My subquery:
select studentName, Course.dataStart
from Student,
Course
where Student.id in (select Course.id from Course);
I need a solution to this (above) subquery (not a join)
Why does the SQL subquery display one date for each name? (task: display the names of students from the Student table and the course start date from the Course table using a subquery)
With the help of Join, I get it as it should: (but I need to do it with a subquery)
You seem to be using implicit join syntax, but really you should be using an explicit inner join:
SELECT s.studentName, c.dataStart
FROM Student s
INNER JOIN Course c
ON c.id = s.course_id;
If you really wanted to use the implicit join syntax, it should be something like this:
SELECT s.studentName, c.dataStart
FROM Student s, Course c
WHERE c.id = s.course_id;
But again, please use the first version as its syntax is considered the best way to do it.
You can apply join :
SELECT S.studentName, C.dataStart
FROM Student S
INNER JOIN Course C
ON C.id = S.course_id;
With Sub query:
Select studentName, (Select Course.dataStart from Course
Where Course.id = course_id)
From Student
Asuming that Course.Id field is Student.Id (although it seems strange to me), I think the only way to get the results you want with a subquery would be using it in the SELECT clause:
select studentName, (SELECT Course.dataStart FROM Course WHERE Course.Id = Student.Id)
from Student
This would fail if you have more than 1 row in Course per Student, in that case you could use (SELECT DISTINCT Course.dataStart...)
Related
I have a query like below
Select Student_ID, Name, School
From Student S
Where S.Student_ID in ( select associate_id from Details D)
The output count is coming as expected
But now I have a new requirement to get additional column (D.Subject)in the data from the Details table
Select S.Student_ID, S.Name, S.School, D.Subject
From Student S
Where S.Student_ID in ( select associate_id from Details D)
When I’m trying to achieve the above by using the join like below the count is not matching . I tried both Left outer and inner join and the count doesn’t come out correctly .
Select S.Student_ID, S.Name, S.School, D.Subject
From Student S
Left outer join Details D on S.Student_ID = D.associate_id
Where S.Student_ID in ( select associate_id from Details D)
Please let me know how to achieve this
Since we dont have the sample data, I tried to put something together based on your question
problem is with the distinct count.
You should use distinct values in select. Dont really need the where condition, as that will be covered in your join condition.
Select distinct S.Student_ID, S.Name, S.School, D.Subject
From Student S join Details D on S.Student_ID = D.associate_id;
this will give you the unique counts there are. As there may be more join conditions between these tables and not just the column student_id to associate_id
for ex-
If i take the example below
and
in line query will give you a different count then inner join.
So either you find a correct join conditions or distinct the values to keep you close.
I got two tables :
STUDENT with the columns ID, name, surname, class
SCORE with columns Student ID, Score_date, Score, Discipline
Display all students and their scores in Mathematics (whether or not they have a score):
SELECT Id, Name, Score
FROM STUDENT, SCORE
WHERE SCORE.Studentid = STUDENT.id
AND SCORE.Discipline = 'Mathematics';
How to do these queries using JOIN? I've tried to understand it on several sites, but it's still not clear (I'm new in SQL), can someone explain simply how it works?
This:
FROM student, score WHERE student.id = score.studentid
is the syntax we used in the 1980s. In 1992 the SQL standard invented explicit joins:
FROM student INNER JOIN score ON student.id = score.studentid
If you are in a class where you are taught using the 1980s syntax, I'd say quit that class.
Anyway, both are joins. Inner joins at that. You join the tables student and score on the sudentid and only keep matches (i.e. if a student doesn't have any score row, you don't select that student).
Your second query doesn't contain a join, but a lookup with NOT EXISTS.
Your third query contains an inner join again. This, however, does not match the task where you are asked to also show students that are not in Mathematics. You'd need an outer join there:
SELECT st.id, st.name, sc.score AS math_score
FROM student st
LEFT OUTER JOIN score sc ON sc.studentid = st.id AND sc.discipline = 'Mathematics';
How would you rewrite the following query into one without subquery as much as possible?
Select dept name,
(Select Count(*)
From instructor
Where department.dept name = instructor.dept name
) As num_instructors
From department;
I came up with the following. Is it a good equivalence to the above?
Select dept name, count(*)
From department, instructor
Where department.dept name = instructor.dept name
Group By department.dept_name;
Thanks.
The proper way to write the query uses explicit JOIN syntax:
select d.dept_name, count(i.dept_name)
from department d left join
instructor i
on d.dept_name = i.dept_name
group by d.dept_name;
If you only care about departments that have at least one instructor, then no join is necessary at all:
select i.dept_name, count(*)
from instructor i
group by i.dept_name;
Your attempt is really close, just a couple things..
You should use explicit joins (ie. JOIN, LEFT JOIN etc.) instead of implicit joins (commas in the FROM clause). Implicit joins are 25+ years depreciated.
Also, in this case you will want a LEFT JOIN or no departments will be displayed that don't have instructors. LEFT JOIN will retain departments without instructors and give you a 0 count (like the first query), where a JOIN would not display those at all.
SELECT d.dept_name, COUNT(i.dept_name) as num_instructors
FROM department d
LEFT JOIN instructors i on d.dept_name = i.dept_name
GROUP BY d.dept_name
I'm writing a query to multiply the count that I receive from subquery to fees amount, But I don't know how to do that. Any help/suggestion?
Oracle query is:
select courseid,coursename,fees*tmp
from course c join registration r on
r.courseid=c.courseid
and tmp IN (select count(*)
from course c join registration r on
r.courseid=c.courseid group by coursename);
I tried to use like a variable tmp ,But i don't think it works in oracle query. Is there an alternative way to do so?
You can't do that, because you can only select data from tables that appeared between FROM and WHERE. The IN operator is a quick way to save having to write a bunch of OR statements, it is not something that can establish a variable in the outer query.
Instead do something like:
select courseid,coursename,fees * COUNT(r.courseID) OVER(PARTITION BY c.coursename)
from course c join registration r on
r.courseid=c.courseid
Edit/update: you noted that this query produces too many rows and you only want to see distinct course names. In that case it would be better to just use the registrations table to count the number of people on the course and then multiply the fees:
SELECT
c.courseid, c.coursename, c.fees * COALESCE(r.numberOfstudents, 0) as courseWorth
FROM
course c
LEFT OUTER JOIN
(select courseid, COUNT(*) as numberofstudents FROM registration GROUP BY courseid) r
ON c.courseID = r.courseid
You can use a windowing function like Caius or you can use a join like this:
select courseid,coursename, fees * COALESCE(sub.cnt,0)
from course c
join registration r on r.courseid=c.courseid
left join (
select coursename, count(*) as cnt
from course c2
join registration r2 on r2.courseid=c2.courseid
group by coursename
) as sub;
note: I make no claim your joins are correct -- I'm basing this query off of your example not on any knowledge of your data model.
I have two tables:
students(id, name, school_id)
schools(id, name)
I'm trying to use EXISTS in order to learn if there are any students that go to specific school, say "Harvard" for example. I know that EXISTS is used after WHERE but I'm wondering if I can do this:
SELECT EXISTS
(SELECT *
FROM students st, schools sch
WHERE st.school_id=sch.id AND sch.name="Harvard");
Is this query correct? I am working on MySQL Workbench and I don't get an error. But I don't know if it does what it's supposed to do.
If it's not, then what should I change? I just want to know if it's correct and if I can use this syntax in the future.
Note that the desired result is either yes or no (1 or 0).
How do I get this result?
Sorry if my question was unclear, I can edit it again if you still don't understand.
You could just get the count, then you would know if any exist - and if so, how many...
SELECT COUNT(*)
FROM students st, schools sch
WHERE st.school_id=sch.id AND sch.name="Harvard"
The EXISTS keyword is normally used as a pre-condition... i.e. "if not exists, then add this..."
This is a simple Select with a Join.
SELECT *
FROM Students S
JOIN Schools SC
ON S.School_id = SC.id
Where SC.Name = 'Harvard'
This will give you all the rows for the students that go to Harvard, if any. If you want to do a count you can do SELECT COUNT(*) instead or limit which columns are returned by indicating the specific columns in the SELECT statement
This will give you student names that go to Harvard. If you wish to see how many students learn at Harvard replace st.name with count(*).
Note that it doesn't matter what you put in SELECT list inside EXISTS statement, so choosing a constant value provides better performance than selecting columns.
SELECT
st.name
FROM
students st
WHERE
EXISTS(
SELECT 1 FROM schools s WHERE s.name= 'Harvard' AND s.id = st.school_id)
OR
SELECT
st.name
FROM
students st
INNER JOIN schools s ON
st.school_id = s.id
WHERE
s.name = 'Harvard'
Additional note: Code below would only yield result either true or false.
SELECT EXISTS ( <query> )
This means that below query would return true if there are any students that learn at Harvard.
SELECT EXISTS (
SELECT 1
FROM students st
INNER JOIN schools s ON st.school_id = s.id
WHERE s.name = 'Harvard' )