Write a query to display list of students name who all are from IT department. Sort the result based on students name in descending.
This query will work for any rdbms and any schema (I assume that's what you are after here)
select studentid, firstname, surname
from tblStudent s
inner join tblStudentDepartment sd on s.studentid = sd.studentid
inner join tblDepartment d on sd.departmentid = d.departmentid
where d.department = 'IT'
Related
I am trying to write a sql query to show student list for each course.
The diagram below show the database relationship.
The SQL query I have written is:
select * from Courses
inner join Enrollments on Enrollments.CourseId = Courses.CourseId
inner join Student on Enrollments.StudentId = StudentId
where Courses.CourseId = 1
The issue is that i am getting returned alot more data than I expected as only one student is registered for the course but i get ten entries. I am not sure if i have done somethings fundamental wrong or is my query the issue.
This is the data
This is the result
I expected only two rows to be returned.
Thanks
Every column in your query must be qualified with the table's name.
You did not qualify the column StudentId in this join:
inner join Student on Enrollments.StudentId = StudentId
If you did you would find the error which is that there is no column StudentId in the table Student and you should use the column Id:
select * from Courses
inner join Enrollments on Enrollments.CourseId = Courses.CourseId
inner join Student on Enrollments.StudentId = Student.Id
where Courses.CourseId = 1
or better with aliases for the tables:
select *
from Courses as c
inner join Enrollments as e on e.CourseId = c.CourseId
inner join Student as s on e.StudentId = s.Id
where c.CourseId = 1
The primary key of table Student is Id, not StudentId.
So the correct query is:
select * from Courses
inner join Enrollments on Enrollments.CourseId = Courses.CourseId
inner join Student on Enrollments.StudentId = Student.Id
where Courses.CourseId = 1
Can you please help me, I don't know how to create this query. I am a front end dev.
Expected result should be table with 2 columns
Name(one) DepartmentName(many)
The tables and their relationship are shown in this image:
You can do this with a couple of INNER JOINS. You don't need to reference the Location table in this query because Employee.LocationId is the same as EmployeeDepartment.LocationId.
This simple query will return all employee names and all department names they are related to. The employee name may be repeated, as this query puts only one department name in the column, so if an employee is in two departments, you get the employee name twice.
SELECT
EmployeeName = e.Name
,DepartmentName = d.Name
FROM Employee e
INNER JOIN EmployeeDepartment ed ON ed.LocationId = e.LocationId
INNER JOIN Department d ON d.id = ed.DepartmentId
This query is a bit more complicated, but returns each employee name only once, and the department names will be a comma-separated string of names. This is accomplished by using STUFF() in conjunction with FOR XML.
;WITH EmployeesAndDepartments AS
(
SELECT
EmployeeName = e.Name
,DepartmentName = d.Name
FROM Employee e
INNER JOIN EmployeeDepartment ed ON ed.LocationId = e.LocationId
INNER JOIN Department d ON d.id = ed.DepartmentId
)
SELECT
ead.EmployeeName
,Departments = STUFF((
SELECT ',' + DepartmentName
FROM EmployeesAndDepartments
FOR XML PATH('')
) , 1, 1, ''
)
FROM EmployeesAndDepartments ead
GROUP BY ead.EmployeeName
This should work
SELECT
e.Name
, d.Name AS DepartmentName
FROM
Employee e
LEFT OUTER JOIN
EmployeeDepartment ed
ON e.LocationId = ed.LocationId
LEFT OUTER JOIN
Department d
ON ed.DepartmentId = d.id
Note that the use of LEFT OUTER JOIN will return the employee even if they don't have a corresponding record in EmployeeDeparatment and/or Department. If you only want to retrieve employees that do have corresponding EmployeeDepartment and Department records, use INNER JOIN instead
I need to print all persons which are not students and not teachers. I have three tables. Oracle database. Code so far:
SELECT PersonID, FirstName, LastName, Gender, DateOfBirth
FROM PERSON
INNER JOIN STUDENT S ON PERSON.PersonID = S.StudentID
INNER JOIN TEACHER T ON PERSON.PersonID = T.TeacherID
WHERE PERSON.PersonID != S.StudentID
AND PERSON.PersonID != T.TeacherID;
I guess my query is wrong because it returns 0 results. Do you have any idea what must I change?
" my query is wrong because it returns 0 results"
Inner joins return records when there is a match. You're trying to find PERSON records which join to neither STUDENT nor TEACHER. So, change your query to use outer joins:
SELECT PersonID, FirstName, LastName, Gender, DateOfBirth
FROM PERSON
LEFT OUTER JOIN STUDENT S ON PERSON.PersonID = S.StudentID
LEFT OUTER JOIN TEACHER T ON PERSON.PersonID = T.TeacherID
WHERE S.StudentID is null
AND T.TeacherID is null;
This is an anti-join: it returns records from PERSON which don't match records in STUDENT and TEACHER.
Use not exists:
select p.*
from person p
where not exists (select 1 from teacher t where t.teacherid = p.personid) and
not exists (select 1 from students s where s.studentid = p.personid);
Although you can write this query with left join, I think the version using not exists is almost a direct translation of the question, making it easier to understand.
In Oracle, you can also write this using minus -- if you want only the id:
select personid
from person
minus
select teacherid
from teacher
minus
select studentid
from student;
--Try this with Left Outer Join...
SELECT
P.PersonID,
P.FirstName,
P.LastName,
P.Gender,
P.DateOfBirth
FROM PERSON P
LEFT OUTER JOIN STUDENT S ON P.PersonID = S.StudentID
LEFT OUTER JOIN TEACHER T ON P.PersonID = T.TeacherID
WHERE S.PersonID is null and T.PersonID is null
I am trying to get teacher FIRSTNAME, LASTNAME and count of courses he got. Oracle database. CLASS table has columns: CLASSID, TEACHERID, CLASSNAME Current code:
SELECT DISTINCT FIRSTNAME, LASTNAME, COUNT(TEACHERID)
FROM PERSON
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID
INNER JOIN CLASS C ON T.TEACHERID = C.TEACHERID
WHERE T.TEACHERID = C.TEACHERID;
Where is my mistake?
Sounds like you need a GroupBy:
SELECT FIRSTNAME, LASTNAME, COUNT(T.TEACHERID)
FROM PERSON
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID
INNER JOIN CLASS C ON T.TEACHERID = C.TEACHERID
GROUP BY LASTNAME,FIRSTNAME
By the way, I initially deleted this answer because it's not a good way to do the grouping. I'd instead prefer grouping by TEACHERID, then joining back to get the names, rather than grouping by names.
I think this is a better approach:
SELECT FIRSTNAME, LASTNAME, C.NUMCLASSES
FROM PERSON
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID
INNER JOIN (SELECT TEACHERID, COUNT(CLASSID) AS NUMCLASSES FROM CLASS GROUP BY TEACHERID) C
ON C.TEACHERID=T.TEACHERID
you are missing the GROUP BY clause after the WHERE clause
GROUP BY FIRSTNAME, LASTNAME --all the selected columns except those in aggregate functions such as COUNT
May be you can try
SELECT FIRSTNAME, LASTNAME,
(select COUNT(C.TEACHERID) form CLASS C ON T.TEACHERID = C.TEACHERID)
as COURSECOUNT
FROM PERSON
INNER JOIN TEACHER T ON PERSON.PERSONID = T.TEACHERID
I'm doing some SQL practice and have been stumped by the following question.
I'm given the database schema:
Course (Course#, title, dept)
Student (Student#, name, program)
Enrolled (Student#, Course#, grade)
I'm trying the translate the following statement to SQL:
List the names of all students who takes Computer courses or Science courses.
Initially I thought the answer might be something like this:
SELECT Sname
FROM Course,Student,Enrolled
WHERE Course.dept = "Computer" OR Course.dept = "Science"
However, I feel like the rows in the table are not joined quite how I imagined, and that there is something off with this. How far off am I?
This is not that simple: first, you need to join the tables, and then you need to group by name to eliminate duplicates:
SELECT s.name
FROM Student s
JOIN Enrolled e ON s.Student#=e.Student#
JOIN Course c ON e.Course#=c.Course#
WHERE c.dept = 'Computer' OR c.dept = 'Science'
GROUP BY s.name
GROUP BY is necessary because the same student may be taking both "Computer" and "Science" courses, in which case JOIN would produce multiple records for the same student. In this case you have an option of replacing it with DISTINCT.
If you have 2 courses Computing (1) and Science (2) with the IDs 1 and 2, you need to do a query like this:
SELECT s.first_name, s.last_name FROM students s JOIN enrolled e ON e.student_id = s.id WHERE e.course_id IN(1, 2)
Sorry may have misread, if you need to do it by course type and the courses are tagged as dept = Computer, Science, Literacy etc... Do the following query:
SELECT s.first_name, s.last_name FROM students s JOIN enrolled e ON e.student_id = s.id JOIN courses c ON c.id = e.course_id WHERE c.dept IN('Computing', 'Science')
Or if you want to do an OR:
SELECT s.first_name, s.last_name FROM students s JOIN enrolled e ON e.student_id = s.id JOIN courses c ON c.id = e.course_id WHERE c.dept = 'Computing' OR c.dept = 'Science'