database design - Can a 2NF relation be M:M? - Database Administrators Stack Exchange
Amanda, we have two responses for you: Dear Amanda, One mm is one " millimeter" or one one-thousandth of a meter (1 mm = 1/ m). One cm is one. As a specialist consultant with extensive experience in SAP ERP implementation projects, I constantly face challenges in desing Gaps or. The normal forms don't really have anything to do with many-to-many relationships. If you happen to lose some as a byproduct of the.
If you happen to lose some as a byproduct of the normalization process, that's fine, but you won't generally do so. If we consider that we have two tables: Salesman and Product which each have ID fields as their primary key and we have a third table called Specializes which shows which Salesmen specialize in selling which products. This Specializes table would represent the many-to-many relationship since each salesman can specialize in multiple products and each product can be specialized in by more than one salesmen.
It would probably look something like this excuse the awkward formatting, we can't do real tables on StackExchange: SalesmanID ProductID 1 1 1 2 2 1 2 2 2 3 Obviously, the lack of nulls and repeated rows means that this table is in 1NF. It also contains no non-trivial functional dependencies.
Data Modeling and Entity Relationship Diagram (ERD)
So really, no normal form removes many-to-many relationships, nor are they meant to. The purpose of normal forms is not to remove many-to-many relationships and I am not actually clear on why you would want to but rather to remove potential insert anomalies, update anomalies, and deletion anomalies. The primary role of the normal forms is to ensure that each piece of information is represented in a database table precisely once.
Having the same information embedded in multiple places leads to problems. But that has nothing to do with many-to-many relationships. I think that you mean to be asking a slightly different question about something like situations where a many-to-many relationship is embedded in a table which also tries to contain other information, such as if the table above contained the product name in addition to the product number where name is functionally determined by number.
A table like that would either violate 2NF if the name did not also functionally determine the number or Boyce Codd Normal Form if the name did functionally determine the number. You could also perhaps be thinking of a different situation: M relationships in the same table, such as if we were to add a third column to identify which language or languages each salesman speaks.
SalesmanID ProductID Language 1 1 English 1 2 English 2 1 Spanish 2 2 Spanish 2 3 Spanish 2 1 French 2 2 French 2 3 French As you can see, that table is quite problematic, since we need 6 entries to express that Salesman 2 specializes in 3 products and speaks 2 languages.
A Relationship (basic) of MM and FI tables
This is a fourth normal form violation. Verbs often describe relationships between entities. We will use Crow's Foot Symbols to represent the relationships. Three types of relationships are discussed in this lab. If you read or hear cardinality ratios, it also refers to types of relationships.
Converting mm to cm - Math Central
One to One Relationship 1: Each student fills one seat and one seat is assigned to only one student. Each professor has one office space. One to Many Relationship 1: M A single entity instance in one entity class parent is related to multiple entity instances in another entity class child For example: One instructor can teach many courses, but one course can only be taught by one instructor. One instructor may teach many students in one class, but all the students have one instructor for that class.
Many to Many Relationship M: M Each entity instance in one entity class is related to multiple entity instances in another entity class; and vice versa.
Each student can take many classes, and each class can be taken by many students. Each consumer can buy many products, and each product can be bought by many consumers.
- Data Modeling and Entity Relationship Diagram (ERD)
The detailed Crow's Foot Relationship symbols can be found here. Crow's Foot Relationship Symbols Many to many relationships are difficult to represent.MM: Initiating Relationship and Connection
We need to decompose a many to many M: M relationship into two one-to-many 1: Attributes Attributes are facts or description of entities. They are also often nouns and become the columns of the table. For example, for entity student, the attributes can be first name, last name, email, address and phone numbers.
For example, for a student entity, student number is the primary key since no two students have the same student number. We can have only one primary key in a table. It identify uniquely every row and it cannot be null.