Here is the problem I'm trying to solve:
Entity A : a_id
Entity B : b_id
One A can use Many B's. However, not all Bs are used by all As.
Here is the best example I can think of :
One teacher has many students.
Some students are taught by more than one teacher.
What is a relationship so I can add/remove students being taught by one teacher, but not affect the teachers already teaching said students?
you need a third table known as mapping between this two tables.create this table so:
Table Student_Teacher_Mapping
Id (Int)
TeacherId(Int) // foreign key for teacher table
StudentId(Int) // foreign key for student table
i think this is what you want.
You need a StudentTeacher entity that will relate the two together. It would have an a_id column and a b_id column.
Related
Lets say I have a table of Classrooms.
Classroom Table
Each Classroom has its own set of Students specific to that Classroom. What would be the best way to set this up? Should I…
A. Make a separate Student table for each Classroom? How would I assign a Classroom to a table though?
B. Make one big list of Students each with their own Classroom FK? What if there are millions of Students and you are only looking for Students of a specific Classroom?
I am new to SQL btw
I would suggest one table for each entity and a join table to describe the relationship between the two. So one table for all the students, another table for all the classrooms and another table for joining.
Minimal example
STUDENT table has columns id (integer, primary key) and name (varchar).
CLASSROOM table has columns id (integer, primary key) and description (varchar).
STUDENT_CLASSROOM has integer columns id (primary key), student_id and classroom_id.
This way students can be assigned to classrooms (or classrooms can be assigned to students) and you can declare your foreign keys as appropriate.
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
I'm having a little bit of trouble with the following relationship between two tables:
Say, there's two tables, one being Student, the other being Module. 1 student can have 1 or many modules, however, 1 module can technically have 0 or more students. How would I implement this in a database? Thanks.
This is an N:N relationship. The answer is that you have to create a middle table that will create a link between your Student table and your Module table.
The middle table, which you can name StudentByModule for example, will hold a key identifying the student and another key identifying the module. This assumes that you have created a proper key in both tables.
http://en.tekstenuitleg.net/articles/software/database-design-tutorial/many-to-many.html
As for the case where a module is not assigned to any student this will be modelled by the absence of rows in the middle table linking the module to a student.
Also note that in order to associate a student to a module, you will first have to make sure that both the student and the modules are created first.
It's actually a many-to-many relationship. You implement it using a third table to link your tables
Student table
--
StudentId PK
...
Module table
---
ModuleId PK
...
StudentModule table
---
StudentId FK
ModuleId FK
The last table has a record only if there's a link between a student and a module. You insert primary keys of both Student and Module tables to make them linked.