If we have two entities in an ER model.
Sport and Organization.
We have a relationship between these two. it is 1 to 1 relationship.
One sport belongs to one Org. one Org. handles one sport.
If i want to covert this to relations, I will have one Sport relation one ORG. relation.
How should i create a relation for the 1 to 1 relationship?
1 to many is easy as is many to many.
Passing a "1 to 1" relationship from ER model to Relational means that you have to create a relation putting a foreign key in either the first table or the second one. You can choose the table that makes more sense to have the foreign key and it will depend on the particular usage.
In these cases, I used to put the foreign key in the table that looks more concrete. For example you could have the foreign key in the sports table. If in the future the requirements changes (you relationship to "1 to N") and it is possible that an organisation could handle several sports, you do not have to change anything as you already have the foreign key in the sports table.
Related
I have some tables already created in my database and now I need to draw ER diagram for these tables.
Identified the primary key and foreign key between the
tables
Determined pk-fk relationship using the keys
Now I need to identify the cardinality between the tables. How do I
do this?? Please let me know if there is any set of rules which I
need to consider while evaluating 1:1, 1:M and M:M relationships.
Let me take an example of two tables where am struck at:
Table A has a composite key made of pid and identitytype.
Table B has a composite key made of pid and maritalid.
Table A and Table B are associated with each other using pid and pid
is not null in both the tables.
Let me know what could be the relationship type between Table A and Table B whether it is 1:1, 1:M or M:M. Also, please let me know the sequence of steps that you followed in arriving at type of relationship conclusion.
Thanks,
Dex.
Relationships in the entity-relationship model are very different from what you've got in mind. Relationships are not represented by foreign key constraints - that's the old network data model, and it's limited to binary relationships. The entity-relationship model represents n-ary relationships between entity sets in tables, not between tables.
Foreign key constraints restrict the values of a set of columns to be a subset of the values of another set of columns. They are effectively used to enforce entity sets (domains) - for example, to ensure that every person_id column is a subset of the one that represents all known persons in the system. FK constraints are only used during updates - you could delete all FKs from a database and your SELECT queries and JOINs would work exactly as before, further demonstrating that they don't represent relationships.
A relationship is an association among two or more entity sets, each represented by a suitable key. Relationship instances are always recorded in rows of a table. For example:
A 1:1 relationship between a driver's license and a person would be represented by having the license key and person key together as two columns of a table, and both (separately) uniquely constrained. Whether this is in a license table, a person table, or a separate driver's license table is an implementation detail.
A 1:N relationship between cars and their owners would be represented by having the car key and person key together as two columns of a table, and the entity set on the many side uniquely constrained. This is often implemented in the table recording the attributes of the entity on the many side.
The preceding relationships are simple enough that we can denormalize them and don't need to record the relationship in a separate table. For higher relationships, though, we need separate tables:
An M:N relationship between students and subjects would be represented by having the student key and the subject key together as two columns of a table, and the combination of the two uniquely constrained.
An M:N:P relationship between suppliers, products and regions would be represented by having the supplier key, product key and region key together as three columns of a table, and the combination of the three uniquely constrained.
An M:N:1 relationship between regions, products and suppliers (a sole mandate) would be represented by having the region key, product key and supplier key together as three columns of a table, and the combination of region and product keys uniquely constrained.
See the pattern?
Every role/component of a relationship can have a foreign key constraint defined if the characteristic predicate of that entity set (its required attributes) is represented in a different table. That means a single n-ary relationship can require n different FK constraints.
To determine the cardinality of a relationship from an existing table:
Determine the entity sets represented in the table. Not all columns represent entity sets - some represent value sets, meaning the values have meaning themselves as labels or measures.
Determine which combination of entity sets are uniquely constrained together. These are the many sides of the relationship, and we'll give them variables like M, N, P, etc.
Every other entity set is dependent on the previous combination and represented by a 1 in cardinality.
It's not quite that simple. It's possible for a table's key to involve value sets in addition to entity sets. In these cases, we've got a weak entity/identifying relationship/subtyping situation. These are usually (but not always) 1:N relationships in which the child entity's key partially overlaps with the parent entity's key.
For more information, I recommend Peter Chen's paper The Entity-Relationship Model - Toward a Unified View of Data.
You should look at this SO question:
postgresql-describe-table
In reality, without looking at the schema definition, without a contra-positive example, you have no way of knowing if the table is a 1:1, 1:n, or n:m relation if n = m = 1.
You can do a scan, but it is the constraints that determine that relationship.
If you don't have that data, then you can only demonstrate 1:n and n:m with examples, but it cannot be proven that 1:1 is not n:m without the constraint definitions.
A 1:1 relationship will look like this:
PK - PK
this can only by one-to-(zero or one), as both tables can only have unique keys, 1:n and n:m are not allowed. This would have to be constrained by software on some level to ensure the PK = PK for the separate tables, or more usually, if you really want 1:1 the data is stored, normalized, in the same table. Otherwise, you need to ensure key coordination by a transactional insert, or whatnot. Auto-generated keys are not advised.
A 1:n relationship will look like this:
PK - FK
the FK (foreign key) defines it to be constrained to a primary key in another table, but can be in multiplicity.
An n:m relationship will look like this:
PK - FK|FK - PK
where there are three tables. Two normalized tables with primary keys (the PKs) and a joining table with FK relationships defining the mapping n:m between the tables.
Of course, all of this could be constrained by code using the database, and hence, the table constraints are the only reliable definition of the data schema.
Foreign Keys must point to Primary Keys in another table, so you can't have FK:FK relationships, and there really is no PK:PK relationship defined by the database. That has to be constrained by transactional insert via code. The usual convention is to store data that is PK:PK in the same table, per a normalized data format.
Okay, so, to add a comment, directed at tables A and B; all you can say for certain is you have primary keys consisting of pid:identitytype and pid:maritalid, and if that is the case, for the sake of discussion, say identitytype and maritalid are ints, then you have int:int and int:int, and it tells you nothing. if identitytype has overlap with maritalid, then there is no way to reliably tell them apart. If you are only going to match on pid, then you have an N:M relationship as you have pid:N-pid:M different possibilities which would lead to an N:M relationship.
I want to make an ER diagram involving a 1 to many relationship. Should I add the attribute of the relationship on the N side too? And if possible write the correct ER diagram.
Take a simple example of many people working in one department with a constraint that a single person can work only in one department. That makes department connected to people in a one-to-many fashion.
The simplest way to represent this is to add the primary key of you department table as a foreign key in your people table.
The schema will look something like this
DEPARTMENT (dept_no, dept_name, dept_location,....)
PEOPLE (p_no, dept_works, p_name, p_dob,....)
Where, dept_works in PEOPLE table is the foreign key that references the dept_no Primary key in the DEPARTMENTS table. This way you can store the ID of the department for each person in which he/she works.
I have a question about the ER diagram below.
Question: If there are 2 relationships born and live from actor to place entity.
Do they both have the same foreign key which is place_no? So does that mean I just put the foreign key in Actors relational model and that's it or do I have to make a table for Born?
Because an actor is born in a place and lives in a place... so how would it be possible to differentiate between 1 place_no FK?
You have to specify the arity of the relationships. You can map the conceptual 1:n relationships directly in relational model using just foreign keys. For n:m relationships you should use junction tables.
Since it seems that the relationships here are both n:1 since a person can be born in just one place and (based on your comments) we can also assume that a person can only live in one place at the time we can map these relationships as foreign keys.
As an example in SQL (assuming we have two relations actor and place, the code is in CREATE code of actor):
[...]
born int REFERENCES place(place_no),
lives int REFERENCES place(place_no),
[...]
When having a one-to-one relationship in a database the other table has a foreign key ID (in this example). And in a one-to-many relationship the table contains many foreign keys.
But does the database know whether this is a one-to-one or one-to-many relationship? Are the relationships that I make in an ER-Diagram only to indicate where there should be foreign keys when making the actual tables?
What is the difference between one-to-one and one-to-many relationship in a database?
In a sense, all the relationships we talk about are not known to the database, they are constructs we have invented to better understand how to design the tables.
The big difference in terms of table structure between one-to-one and one-to-many is that in one-to-one it is possible (but not necessary) to have a bidirectional relationship, meaning table A can have a foreign key into table B, and table B can have a foreign key into the associated record in table A. This is not possible with a one-to-many relationship.
One-to-one relationships associate one record in one table with a single record in the other table. One-to-many relationships associate one record in one table with many records in the other table.
To enable one-to-one relationship you need to add unique constraint to foreign key. It is not possible to have two foreign keys for each table as it will be impossible to create records.
Im having trouble understanding what the actual question is.
Your analysis is for the most part correct, in that if you have a 2 tables, and table2 has a foreign key to table one, it could be either a one-to-one or a many-to-one.
Your sentence "And in a one-to-many relationship the table contains many foreign keys."
The table of the 'many' side still contains one column that is a foreign key, its just that more than one row can have the same foreign key value (many rows point to one parent).
Also note that you can put the foreign key on the parent table, to the child, instead of the other way around. In this way, you can prevent one-to-many if you want to do that. Also note that in this way, more than one parent can share a child, which might or might not be what you want.
The database-level equivalent of a 1:1 vs. 1:m is having a unique index on the foreign key column. Note that this will only work for 1:1, NOT 1:0..1, as null is considered when evaluating uniqueness. There are workarounds for this restriction, but that's it at the basic level.
Similarly by example, a product has only one product code, so it's one-to-one relationship (product <-> ABC123), but a customer can purchase more than one product, so it's one-to-many relationship (person <->>>product).
well, you are right, this relation is important for you, but not for db itself. When you have two tables, one with your basic information, and another one with your detailed information.. for both tables you are you, so it is one-to-one relation, you can not map your data to somebody else.
Now add third table "cities" and one of your information points to city you live in - this is example of one-to-many (one city can be used, and should be used for many people).
one-to-many / one-to-one just show how your tables interact. And all the time, you want to "save" rows/columns in table not duplicating them you will use one-to-many relation with another table. Or many-to-many :)
Let's assume you have a table with two attributes A and B. If A is a candidate key and B is not then the relationship between A and B is 1 to many. If both A and B are candidate keys then the relationship is 1 to 1.
Given table A and B if
A and B have a strict 1 to 1 relationship
For every B instance, there will always be an A instance
The best approach is to make the primary key of B also a foreign key referencing A. This is also called "Table per Type Inheritance" and the "is a" relationship. There are other ways to enforce a unique foreign key, but using the primary key makes the relationship clear in the schema and in ER diagrams.
Of course there are always other scenarios, and if your design doesn't meet both of the criteria above, you'll have to use another approach.
I have a question, if two tables in database are from different entities then how we define relationship between them. i mean can we use some foreign key or any thing to define relationship between them. or we have to create a third table
It depends on what kind of relationship you have. If it´s a 1-1 or 1-N then you will only add foreign key column on the respective table. If you need a N-N (aka N-M) you will need a third table.
There is a design principle that states that a table either defines an entity or a relationship but not both. Therefore, use a third table to model the relationship between your two entities -- yes, even if it is 1-1 or 1-N -- noting that the relationship itself may have its own attributes.