Meaning of n m and 1 n in database design

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In database design what do n m and 1 n mean    Does it have anything to do with keys or relationships

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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

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Many to Many  n m  One to Many  1 n

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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

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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

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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

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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

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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

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m n refers to many to many relationship where as 1 n means one to many relationship forexample  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 its a 1 N relationship

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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

[mysql] Meaning of "n:m" and "1:n" in database design

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