I'm trying to create a view that returns the amount of courses each Californian student is enrolled in. There are 4 CA students listed in my 'Students' table, so it should return that many rows.
create or replace view s2018_courses as
select Students.*, COUNT(Current_Schedule.ID) EnrolledCourses
from Students, Current_Schedule
where Students.ID = Current_Schedule.ID AND state='CA';
This query, however, only returns a single row with one student's information, and the total number of courses all CA students are in (in this case 14, since each student is enrolled in 3-5 classes).
I created a view similar to this recently (in a different DB) and it worked well and executed multiple rows, so I'm not sure what is going wrong? Sorry if this is a confusing question, I'm new to SQL and StackOverflow!! Thank you in advance for any advice!
You are missing an aggregation step in your query/view:
CREATE OR REPLACE VIEW s2018_courses AS
SELECT
s.ID, COUNT(cs.ID) EnrolledCourses
FROM Students s
INNER JOIN Current_Schedule cs
ON s.ID = cs.ID
WHERE
state = 'CA'
GROUP BY
s.ID;
The logical problem with your current query is that you are using COUNT(*) without GROUP BY, and MySQL is interpreting this to mean that you want to take the count of the entire table. Note also that I only select ID in my query above, because this is what is being used to aggregate.
Related
I am new to SQL. I am trying to write a query which requires contains operator. I am looking at many examples for the contains clause, but they seem different than what I need to use. I want a divide operator(relational algebra) equivalent in sql.
a sample of what I am doing is :
Give the students names who are enrolled for computer architecture course and have satisfied all its prerequisites. I have a prereq table and a course table which lists all courses taken by student
so what I ultimately want is
(get all courses taken by a student) contains (get all prerequisits)
for what exactly shall I look for for sql equivalent of divide operation in relational algebra?
You can do this by comparing counts.
select s.StudentName
from StudentCourses s, PrereqCourses p
where s.CourseId = p.CourseId
group by s.StudentName
having count(*) = (select count(*) from PrereqCourses)
See: http://www.dba-oracle.com/t_sql_patterns_relational_division.htm
I want to retrieve the course_id in table course that is not in the table takes. Table takes only contains course_id of courses taken by students. The problem is that if I have:
select count (distinct course.course_id)
from course, takes
where course.course_id = (takes.course_id);
the result is 85 which is smaller than the total number of course_id in table course, which is 200. The result is correct.
But I want to find the number of course_id that are not in the table takes, and I have:
select count (distinct course.course_id)
from course, takes
where course.course_id != (takes.course_id);
The result is 200, which is equal the number of course_id in table course. What is wrong with my code?
This SQL will give you the count of course_id in table course that aren't in the table takes:
select count (*)
from course c
where not exists (select *
from takes t
where c.course_id = t.course_id);
You didn't specify your DBMS, however, this SQL is pretty standard so it should work in the popular DBMSs.
There are a few different ways to accomplish what you're looking for. My personal favorite is the LEFT JOIN condition. Let me walk you through it:
Fact One: You want to return a list of courses
Fact Two: You want to
filter that list to not include anything in the Takes table.
I'd go about this by first mentally selecting a list of courses:
SELECT c.Course_ID
FROM Course c
and then filtering out the ones I don't want. One way to do this is to use a LEFT JOIN to get all the rows from the first table, along with any that happen to match in the second table, and then filter out the rows that actually do match, like so:
SELECT c.Course_ID
FROM
Course c
LEFT JOIN -- note the syntax: 'comma joins' are a bad idea.
Takes t ON
c.Course_ID = t.Course_ID -- at this point, you have all courses
WHERE t.Course_ID IS NULL -- this phrase means that none of the matching records will be returned.
Another note: as mentioned above, comma joins should be avoided. Instead, use the syntax I demonstrated above (INNER JOIN or LEFT JOIN, followed by the table name and an ON condition).
select distinct ID, title, takes.course_id
from course join takes
on course.course_id = takes.course_id
where takes.course_id in
(select takes.course_id
from takes
where ID = '10204');
select ID, title, takes.course_id
from course join takes
on course.course_id = takes.course_id
where ID = '10204';
I want to query the course IDs and the titles of the courses that a student whose ID is 10204 takes. The first gives a result with 5000 rows which is incorrect. The second give a correct result. So what is wrong with the first?
The first query gives you data for all students that happen to take a course that 10204 also takes.
Essentially the first query can be read as "Find all courses and the students that take them, for any course that is also taken by student 10204". You can look at the first query as a 3 way join. The results of the subquery select takes.course_id from takes where ID = '10204' would be the "third" table.
Adding to the pile on since everyone seems to be offering bits and pieces, some of which are oddly irate...
The first query says "Give me information on the students and courses where the courses were also taken by student 10204"
The second query says "Give me information on the students and courses taken by student 10204"
You say you wanted to get the course IDs and Titles for the courses taken by the student 10204, so obviously the second query is the correct one. You don't care about other student's that have taken the same courses.
Perhaps, to put it into perspective, rewriting the first, and incorrect query will help:
select distinct ID, title, takes.course_id
from course
join takes
on course.course_id = takes.course_id
join takes as takes2
on takes.course_id = takes2.course_id
WHERE
takes2.ID = '10204');
Well that is could be because in the first query you are quering where the course_id in takes table is equal to a specific course_id in that table (WHICH CAN BE NOT UNIQUE)
and in the second query you are straightly querying where the course_id is equal to a unique ID in that table!
Thanks you guys. I think my problem is that I did not realize that other students can take the same courses with the student having ID 10204. That this why though the condition is to query only courses take by the students 10204, the results is all about the courses taken by both 10204 and other students.
Because takes.ID != course.ID. The first you use takes.ID in the where clause but the second you use course.ID
I have two tables in a Database
and
I need to retrieve the number of staff per manager in the following format
I've been trying to adapt an answer to another question
SELECT bankNo AS "Bank Number",
COUNT (*) AS "Total Branches"
FROM BankBranch
GROUP BY bankNo
As
SELECT COUNT (*) AS StaffCount ,
Employee.Name AS Name
FROM Employee, Stafflink
GROUP BY Name
As I look at the Group BY I'm thinking I should be grouping by The ManID in the Stafflink Table.
My output with this query looks like this
So it is counting correctly but as you can see it's far off the output I need to get.
Any advice would be appreciated.
You need to join the Employee and Stafflink tables. It appears that your FROM clause should look like this:
FROM Employee INNER JOIN StaffLink ON Employee.ID = StaffLink.ManID
You have to join the Eployee table twice to get the summary of employees under manager
select count(*) as StaffCount,Manager.Name
from Employee join Stafflink on employee.Id = StaffLink.EmpId
join Employee as Manager on StaffLink.ManId = Manager.Id
Group by Manager.Name
The answers that advise you on how to join are correct, assuming that you want to learn how to use SQL in MS Access. But there is a way to accomplish the same thing using the ACCESS GUI for designing queries, and this involves a shorter learning curve than learning SQL.
The key to using the GUI when more than one table is involved is to realize that you have to define the relationships between tables in the relationship manager. Once you do that, designing the query you are after is a piece of cake, just point and click.
The tricky thing in your case is that there are two relationships between the two tables. One relationship links EmpId to ID and the other links ManId to ID.
If, however, you want to learn SQL, then this shortcut will be a digression.
If you don't specify a join between the tables, a so called Cartesian product will be built, i.e., each record from one table will be paired with every record from the other table. If you have 7 records in one table and 10 in the other you will get 70 pairs (i.e. rows) before grouping. This explains why you are getting a count of 7 per manager name.
Besides joining the tables, I would suggest you to group on the manager id instead of the manager name. The manager id is known to be unique per manager, but not the name. This then requires you to either group on the name in addition, because the name is in the select list or to apply an aggregate function on the name. Each additional grouping slows down the query; therefore I prefer the aggregate function.
SELECT
COUNT(*) AS StaffCount,
FIRST(Manager.Name) AS ManagerName
FROM
Stafflink
INNER JOIN Employee AS Manager
ON StaffLink.ManId = Manager.Id
GROUP BY
StaffLink.ManId
I don't know if it makes a performance difference, but I prefer to group on StaffLink.ManId than on Employee.Id, since StaffLink is the main table here and Employee is just used as lookup table in this query.
My directions are: "Print an ordered list of instructors and the number of A's they assigned to the students. Order the output by number of A's (lowest to highest) and ignore instructors with no A's assigned"
This is what I have (http://pastebin.com/pKwc6cSK):
Use Student_Course59
SELECT INSTRUCTOR
FROM Section RIGHT OUTER JOIN Grade_report
ON Section.SECTION_ID = Grade_report.SECTION_ID
WHERE GRADE IS NOT NULL
AND INSTRUCTOR IS NOT NULL
Here are my Tables:
I cant figure out how to count how many A's a specific instructor has assigned.
Thanks so much!!
I think you should use this query to find a specific instructor has assigned.
SELECT INSTRUCTOR FROM Section
RIGHT OUTER JOIN Grade_report
ON Section.SECTION_ID = Grade_report.SECTION_ID
WHERE Grade_report.GRADE IS NOT NULL AND
Section.INSTRUCTOR IS NOT NULL
From the screen shot it looks like you're running a version of SQL Server. If this is SQL Server 2005 or higher you can now use SUM as a "window function" which means it can take the OVER (PARTITION BY) meaning something like this might work:
Note the syntax is taken from your example, but if you'd provided a fiddler example, it would have worked better :)
SELECT DISTINCT INSTRUCTOR, SUM(CASE WHEN Grade = 'A' THEN 1 ELSE 0 END) OVER (PARTITION BY INSTRUCTOR) AS SUM
FROM Section
LEFT JOIN Grade_report ON Section.SECTION_ID = Grade_report.SECTION_ID
WHERE INSTRUCTOR IS NOT NULL
Note that I do not also know how your grade is represented so the value for 'A' might be different of course.
What it is doing is selecting all instructors and their grades. When the grade is 'A' it'll assign 1 otherwise 0 and then it'll SUM them up "partitioned by" (essentially a group by) instructor
You can solve a lot of problems using the OVER (PARTITION BY) in SQL Server so even if my syntax is incorrect for your case, try checking out the window functions and seeing if they cannot help you onwards.