I'm studying computer science and am brushing up on database systems. I'm having difficulties grasping certain parts.
Say I have the following relations:
Lecturers(LecturerID, Name, DeptID)
Course(DeptID, CrsCode, CrsName, Description)
I note that they both share a common attribute, DeptID, therefore they are union-compatible.
How would I go about listing all courses that are taught by lecturers belonging to computer science dept (CS) or electronic engineering dept (eEng)?
My answer would be using intersection with selection. Would the following be correct or near the mark?
πDeptID,CrsName(Course) intersection πDeptID,Name(σDeptID = CS or DeptID = eEng(Lecturers))
I'm sure join could be used here, but I'm unsure how to use the predicate with it.
Thanks for your help. Once I understand what to use in a few situations I'm sure the rest will be easier.
Thanks for any help.
I would use a simple INNER JOIN for this.
SELECT DEPTID, CRSNAME
FROM COURSE A
INNER JOIN LECTURERS B on A.DEPTID=B.DEPTID
WHERE B.DEPTID='eENG' or B.DEPTID='CS'
There must be also a table for Departments, as you have a refference to the DeptID field, which should be an INT. I assume it is DEPARTMENTS with DeptID and Code as fields. In this case:
SELECT
*
FROM
Course C
INNER JOIN
LECTURERS L on C.DeptId = L.DeptID
INNER JOIN
Departments D on C.DeptID = D.DeptID
WHERE
D.code = 'eENG' or D.code = 'CS'
Related
In a webshop, there are two (relevant to this question) tables: UserSnapshot and Purchase. Upon making a purchase, the user's current information is snapshot so that the purchase records are intact even if the user is later removed or changed. This gives a 1:1 relationship, where each purchase has only one user snapshot, and each user snapshot has only one purchase.
My question is, how should I implement this? Should I have a foreign key pointing to the user snapshot in the purchase table, the other way around, or should I use both (redundant)? Should I combine the two (messy)? Serialise the user snapshot (does not obey 'one value per field')?
I'd suggest looking at the likely queries you want to run, and design your model on that basis.
For instance, I guess you want to know "which orders has this customer placed?". The most natural way of expressing that would be something like:
select *
from customer c
inner join customer_snapshot cs
on c.customer_id = cs.customer_id
inner join orders o
on cs.order_id = o.order_id
where c.customer_id = ?
Or: "What is the current status of the customer who placed this order?".
select *
from order o
inner join customer_snapshot cs
on o.order_id = cs.order_id
inner join customer c
on cs.customer_id = c.customer_id
where o.order_id = ?
This feels natural to me, as it almost uses the customer_snapshot table as a "many to many" joining table.
But that's mostly stylistic - the join could just as easily be on o.customer_snapshot_id = cs.customer_snapshot_id.
How about "how many orders were sent to customers living in city x?"
select *
from order o
inner join customer_snapshot cs
on o.order_id = cs.order_id
inner join customer c
on cs.customer_id = c.customer_id
and cs.city = ?
You don't need "redundant" columns - all queries work without jumping through hoops. You could serialize the snapshot data, but then the "which orders were for customers living in city x" query would be painful.
having trouble here with SQL statement for displaying companies that does not supply beverages. Now the problem here is that there are few companies that supply both beverage and other stuff. I need it to only display companies that does not supply beverages at all. When i run my modified SQL statement, i managed to get what i want but, for those companies that supply foods as well as beverages, it is still being displayed because of my WHERE clause i set to search for C.CategoryName != 'Beverages'. I just want to remove those companies that supply both beverages and foods so i can only display those companies that purely supply only foods.
Sorry if i have some grammer mistake or anything or my question is unclear. I just don't really know how to explain professionally as it is hard to describe my problem. Thanks for those who willing to help me, i am really appreciate it. And also i do not know if these info and source codes are sufficient regarding my problem. Will provide more info/source code if needed.
Pictures of Both Category & Products table.
Category Table
Products Table
Here's the Initial statement
SELECT P.ProductName
FROM Products AS P
INNER JOIN
Categories AS C
ON
C.CategoryID = P.CategoryID
AND C.CategoryName = 'Beverages'
Result
So far here's the modified statement
SELECT S.CompanyName, P.CategoryID, C.CategoryID FROM Suppliers AS S
INNER JOIN
Products AS P
ON S.SupplierID = P.SupplierID
LEFT OUTER JOIN
Categories AS C
ON
C.CategoryID = P.CategoryID
AND C.CategoryName != 'Beverages'
Result
Answer Suggested by Martin Smith.
With adding EXCEPT Clause to the SQL statement to my modified SQL statement, i managed to get it done. Here is the Answer to my problem.
SELECT S.CompanyName FROM Suppliers AS S
EXCEPT
SELECT S.CompanyName FROM Suppliers AS S
INNER JOIN
Products AS P
ON S.SupplierID = P.SupplierID
RIGHT JOIN
Categories AS C
ON
C.CategoryID = P.CategoryID
AND C.CategoryName = 'Beverages'
WITH FOODSUPPLIERS AS
(
SELECT DISTINCT SUPPLIERID
FROM PRODUCTS P
JOIN CATEGORY C ON C.CATEGORYID = P.CATEGORYID
WHERE CATEGORYNAME <> 'Beverages'
)
SELECT S.COMPANYNAME FROM SUPPLIERS S
JOIN FOODSUPPLIERS FS ON FS.SUPPLIERID = S.SUPPLIERID
The FOODSUPPLIERS CTE identifies suppliers in PRODUCTS that are not associated with any beverage products, i.e., associated only with food products. It's then a simple matter of joining the CTE to Suppliers to get the records for these suppliers.
I'm extremely new to SQL Sever and so I apologize if the question is worded strange. I am doing a homework assignment, and this is the question:
"A manager wants to know the email address, number or orders, and the total amount of purchases made by each customer. Create a summary query that returns these three items for each customer that has orders."
I have all of the data queried, the problem is when I pull data from each customer, it will show the quantity of items per order, and I need the items to be pooled together into one column. This is my query thus far (again, total noob, please excuse any poor syntax, etc.)
SELECT EmailAddress,
ItemPrice - DiscountAmount * Quantity AS TotalPurchaseAmount,
COUNT(*) AS OrderQty
FROM Customers
JOIN Orders ON Customers.CustomerID = Orders.CustomerID
JOIN OrderItems ON Orders.OrderID = OrderItems.OrderID
GROUP BY Orders.CustomerID,
OrderItems.ItemPrice, OrderItems.DiscountAmount,
OrderItems.Quantity,
Customers.EmailAddress;
The following is a small bit of the result set that I get:
Email Address OrderTotal OrderQty
allan.sherwood#yahoo.com 253.15 2
allan.sherwood#yahoo.com 839.30 2
allan.sherwood#yahoo.com 1208.16 2
barryz#gmail.com 303.79 4
christineb#solarone.com 479.60 2
david.goldstein#hotmail.com 299.00 2
david.goldstein#hotmail.com 489.30 1
david.goldstein#hotmail.com 479.60 1
So as you can see, I have several orders I need to smoosh together into one single row per e-mail, I have looked and looked for an answer but the only thing I can find is how to find duplicates and ignore them, not combine their data. Any help is extremely appreciate, thanks so much for taking the time to read this :) If my question doesn't make sense please let me know so I can clear up any bad wording I may have used!
Just do GROUP BY CustomerID, EmailAddress:
SELECT
c.EmailAddress,
SUM((i.ItemPrice - i.DiscountAmount) * Quantity) AS TotalPurchaseAmount,
COUNT(*) AS OrderQty
FROM Customers c
INNER JOIN Orders o
ON c.CustomerID = o.CustomerID
INNER JOIN OrderItems i
ON o.OrderID = i.OrderID
GROUP BY
c.CustomerID, c.EmailAddress
Additional note: Use aliases for your tables
You need to change your formula and remove columns that you dont want to group by from select query..
for example your query should be something like this
SELECT EmailAddress,
--do your aggregation here
blah AS TotalPurchaseAmount,
COUNT(*) AS OrderQty
FROM Customers
JOIN Orders ON Customers.CustomerID = Orders.CustomerID
JOIN OrderItems ON Orders.OrderID = OrderItems.OrderID
GROUP BY Orders.CustomerID,
Customers.EmailAddress;
I am hoping someone might be able to help me with this issue I am having.
I have a table of customers - let's call it Table C. And I have a second table of customers that are not to be called - let's call it Table D.
I would like to pull all of the needed information (name, address, phone, etc...) from Table C unless the customer appears in Table D.
In the example shown below, I'd like to have data returned for all customers except John Doe (ID: 1) and Fred Savage (ID: 5)
I think a RIGHT OUTER JOIN might be applicable here, but I have not used this type of join before.
Use NOT EXISTS to do this:
SELECT c.*
FROM tableC c
WHERE NOT EXISTS (
SELECT *
FROM tableD d
WHERE c.customerID = d.customerid
);
If you want to use a join then it's a left join you want with a filter for null values in the d table. A right join would have gotten you all the rows from the d table, plus the matching ones from the c table - quite the opposite of what you want, but if you had switched the tables around then you would have gotten the same result, so this:
select c.* from c
left join d on c.CustomerID = d.CustomerID
where d.CustomerID is null
is equivalent to:
select c.* from d
right join c on c.CustomerID = d.CustomerID
where d.CustomerID is null;
Personally I would prefer using either a correlated not exists query or not in (but beware of null values) as I think those convey the intent more clearly.
Select * from table.c where customer_id not in (select distinct customer_id from table.d);
Yes you want an outer join.
Try this: https://technet.microsoft.com/en-US/library/ms187518(v=SQL.105).aspx
I am having a difficult time deciding how to handle a business requirement in my database schema. I have a lot of tables in the database, but there are only three I need to deal with for this problem: Courses, PersonnelCourses, and Personnel.
Courses is a list of Courses
Personel
is a list of Personnel
PersonnelCourses is a list of Courses
that Personnel have taken.
In courses there is a column called Universal. If a course is universal, that means all Personnel must take that course.
I need to generate a list of all the universal courses that Personnel must take, but the only way I am able to generate this list is with a cross join / cartesian join:
select P.LastName, C.Name
from Courses C, Personnel P
where Universal = 1
From that I want to do a left join onto PersonnelCourses so that I can have a list of all the Personnel and the Courses they must take as well as the courses they have taken. I'm thinking this would all be easier if there was a many to many table between Personnel and Courses. But if all Personnel are going to be in this middle table anyway, isn't that a bit redundant?
Is there a better way to handle this?
Much appreciated,
-Matt
There is a list of courses that everybody has to take. Why not just take this list and work with it, instead of repeating the same list for every personnel row? I don't understand why you are trying to multiply your result set.
Isn't your PersonnelCourses establishing a many to many relationship between a Persons and Courses? If it isn't then I am not sure, If it is then...
select *
from Personnel_Courses
inner join Person on... /*get the Person details*/
inner join Courses on... /*get the Course details*/
where Course.Universal = 1 and Person.Id = #Id
would tell you what universal courses they have taken...
and then
select *
from Courses
where Courses.Universal = 1 and Course.Id not in (
select Course.Id from Personnel_Courses
inner join Person on... /*get the Person details*/
inner join Courses on... /*get the Course details*/
where Course.Universal = 1 and Person.Id = #Id
)
Would give you the universal courses that they haven't taken...
To me it might be easier to do the 2nd in your code (Get the first query results, Do a select from the Course table to get all the universal and then do a comparison...)
This is a topic Database Normalization that books have been written on
and part of why you want to do this is DRY or don't repeat yourself.
So to answer your question about a better way - I would answer no.
How about something like this (using the existing structure)?
SELECT P.LastName, C.Name, 1 as Taken
FROM Courses C
INNER JOIN PersonnelCourses PC ON (C.CourseID=PC.CourseID)
INNER JOIN Personnel P ON (P.PersonID=PC.PersonID)
WHERE(C.Universal = 1)
UNION
SELECTP.LastName, C.name, 0 as Taken
FROM Courses C, Personnel P
WHERE (Universal = 1) and
NOT EXISTS(SELECT * FROM Courses C2
INNER JOIN PersonnelCourses PC2 ON (C.CourseID=PC.CourseID)
INNER JOIN Personnel P2 ON (P.PersonID=PC.PersonID)
WHERE (Universal = 1) and
(PC2.CourseID=C.CourseID) and
(P2.PersonID=PC2.PersonID)
)
Universal is a two-value (boolean) attribute of Course, right? In that case, consider normalizing further. Redesign so that UniversalCourse is a table, not a column on Course. That table would have a single column referencing the course. To find all universal courses, simply select everything from this table. Now you can shorten your cartesian join considerably since you have to multiply Personnel only by the UniversalCourse table, having eliminated the where Universal = 1 clause.
All personnel will not be in the personnelcourses table, only personnel who have taken courses.
I think you design is fine. You just need to tweak your query to get what your after.
In a subquery pull the courses the personnel have taken. Then in an outer query select all the courses that the personnel must take and do a left outer join with the subquery.
Select a.CourseName, b.PersonName from Courses a,
(select P.LastName, C.Name from Courses C, Personnel P, PersonnelCourses pc
c.courseid = pc.courseid and
p.personnelid = pc.personnelid and
c.Universal = 1) b
where
a.courseid += b.courseid order by courseid
It would probably be best to filter by personnel, if this is for a report. That way you would see all of the courses required including the ones taken per person.