I have two tables: Student and Shop and I would like to record information about which student visited which shop and if the number of visits is above n, they should received a discount:
This is how I did it:
All of attributes (studentID, shopID, time, date) in table StudentShop makes the primary key for this table. I just wanted to know if this design is good?
As relation between entieties Student - Shop is many to many relationship,
it is always implemented using associative table (consists of primary keys from both relations), so StudentShop table - is good choice for implementation of such relation.
Related
Perhaps, this could seem like the basics of database design.But,certainly i find these concepts a lil tricky in the sense that how they relate to each other.
Theory as per my understanding.
One to One :
When each particular entity has one record.
One to Many :
When each particular entity has many record
Many to Many :
Multiple entities have multiple records.
As per the above if i could relate an example as
ONE TO ONE
Each employee having a unique passport number.
Table Employee
Empid(pk)
empname
passpordid(fk to passport)
Table passport
passportid(pk)
passportno
ONE TO MANY
An organisation having multiple employees
TABLE ORGANISATION
ORGID (PK)
ORGNAME
EMPID (FK TO EMPLOYEE)
TABLE EMPLOYEE
EMPID (PK)
EMPNAME
SALARY
This is the part that i want to know more that is many to many. I mean if we see one to many here. As a whole it could be said as many to many as many organisations having many employees but does that mean the whole relationship is many to many not one to many or one to many is a subset of many to many in the above example.
I wanna know the difference mainly between one to many and many to many both theoritically and by implementation.
An example of a many-to-many relationship would be EMPLOYEES and SKILLS, where SKILLS are things like "SQL", "Javascript", "Management" etc. An employee may have many skills (e.g. may know SQL and Javascript), and a particular skill by be possessed by many employees (e.g. Jack and Jill both know SQL).
In a database like Oracle, you need a third table to express the many-to-many relationship between EMPLOYEES and SKILLS:
create table EMPLOYEE_SKILLS
( empid references EMPLOYEES
, skillid references SKILLS
, constraint EMPLOYEE_SKILLS_PK primary key (empid, skillid)
);
Note that this third table has a foreign key to both of the other tables.
The table can also hold further information about the relationship - for example:
create table EMPLOYEE_SKILLS
( empid references EMPLOYEES
, skillid references SKILLS
, rating number
, date_certified date
, constraint EMPLOYEE_SKILLS_PK primary key (empid, skillid)
);
This is more a theory question that a programming one, but why not.
You could imagine a "many to many" as 1 table being a list of employees, and another table being a list of products sold.
Each employee is likely to "handle" more than 1 product, and each product could very well be handled by multiple employees, and it's quite possible that products A and B can be handled by, at the same time, employees C and D.
You seem to have the basic idea of one-to-one and one-to many down, in theory.
One-to-one: each item A has a separate and unique item B
In a database, you could include the new data in the same table. You could put it in a separate table, and enforce the one-to-one constraint by making the id into the remote table "unique" (indexing constraint), forcing the id for item B to not be repeated in the table. In your example, this would be a constraint added to the passportid.
One to many: each item A has some number of item Bs
In a database, you put the key in the table on the "many" side. So many Bs would have the same foreign key into table A. In your example, put an orgid as a foireign key into the employee table. Many employees can then point to a single organization, and an employee can belong to one organization.
Many to many: some number of As can be linked to any number of Bs and vice-versa
In a database, this is usually implemented as a link table, having just the IDs from both A and B. Following you example, consider that an employee can be part of multiple groups (or projects), multitasking across those groups. You could have a group table:
GROUPID (PK)
GROUPNAME
And the link table would then look like
LINKID (PK)
EMPID (FK TO EMPLOYEE)
GROUPID (FK TO GROUP)
This lets any employee be part of any number of groups, and groups to have any number of employees, AKA many-to-many.
A side comment:
Technically, you don't HAVE to have a LINKID in the link table. But it makes it easier to manage the data in that table, and is generally considered "best practice".
i have a database for university so the students take many courses but each student have id as primary key also each course have an id as primary key its not allowed to have duplicate id at same table what should i use to make a table that same id but different courses id
In a normalized relational database, when you have a situation where items from one table can be associated with many items in a second table, and likewise those related items can be associated with many items in the first table, you should use a third table to store those relationships. In your case, that table might be called Enrollments.
You can use JOIN operations on that third table to relate, group, and aggregate across that relationship. You can also store any metadata associated with that relationship in the JOIN- table, for example you might store the enrollment date there.
Like vicatcu said, create a third table
Course_id | Student_id
The ID's of the courses and the students are uniq in their own tables. The relation between course < - > student is stored in that third table.
I’m building a hobby site about cars. If a user clicks on a car I want to display a list of similar cars for the chosen car.
I have a main table witch stores the basic information about each car, CarDataMain, two of the columns in the table are CarID (Pk) and SimilarCarsID (Fk).
I have another table called “SimilarCars”, it has three columns; SimilarCarsID (Pk), CarGroupID and CarID.
So the SimilarCarsID-column in the SimilarCars-table has a relationship with the column SimilarCarsID in the CarDataMain-table.
Is this the correct approach, or “best practice”?
Each car can only belong to one CarGroup (CarGroupID).
Another solution would be to create a third table witch holds the relationship between CarDataMain and SimilarCars-data, but as there is a one-to-many relationship I guess this is over kill? That way I could however put the same foreign key-value in CarDataMain for all cars witch belong to the same CarGroup, witch somehow feels appealing…
A third solution would be to skip the SimilarCarsID column in CarDataMain and make the CarID a foreign key in the SimilarCars table, if you understand what I mean. But I guess there are some drawbacks with that solution…
Sorry for newbie questions (and if this question has been up before), but I want to get it right before I start :)
Ludde
Note: re-written
Design tables that describe things like Car, Group, others?
Use "join tables" to give the desired one-to-many relationships.
Tables
CarID - primary key
Group
GroupID - primary key.
Name - the name of the group.
The GroupID may not be necessary. If Name is unique then use that as the primary key.
CarGroup
CarID - foreign key to Car Table
GroupID - foreign key to Group table
This is a join table. Here you can map a given car to many groups, and map a given group to many cars.
SimilarCar
I may be misunderstanding, but is seems like you want to match cars as similar, for no particular reason. If you want to match by group, that is above; and if you want "these 2 cars are similar, period", here you go. This is not mutually exclusive to the groups.
CarID - FK to Car table
SimilarCarID - Fk to Car table
A constraint so CarId cannot match SimilarCarID -- unless you do want a car to be similar to itself. But if so then just read the car table instead.
I think your concepts need to be cleared up first. Isn't a "similar car" a car that is in the same "group"? If so, you only need 1 term. Since "group" is a reserved word it is better to use something else.
I would go with the term "category". So my Cars table would have a CategoryId column. Picture the page on the front-end where someone would add a new car, wouldn't they have a drop down list to select the Category?
So my model would be:
Cars
-Id (PK)
-Name
-CategoryId (FK to Categories)
...
Categories
-Id (PK)
-Name
I have a major problem with my SQL design.
I need to create a database which models the following situation:
I have a list of capitals, it looks like this: Berlin, London, Prague, you get it. The other list is a list of travellers: John, Kyle, Peter etc.
The database needs to be able to answer queries such as: List of cities a given Traveller has visited, what Travellers has visited a given City and so on.
My problem is that when I create the two tables I just freeze and am unable to figure out the connection between them that would allow me to implement the intended behaviour.
I read up on it on the internet and I was told to use intersection entities, which I can, but I just don't see how that would help me. Sorry if this is a FAQ, but I just could not get my head around the proper keywords for a search.
Isn't it easier to create third table like travelers_cities with to foreign keys traveler and city, than you jan join that table with table you need and look for result there?
Solution:
Follow the following schema
First Table: Capital
Let say two columns: cid (Primary Key), Name
Second Table: Travellers
Let say two columns: tid (Primary Key), Traveller_Name
Now there is a many to many relationship that one traveller can travel/visit one or many capitals and one capital can be visited by one or many visitors.
Many to many relationship can be acheived by creating a new table which will act as reference/mapping table. let say "capital_travellers"
So, This third table will have following columns
capital_travellers:
id (Primary key): Its optional
cid (Primary key of Capital Table will work as Foreign key)
tid (Primary key of traveller Table will work as Foreign key)
Now when you want to fetch records, you will look into this table(capital_travellers).
I hope it helps.
In a many to many relationship it is necessary to implement a third junction table between the two entities. We could, say, name it travel. This table uses two foreign keys, one from each entity, to form one compound key.
Now you have 3 tables. One table called 'Traveller', one called called 'City' and a third junction table called 'Travel'. Lets assume the primary key for traveller is travellerId and for city it's cityId. The junction table takes these two keys to form a primary key for the entity 'Travel'. When u query the database for which cities a traveller with travelId '031' has travelled to it would make use of this junction table and return the cityId's to you.
If this doesn't help or if you need more clarification I recommend searching these terms:
Many-to-many
Cardinality
Ok, I'm brand new to SQL etc so apologies if this is totally wrong..
I have designed an ER Model which I feel is right and I am trying to convert it to relational model and want any advice as to where I am gone wrong on converting it or any tips. Racking my brain.
As I believe it..
1-1 relationships
entities are either combined or the primary key of one entity type is placed as a foreign key in the other relation.
1-m relationships
The primary key from the `one side' is placed as a foreign key in the many side.
m-n relationships
A new relation is created with the primary keys from each entity forming a composite key.
multivalue attributes
a new table is created, primary key used from 1st table and attribute used in the second table alognside primary key.
So here's my go at Relational model, PK in bold, FK in italics
USER:
USERID FNAME LNAME USERNAME PASSWORD USERTYPE EMAIL
CUSTOMER:
USERID, CUST_ID, BIO
ADMIN:
USERID ADMIN_ID
ARTIST
USERID, ARTIST_ID, BIO REC_ID
PRODUCER:
PROD_ID, Name, Email
RECORD LABEL:
RECORD_ID , NAME, DESCRIPTION
ALBUM:
ALBUMID NAME , COST, TITLE, NOOFSONGS
TRACK:
TRACK ID, NAME, COST, TITLE, DESCRIPTION
TRACK REVIEW: DEPENDENT on TRACK SO TRACK ID comes into this table =
REVIEW_ID(PK), TRK_ID(PK) NAME
TRACK PURCHASE TABLE (USER id comes into this table as foreign key)
TrackPuchaseID user_id, date
ALBUM PURCHASE TABLE
AlbumPuchaseID user_id, date, quantity
GENRE TABLE?:
Not sure??
BPM: is mutli value atribute so becoems seperate table so it.s
GenreID BPM
I know this all might be wrong. but any help would be great.. with explanation which should be FK or composite PK etc or what tables I am missing..
On album purchase you should have: user_id, date, album_id.
At track purchase I don't see why you care about the quantity and
you're missing the track_id.
review, track, album, user should all include a date. Might
be as well interesting to add an last_update_date to the artist
and customer.
review should include an user_id.
I guess there's other thing left. You will need an Invoice / Invoice_Purchase table to be able to say "customer xyz bought the items 1,2,3,4...", it should be like:
Table Invoice
invoice_id
user_id
date
status
Table Invoice_Purchase
invoice_id // Id of the invoice
purchase_type // the type of the purchase (for eg. 0 means track and 1 means album)
Maybe you should also add a status to your albuns and tracks, this way you can set if a item can be bought or not. And yes, you shouldn't delete them because you'll have the old invoices relaying on it...
Anyways, you should consider later using a software like Navicat do draw the relation part and deploy your DB.
From an initial look, here are some of my impressions, in no particular order.
1-1 relationships: not only is the PK of the entity defined as a FK in the related table, it is also the related table's PK. This preserves the unity of the relationship.
multivalue attributes: in other words, a 1-m relationship.
There is no reason to have separate ID values for Customer, Admin or Artist. In fact, call them all ID. I would say this is a m-1 relationship as a user could be both a customer and an artist. But the relationship between User table and the others individually are 1-1.
Isn't Track related to Album? So wouldn't AlbumID be a Track attribute?
Purchase tables should have CustID not UserID and be FK to Customer table. This enforces the need that a purchaser be defined as a customer.
Genre could be an attribute of track or album or both. An album could be generally "Country" but have a "Blues" track, a "Pop" track, etc.
Same for producer. There will be an album producer but could be one or more tracks produced by someone else.
Same for artist. There may be one artist for the entire album but what about "The Greatest Top 40 Hits of the 1980s"?
You may want an intersection table between Artist and Label. A label can sign up many artists and an artist could work for many labels (though generally not at the same time).
I have no idea what a BPM is.
That should keep you busy for a while.