In database design what do n:m and 1:n mean?
Does it have anything to do with keys or relationships?
m:n is used to denote a many-to-many relationship (m objects on the other side related to n on the other) while 1:n refers to a one-to-many relationship (1 object on the other side related to n on the other).
1:n means 'one-to-many'; you have two tables, and each row of table A may be referenced by any number of rows in table B, but each row in table B can only reference one row in table A (or none at all).
n:m (or n:n) means 'many-to-many'; each row in table A can reference many rows in table B, and each row in table B can reference many rows in table A.
A 1:n relationship is typically modelled using a simple foreign key - one column in table A references a similar column in table B, typically the primary key. Since the primary key uniquely identifies exactly one row, this row can be referenced by many rows in table A, but each row in table A can only reference one row in table B.
A n:m relationship cannot be done this way; a common solution is to use a link table that contains two foreign key columns, one for each table it links. For each reference between table A and table B, one row is inserted into the link table, containing the IDs of the corresponding rows.
n:m --> if you dont know both n and m it is simply many to many and it is represented by a bridge table between 2 other tables like
-- This table will hold our phone calls.
CREATE TABLE dbo.PhoneCalls
(
ID INT IDENTITY(1, 1) NOT NULL,
CallTime DATETIME NOT NULL DEFAULT GETDATE(),
CallerPhoneNumber CHAR(10) NOT NULL
)
-- This table will hold our "tickets" (or cases).
CREATE TABLE dbo.Tickets
(
ID INT IDENTITY(1, 1) NOT NULL,
CreatedTime DATETIME NOT NULL DEFAULT GETDATE(),
Subject VARCHAR(250) NOT NULL,
Notes VARCHAR(8000) NOT NULL,
Completed BIT NOT NULL DEFAULT 0
)
this is the bridge table for implementing Mapping between 2 tables
CREATE TABLE dbo.PhoneCalls_Tickets
(
PhoneCallID INT NOT NULL,
TicketID INT NOT NULL
)
One to Many (1:n) is simply one table which has a column as primary key and another table which has this column as a foreign key relationship
Kind of like Product and Product Category where one product Category can have Many products
In a relational database all types of relationships are represented in the same way: as relations. The candidate key(s) of each relation (and possibly other constraints as well) determine what kind of relationship is being represented. 1:n and m:n are two kinds of binary relationship:
C {Employee*,Company}
B {Book*,Author*}
In each case * designates the key attribute(s). {Book,Author} is a compound key.
C is a relation where each employee works for only one company but each company may have many employees (1:n):
B is a relation where a book can have many authors and an author may write many books (m:n):
Notice that the key constraints ensure that each employee can only be associated with one company whereas any combination of books and authors is permitted.
Other kinds of relationship are possible as well: n-ary (having more than two components); fixed cardinality (m:n where m and n are fixed constants or ranges); directional; and so on. William Kent in his book "Data and Reality" identifies at least 432 kinds - and that's just for binary relationships. In practice, the binary relationships 1:n and m:n are very common and are usually singled out as specially important in designing and understanding data models.
To explain the two concepts by example, imagine you have an order entry system for a bookstore. The mapping of orders to items is many to many (n:m) because each order can have multiple items, and each item can be ordered by multiple orders. On the other hand, a lookup between customers and order is one to many (1:n) because a customer can place more than one order, but an order is never for more than one customer.
What does the letter 'N' on a relationship line in an Entity Relationship diagram mean?
Any number
M:N
M - ordinality - describes the minimum (ordinal vs mandatory)
N - cardinality - describes the miximum
1:N (n=0,1,2,3...) one to zero or more
M:N (m and n=0,1,2,3...) zero or more to zero or more (many to many)
1:1 one to one
Find more here:
https://www.smartdraw.com/entity-relationship-diagram/
Many to Many (n:m)
One to Many (1:n)
Imagine you have have a Book model and a Page model,
1:N means:
One book can have **many** pages. One page can only be in **one** book.
N:N means:
One book can have **many** pages. And one page can be in **many** books.
m:n refers to a many to many relationship whereas 1:n means one to many relationship.
For example:
employee(id,name,skillset)
skillset(id,skillname,qualifications)
in this case the one employee can have many skills and ignoring other cases you can say that it's a 1:N relationship
Related
I am trying to simplify an application's database. In that database I have two tables let's say Patient and MedicalRecord. I know that two tables are said to be in One-to-One relationship iff that any given row from Table-A can have at most one row ine Table-B(It means there can be zero matchings).
But in my case, it is not at most, it is exactly. i.e., Every row in Patient should have exactly one row in MedicalRecord(no patient exist without a medical record).
Patient table has all personal details of the patient with his id as PK.
MedicalRecord talbe has details like his blood-group, haemoglobin, bp etc with his id as both PK and FK to the Patient.
My Question is, can I merge those two tables and create one table like,
PatientDetails : personal_details and blood-group, haemoglobin, bp etc
"bp" = "Blood pressure"? Then you must not combine the tables. Instead, it is 1:many -- each patient can have many sets of readings. It is very important to record and plot trends in the readings.
Put only truly constant values in the Patient -- name, birthdate (not age; compute that), sex, race (some races are more prone to certain diseases than others), not height/weight. Etc.
Sure, a patient may have a name change (marriage, legal action, etc), but that is an exception that does not affect the schema design, except to force you to use patient_id, not patient_name as a unique key.
Every patient must have a MedicalRecord? That is "business logic"; test it in the application; do not depend (in this case) on anything in the Database.
Both tables would have patient_id. Patients would have it as the PRIMARY KEY; MedicalRecord would haveINDEXed`. That's all it takes to have 1:many.
In situations where the tables are really 1:1 (optionally 1:0/1), I do recommend merging the table. (There are exceptions.)
If two tables have the same set of subrow values for a shared set of columns that is a superkey in both (SQL PRIMARY KEY or UNIQUE) then you can replace the two tables by their natural join. ("Natural join" is probably what you mean by "merge" but that is not a defined technical term.) Each original table will equal the projection of the join on that original's columns.
(1:1 means total on both sides, it does not mean 1:0-or-1, although most writing about cardinalities is sloppy & unclear.)
Let’s assume there are some rows in a table cars, and each of these rows has an owner. If this owner were always a person (conveniently situated in a table persons), this would be your standard one-to-many relation.
However, what if the owner could not only be a person, but also a company (in a table companies)? How would this relationship be modeled and how would it be handled in PHP?
My first idea was to create a column person and a column company and check that one of them always stays NULL, while the other is filled – however, that seems somewhat inelegant and becomes impractical once there is a higher number of possible related tables.
My current assumption would be to not simply create the foreign key as an integer column person in the table, but to create a further table called tables, which gives IDs to the tables, and then split the foreign key into two integer columns: owner_table, containing the ID of the table (e.g. 0 for persons and 1 for companies), and owner_id, containing the owner ID.
Is this a viable and practical solution or is there some standard design pattern regarding such issues? Is there a name for this type of problem? And are there any PHP frameworks supporting such relations?
EDIT: Found a solution: Such structures are called polymorphic relations, and Laravel supports them.
There are multiple ways to do it.
You can go with two nullable foreign keys: one referencing company and the other user. Then you can have a check constraint which assure you one is null. With PostgreSQL:
CREATE TABLE car{
<your car fields>
company_id INT REFERENCES car,
person_id INT REFERENCES person,
CHECK(company_id IS NULL AND person_id IS NOT NULL
OR company_id IS NOT NULL AND person_id IS NULL)
};
Or you can use table inheritance (beware their limitations)
CREATE TABLE car_owner{
car_owner_id SERIAL
};
CREATE TABLE company{
<company fields>
} INHERITS(car_owner);
CREATE TABLE person{
<person fields>
} INHERITS(car_owner);
CREATE TABLE car{
<car fields>
car_owner_id INT REFERENCES car_owner
};
I have some tables like this :
batches(id, name)
terms(id, name)
subjects(id, name)
Batches is having many-to-many relation with Terms in a table called batches_x_terms.
I wanted to create a table to assign Subjects to many terms and those subjects be traceable from Batches too, so I thought of creating a table like this :
batches_x_terms_x_subjects(id, batch_id, term_id, subject_id)
But upon further thinking I concluded that I will have lots of rows in this table for less data and data redundancy will be there too.
So I want to know if I can use M2M table's PK as a FK in M2M relation between :
'batches_x_terms' and 'subjects'
Update 1:
Batches table is having another column 'year'
Batches will have same batches from different years, ex
'Science', 2010
'Science', 2011
Now, suppose every Batch is having 4 terms(semesters) and in each term they have different subjects but some subjects are common between these 2 batches, right?
If I follow 'batches_x_terms' and 'batches_x_subjects' then I won't be able to figure out which subject is taught to which batch in a specific term. I need to classify my data like this :
Batches have how many terms?
Which subjects are assigned to which Batch in a specific term.
Same subjects can be assigned to another Batch in some other term.
Moreover, I have a constraint that I can't assign a different Term ID to every Batch, for a single semester every Batch will have a common Term ID.
I hope this much detail is useful.
Declare a column set whose subrow value is unique but doesn't contain any smaller column set whose subrow value is unique as UNIQUE NOT NULL. (I'll limit myself to when NOT NULL is appropriate.) You can define one such set per table as PRIMARY KEY instead. (That's equivalent as a constraint to UNIQUE NOT NULL.)
When a column set subrow value must appear in another table as a subrow value for a unique column set declare a FOREIGN KEY. If the unique column set isn't already declared UNIQUE or PK, do so.
Not only "can" you declare a PK, UNIQUE and/or FK per above, you should declare every one that could be. (Some DBMSs will prevent you from declaring FK cycles though.)
I have seen ,implementing the relation between "States" and "Districts" in two ways:
The relation between States and Districts is one to many relationship respectively..
First way:
In this implementaion,take two tables "States" and "Districts" and implement the one to many relationship between States to District as put the foreign key in Districts table.
In my "States" table the columns are: state_id(pk) & state_name.
In my "Districts" table the columns are: district_id(pk) district_name state_id(fk).
Second Way:
In this implementaion,take two tables "States" and "Districts" and implement the one to many relationship between States to District as creating the third table "state_district" and implementing as follows.
In my "States" table the columns are: state_id(pk) & state_name.
In my "Districts" table the columns are: district_id(pk) district_name .
The third table is "state_district",the columns are s_did(pk), district_id(fk),state_id(fk).
What is the difference betwen these two mechanisms.
The difference is that in the first case there can be only one state per district, wheras in the second there can be many states per district.
Which one you should use depends entirely on whether a district can be associated with multiple states or not. If they can then you have to use the second many-to-many model. If they cannot then while in practice you could use the second model, it would be incorrect to do so -- you should use the first one-to-many model.
for one to many relationship we use a table's primary key as foreign key in another table - Which is your first approach ans correct in this case
For many to many relationship we use a third table to store the relationship between first 2 tables- which is not required in your case as state to district has one to many relationship
What is the difference between these two mechanisms.
The difference is that the second method allows a district to be associated with more than one state. You can do this by just adding another row for a given district in the third table.
INSERT INTO state_district (district_id, state_id) VALUES
(1234, 49), (1234, 50);
Now you have the same district 1234 associated with both Alaska (49) and Hawaii (50).
I would assume you don't really need this. In fact, it would be better to ensure that each district belongs to exactly one state. You should have only a one-to-many relationship between states and districts. So you should use the first design.
Your second way should be done if there is a many to many relationship between states to districts.
You first way is correct and you should implement.
I would propose the following table structures:
States: --don't need extra metadata such as the sequence generated value
state_name varchar2(50) PRIMARY KEY
Districts: -- don't need extra metadata
district_name varchar2(100) PRIMARY KEY
State_Districts
state_name varchar2(50)
district_name varchar2(100)
primary key (state_name,district_name)
This ensures that you don't have duplicate district names which are real unique identifiers, regardless of if Wyoming and Pennsylvania have the same district name, the data is independent of each other. This also ensures that there will be no null values in any of the three tables, this is pretty important when we think about normalization techniques.
It would appear from your first table definition of the two tables that there is a State ID field in the District table. This indicates to me that there are one or more districts per state. In this case, a third table would be redundant.
Been reading the tutorial How to handle a Many-to-Many relationship with PHP and MySQL .
In this question I refer to the "Database schema" section which states the following rules:
This new table must be constructed to
allow the following:
* It must have a column which links back to table 'A'.
* It must have a column which links back to table 'B'.
* It must allow no more than one row to exist for any combination of rows from table 'A' and table 'B'.
* It must have a primary key.
Now it's crystal clear so far.
The only problem I'm having is with the 3rd rule ("It must allow no more than one row to exist for any combination").
I want this to be applied as well, but it doesn't seem to work this way.
On my test instance of mysql (5.XX) I'm able to add two rows which reflect the same relationship!
For example, if I make this relation (by adding a row):
A to B
It also allows me to make this relation as well:
B to A
So the question is two questions actually:
1) How do I enfore the 3rd rule which will not allow to do the above? Have only one unique relation regardless of the combination.
2) When I'll want to search for all the relations of 'A', how would the SQL query look like?
Note #1: Basically my final goal is to create a "friendship" system, and as far as I understand the solution is a many-to-many table. Suggest otherwise if possible.
Note #2: The users table is on a different database from the relations (call it friendships) table. Therefore I cannot use foreign keys.
For the first question:
Create a unique constraint on both
columns
Make sure you always sort the columns. So if your table has the
colummns a and b than make sure
that a is less than or equal to
b
For the second question:
SELECT
*
FROM
many_to_many_table
WHERE
a = A or b = A
It sounds like you want a composite primary key.
CREATE TABLE relationship (
A_id INTEGER UNSIGNED NOT NULL,
B_id INTEGER UNSIGNED NOT NULL,
PRIMARY KEY (A_id, B_id)
);
This is how you setup a table so that there can only ever be one row that defines tables A and B as related. It works because a primary key has to be unique in a table so therefore the database will allow only one row with any specific pair of values. You can create composite keys that aren't a primary key and they don't have to be unique (but you can create a unique non-primary key, composite or not), but your specification requested a primary key, so that's what I suggested.
You can, of course, add other columns to store information about this specific relationship.
Ok WoLpH was faster, I basically agree (note that you have to create a single constraint on both columns at the same time!). And just to explain why you collide with the rules you mentioned: Typically, A and B are different tables. So the typical example for n:m relations would allow entries (1,0) and (0,1) because they'd be refering to different pairs. Having table A=table B is a different situation (you use A and B as users, but in the example they're tables).