Are there any hard and fast rules against creating a junction table out of a table's primary key? Say I have a table with a structure similar to:
In this instance there are a list of items that can be sold together, but should be marked as dangerous. Any one item can have multiple other items with which it is dangerous. All of the items are uniquely identified using their itemId. Is it OK to refer a table to itself in a many-to-many relationship? I've seen other examples on SO, but they weren't SQL specific really.
This is the correct design for your problem, as long as your combinations can only be two-item combos.
In database design a conceptual design that renders a relation in many-to-many form is converted into 2 one-to-many in physical design. Say for example a Student could take one or many courses and a course could have many students, so that's many to many. So, in actual design it would be a Student table, Course table then CourseTaken table that has both the primary key of Student and Course table thus creating 2 one to many relayionship. In your case altough the two tables are one and the same but you have the virtual third table to facilitate the 2 one to many relationship so that to me is still very viable approach.
I have a trip (primary: idTrip), where I can link more packages (primary: idPackage), so, I got a relationship table to link trips with packages (primary: idRelTripPackage). (relationship n-to-n)
And next I got a registrations table (primary: idRegistration). How do I best link those (1-to-1 relationship)?
I add two columns in the registrations table (idTrip, idPackage)?
I add a relationship table where i link idRegistration, idTrip, idPackage?
I add a relationship table where i link idRegistration, idRelTripPackage?
Am I right in thinking the relation from Registrations is to RelTripPackage, and its definitely one-to-one. There are a couple of options:
1: As it really is a one-to-one there's not really anything to stop you putting the Registrations data directly onto RelTripPackage, or doing the vice-versa and putting idPackage and idTrip straight onto Registrations as FKs, with a unique key across the two FK columns to ensure there aren't duplicates.
2: If do want the two separate tables then just add idRetTripPackage to Registrations as an FK, and then add a unique constraint on it - again to ensure uniqueness.
There's no need for a separate relationship table as its a 1-1 relationship - They only really become relevant when you are using an n-n. The rest of the time FKs should be placed directly on the child table.
If you follow that logic, you will
add tables and Relations every time you need to add Relations
end up with confusing or duplicate Relations (multiple paths between any two tables)
However the problem (limiting factor) is that the tables you are starting with are not actually normalised. Since the starting position does not have a good basis, you will end up with far more Relations (in tables) than there actually are between the Entities. So the best advice is, the Best practice is, before you attempt this current extension, step back and normalise the data, the existing tables. Then the extension will be much easier, and you will end up with less tables.
if you provide info re the tables (Person, Trip, Package, etc); what exactly is a Registration, etc ... I can provide more explicit answers.
Generally any attribute that is 1::1 with the PK of an Entity should be an attribute in that entity. Any attribute that is 1::0-1 with the PK of an Entity should be in a separate table.
ER Diagram
Based on the information provided, this is your ▶Entity Relation Diagram◀. As long as you use Relational Identifiers, all the Relations you have identified thus far are supported directly (otherwise, if you use IDs, you will need more Relations and tables).
Readers who are unfamiliar with the Relational Database Modelling standard may find ▶IDEF1X Notation◀ useful.
Coming from question “Relation” versus “relationship”
What are definitions of "relation" vs. "relationship" in RDBMS (or database theory)?
Update:
I was somewhat perplexed by comment to my question:
"relation is a synonym for table, and
thus has a very precise meaning in
terms of the schema stored in the
computer"
Update2:
Had I answered incorrectly that question , in terms of RDBMS, having written that relation is one-side direction singular connection-dependence-link,
i.e. from one table to another while relationship implies (not necessarily explicitly) more than one link connection in one direction (from one table to another)?
A RELATION is a subset of the cartesian product of a set of domains (http://mathworld.wolfram.com/Relation.html). In everyday terms a relation (or more specifically a relation variable) is the data structure that most people refer to as a table (although tables in SQL do not necessarily qualify as relations).
Relations are the basis of the relational database model.
Relationships are something different. A relationship is a semantic "association among things".
Relation is a mathematical term referring to a concept from set theory. Basically, in RDBMS world, the "relational" aspect is that data is organized into tables which reflect the fact that each row (tuple) is related to all the others. They are all the same type of info.
But then, your have ER (Entity Relationship) which is a modeling methodology in which you identify objects and their relationships in the real world. Then each object is modelled as a table, and each relationship is modelled as a table that contains only foreign keys.
For instance, if you have 3 entities: Teacher, Student, Class; then you might also create a couple of tables to record these 2 relationships: TaughtBy and StudyingIn. The TaughtBy table would have a record with a Teacher ID and a Class ID to record that this class is taught by this teacher. And the StudyingIn table would have a Student ID and a Class ID to reflect that the student is taking this class.
That way, each student can be in many Classes, and each Teacher can be in many classes without needing to have a field which contains a list of class ids in any records. SQL cannot deal with field containing a list of things.
A relation is a table with columns and rows.
and
relationship is association between relations/tables
for example employee table has relation in branch its called relationship between employee table and branch table
What is the real difference between one-to-many and many-to-one relationship? It is only reversed, kind of?
I can't find any 'good-and-easy-to-understand' tutorial about this topic other than this one: SQL for Beginners: Part 3 - Database Relationships
Yes, it is vice versa. It depends on which side of the relationship the entity is present on.
For example, if one department can employ several employees then department to employee is a one-to-many relationship (1 department employs many employees), while employee to department relationship is many-to-one (many employees work in one department).
More info on the relationship types:
Database Relationships - IBM DB2 documentation
From this page about Database Terminology
Most relations between tables are one-to-many.
Example:
One area can be the habitat of many readers.
One reader can have many subscriptions.
One newspaper can have many subscriptions.
A Many to One relation is the same as one-to-many, but from a different viewpoint.
Many readers live in one area.
Many subscriptions can be of one and the same reader.
Many subscriptions are for one and the same newspaper.
What is the real difference between one-to-many and many-to-one relationship?
There are conceptual differences between these terms that should help you visualize the data and also possible differences in the generated schema that should be fully understood. Mostly the difference is one of perspective though.
In a one-to-many relationship, the local table has one row that may be associated with many rows in another table. In the example from SQL for beginners, one Customer may be associated to many Orders.
In the opposite many-to-one relationship, the local table may have many rows that are associated with one row in another table. In our example, many Orders may be associated to one Customer. This conceptual difference is important for mental representation.
In addition, the schema which supports the relationship may be represented differently in the Customer and Order tables. For example, if the customer has columns id and name:
id,name
1,Bill Smith
2,Jim Kenshaw
Then for a Order to be associated with a Customer, many SQL implementations add to the Order table a column which stores the id of the associated Customer (in this schema customer_id:
id,date,amount,customer_id
10,20160620,12.34,1
11,20160620,7.58,1
12,20160621,158.01,2
In the above data rows, if we look at the customer_id id column, we see that Bill Smith (customer-id #1) has 2 orders associated with him: one for $12.34 and one for $7.58. Jim Kenshaw (customer-id #2) has only 1 order for $158.01.
What is important to realize is that typically the one-to-many relationship doesn't actually add any columns to the table that is the "one". The Customer has no extra columns which describe the relationship with Order. In fact the Customer might also have a one-to-many relationship with ShippingAddress and SalesCall tables and yet have no additional columns added to the Customer table.
However, for a many-to-one relationship to be described, often an id column is added to the "many" table which is a foreign-key to the "one" table -- in this case a customer_id column is added to the Order. To associated order #10 for $12.34 to Bill Smith, we assign the customer_id column to Bill Smith's id 1.
However, it is also possible for there to be another table that describes the Customer and Order relationship, so that no additional fields need to be added to the Order table. Instead of adding a customer_id field to the Order table, there could be Customer_Order table that contains keys for both the Customer and Order.
customer_id,order_id
1,10
1,11
2,12
In this case, the one-to-many and many-to-one is all conceptual since there are no schema changes between them. Which mechanism depends on your schema and SQL implementation.
Hope this helps.
SQL
In SQL, there is only one kind of relationship, it is called a Reference. (Your front end may do helpful or confusing things [such as in some of the Answers], but that is a different story.)
A Foreign Key in one table (the referencing table)
References
a Primary Key in another table (the referenced table)
In SQL terms, Bar references Foo
Not the other way around
CREATE TABLE Foo (
Foo CHAR(10) NOT NULL, -- primary key
Name CHAR(30) NOT NULL
CONSTRAINT PK -- constraint name
PRIMARY KEY (Foo) -- pk
)
CREATE TABLE Bar (
Bar CHAR(10) NOT NULL, -- primary key
Foo CHAR(10) NOT NULL, -- foreign key to Foo
Name CHAR(30) NOT NULL
CONSTRAINT PK -- constraint name
PRIMARY KEY (Bar), -- pk
CONSTRAINT Foo_HasMany_Bars -- constraint name
FOREIGN KEY (Foo) -- fk in (this) referencing table
REFERENCES Foo(Foo) -- pk in referenced table
)
Since Foo.Foo is a Primary Key, it is unique, there is only one row for any given value of Foo
Since Bar.Foo is a Reference, a Foreign Key, and there is no unique index on it, there can be many rows for any given value of Foo
Therefore the relation Foo::Bar is one-to-many
Now you can perceive (look at) the relation the other way around, Bar::Foo is many-to-one
But do not let that confuse you: for any one Bar row, there is just one Foo row that it References
In SQL, that is all we have. That is all that is necessary.
What is the real difference between one to many and many to one relationship?
There is only one relation, therefore there is no difference. Perception (from one "end" or the other "end") or reading it backwards, does not change the relation.
Cardinality
Cardinality is declared first in the data model, which means Logical and Physical (the intent), and then in the implementation (the intent realised).
One to zero-to-many
In SQL that (the above) is all that is required.
One to one-to-many
You need a Transaction to enforce the one in the Referencing table.
One to zero-to-one
You need in Bar:
CONSTRAINT AK -- constraint name
UNIQUE (Foo) -- unique column, which makes it an Alternate Key
One to one
You need a Transaction to enforce the one in the Referencing table.
Many-to-Many
There is no such thing at the Physical level (recall, there is only one type of relation in SQL).
At the early Logical levels during the modelling exercise, it is convenient to draw such a relation. Before the model gets close to implementation, it had better be elevated to using only things that can exist. Such a relation is resolved by implementing an Associative Table at the physical [DDL] level.
There is no difference. It's just a matter of language and preference as to which way round you state the relationship.
Answer to your first question is : both are similar,
Answer to your second question is: one-to-many --> a MAN(MAN table) may have more than one wife(WOMEN table) many-to-one --> more than one women have married one MAN.
Now if you want to relate this relation with two tables MAN and WOMEN, one MAN table row may have many relations with rows in the WOMEN table. hope it clear.
One-to-Many and Many-to-One are similar in Multiplicity but not Aspect (i.e. Directionality).
The mapping of Associations between entity classes and the Relationships between tables. There are two categories of Relationships:
Multiplicity (ER term: cardinality)
One-to-one relationships (abbreviated 1:1): Example Husband and Wife
One-to-Many relationships (abbreviated 1:N): Example Mother and Children
Many-to-Many relationships (abbreviated M:N): Example Student and Subject
Directionality : Not affect on mapping but makes difference on how we can access data.
Uni-directional relationships: A relationship field or property that refers to the other entity.
Bi-directional relationships: Each entity has a relationship field or property that refers to the other entity.
This is an excellent question, according to my experience, in ERD diagrams and relational databases direction is implied. In RDBMS you always define Many-To->One (trivial case One-To->One) relationships. The Many side of the relationship, a.k.a children, references the One side, a.k.a parent and you implement this with a Foreign Key constraint. Technically speaking you have to access an index, fetch the Primary Key record of the One side and then visit this record to get more information.
You cannot do this the other way around unless we are speaking about Object-Relational DBMS such as Postgres, Intersystems Cache, etc. These DBMS allow you to define a bi-directional relationship between the two entities (tables). In that case accessing records the other way around, i.e. One--To-->Many is achieved by using an array of references (children). In ORMs you have classes that reference each other the same way we described here.
WARNING: Most RDBMS in the IT market are NOT relational database management systems in the strict sense, think about null values, duplicate records etc, many of these allowed features break the definition of what a Relation is.
There's no practical difference. Just use the relationship which makes the most sense given the way you see your problem as Devendra illustrated.
One-to-many and Many-to-one relationship is talking about the same logical relationship, eg an Owner may have many Homes, but a Home can only have one Owner.
So in this example Owner is the One, and Homes are the Many.
Each Home always has an owner_id (eg the Foreign Key) as an extra column.
The difference in implementation between these two, is which table defines the relationship.
In One-to-Many, the Owner is where the relationship is defined. Eg, owner1.homes lists all the homes with owner1's owner_id
In Many-to-One, the Home is where the relationship is defined. Eg, home1.owner lists owner1's owner_id.
I dont actually know in what instance you would implement the many-to-one arrangement, because it seems a bit redundant as you already know the owner_id. Perhaps its related to cleanness of deletions and changes.
---One to Many--- A Parent can have two or more children.
---Many to one--- Those 3 children can have a single Parent
Both are similar. This can be used according to the need. If you want to find children for a particular parent, then you can go with One-To-Many. Or else, want to find parents for twins, you may go with Many-To-One.
The easiest explanation I can give for this relationship is by piggybacking on evendra D. Chavan'sanswer.
Using the department and employee relationship
A department can have multiple employees, so from the employee side, it's one-to-many relationship, and from the department side it's many-to-one relationship
But if an employee can also belong to more than one department, we can also say from the employee side it's now many as opposed to one, so the relationship becomes many-to-many
In order words, a simple understanding would be, we can state that a relationship is many-to-many if one-to-many can be viewed from both sides
that is if;
one employee can belong to many departments (one-to-many)
one department can have many employees (one-to-many)
I am new to SQL and only have experience using SQLAlchemy. The documentation on relationships in SQLAlchemy does a good job explaining this, in my opinion.
You may find some clarity by reading this part
Also, I had to come up with my own example to think through this. I'll try to explain without writing a bunch of code for simplicity.
table Vehicle
column (name)
table Manufacturer
column (name)
A Vehicle can only have One manufacturer (Ford, Tesla, BMW etc.)
Manufacturers can make many Vehicles
Ford
Ford makes Mustang
Ford makes F-150
Ford makes Focus
Tesla
Tesla makes Model S
Tesla makes Model X
Tesla makes Roadster
When looking at the database rows you will want to decide if you want a column that references the other side of the relationship. This is where the SQLAlchemy documentation brings in the difference between backref vs. back_populates. I understand that is the difference between having a column in the table to reference the other side of the relationship or not having a column to reference the other side.
I hope this helps, and even more so, I hope I am accurate in the way I learned and understand this.
I have read most of the answer. The problem is not the relationship here at all. If you look at One to Many or Many to One conceptually, it is just a reversible relationship. HOWEVER, while implementing the concept in your software or application it differs a lot.
In case of Many to One, we often desire the table that has Many aspect to be written first and we desire it to associate with the table containing One aspect. If you convert Many to One concept into One to Many, you will have hard time writing the One aspect table first in your code. Since, the relationship is defined while you engineer the database, Many aspect table will seek for the One aspect table data for integrity. So if you are planning to do it by using foreign key -> unique key or foreign key -> primary key, Many to One implementation will be different even if you consider it as a One to Many.
I personally make associations without using actual relationship concepts in many cases. There is no such boundaries as to use Database concept to form relationship every time. Just make sure that your database integrity is maintained as you want, it is indexed properly for your search needs and is decently normalized.
one-to-many has parent class contains n number of childrens so it is a collection mapping.
many-to-one has n number of childrens contains one parent so it is a object mapping
I'm reading Pro JPA 2. The book talks begins by talking about ORM in the first few pages.
It talks about mapping a single Java class named Employee with the following instance variables - id,name,startDate, salary.
It then goes on to the issue of how this class can be represented in a relational database and suggests the following scheme.
table A: emp
id - primary key
startDate
table B: emp_sal
id - primary key in this table, which is also a foreign key referencing the 'id' column in table A.
It thus seems to suggest that persisting an Employee instance to the database would require operations on two(multiple) tables.
Should the Employee class have an instance variable 'salary' in the first place?
I think it should possibly belong to a separate class (Class salary maybe?) representing salary and thus the example doesn't seem very intuitive.
What am I missing here?
First, the author explains that there are multiples ways to represent a class in a database: sometimes the mapping of a class to a table is straightforward, sometimes you don't have a direct correspondence between attributes and columns, sometimes a single class is represented by multiples tables:
In scenario (C), the EMP table has
been split so that the salary
information is stored in a separate
EMP_SAL table. This allows the
database administrator to restrict
SELECT access on salary information to
those users who genuinely require it.
With such a mapping, even a single
store operation for the Employee class
now requires inserts or updates to two
different tables.
So even storing the data from a single class in a database can be a challenging exercise.
Then, he describes how relationships are different. At the object level model, you traverse objects via their relations. At the relational model level, you use foreign keys and joins (sometimes via a join table that doesn't even exist at the object model level).
Inheritance is another "problem" and can be "simulated" in various ways at the relational model level: you can map an entire hierarchy into a single table, you can map each concrete class to its own table, you can map each class to its own table.
In other words, there is no direct and unique correspondence between an object model and a relational model. Both rely on different paradigms and the fit is not perfect. The difference between both is known as the impedance mismatch, which is something ORM have to deal with (allowing the mapping between an object model and the many possible representations in a relation model). And this is what the whole section you're reading is about. This is also what you missed :)