id = 1
...then you get the amount of results, and with that the result. But I only want the amount of results but then for every teacher.
Is that possible in mysql like sizeof or something?
Also, if there are teachers that have no students is it true that there not in the list? or does it have to say 0?
Do you mean you want the number of students for every teacher? You can use something like this:
SELECT teacher_id, count(*) AS student_count
FROM student
GROUP BY teacher_id
This will not include teachers that don't have any students. To include them in the results, you need to use a JOIN (assuming you have a teacher table):
SELECT teacher.id, sum(if(student.id IS NULL, 0, 1)) AS student_count
FROM teacher
LEFT JOIN student ON student.teacher_id=teacher.id
GROUP BY teacher.id
SELECT teacher_id, count(*) as count FROM student GROUP BY teacher_id;
Use:
SELECT t.teacher_id,
IFNULL(ns.num_students, 0)
FROM TEACHERS t
LEFT JOIN (SELECT s.teacher_id,
COUNT(s.student_id) 'num_students'
FROM STUDENTS s
GROUP BY s.teacher_id) ns ON ns.teacher_id = t.teacher_id