Relational Data Modeling - Many-to-many Relationship (Two-way relationship)

Data System Architecture


A many-to-many relationship is a relationship (an m:n mapping) between two relation A and B in which :

  • A may contain a parent row for which there are many children in B
  • and vice versa.

The data warehouse world does not function with normalized fact tables since you need to start doing lots of select count(distinct) resulting in facts columns not being additive.

Many To Many Relationship


  • each project may have zero, one, or more employees assigned to it and each employee may be assigned to zero, one, or more projects.
  • relationship between users and groups:
    • Users can belong to a group,
    • and groups comprise a list of users.
  • a author has contributed to many book, a book is written by many authors
  • in a Sales database, each product belongs to one or more groups, and each group contains multiple products
  • in banking, a customer can have different accounts, and an account can belong to different customers.
  • in insurance, a customer (or household) can have different policies, but a policy can support multiple customers (or households). In many businesses, a parent customer can have subsidiaries.

When you need to model a many-to-many relationship in a Dimensional Data Modeling - Dimensional Schemas, you fall in a snowflake schema.

How to model a many-to-many relationship ?

Many To Many Sport User Interest

There are a few ways of implementing many-to-many relationships :

  • the Bridge Table,
  • the Boolean Column,
  • or the Multiple Column method

The Bridge Table Method

See the many-to-many section of the Association table (bridge,…)

Boolean Column Method

The boolean column method consists of creating a column for each possible value, like so:

Each boolean column can be a numeric (1 or 0), or character (Y or N).

Many To Many Sport User Interest Boolean

Multiple Column Method

The multiple column method consists of having columns for the number match the number of choices one can make. This has its limitations since it is tightly coupled to the application; but is easily transformed. The typical data model would look like:

Many To Many Sport User Interest Multiple Column

In this case, you could only have five sports even though you could have several dozen choices.

Boolean Method / Bridge

The Boolean Method is far superior in every category except the breakdown, and the Bridge is a close second. You have to use the Bridge method if the number of potential values in the dimension exceeds 100 columns or so. You can do it, but it looks ugly. The best of both worlds would be to merge the Boolean and Bridge column methods. Its fairly straight–forward: add the boolean columns to the bridge table as follows:

Many To Many Sport User Interest Boolean Bridge

How to avoid a bridge table?

There are several approaches:

  • Hide the many-to-many relationship. You can publish two versions of the schema: the full one for use by structured reporting and a handful of power users, and a version that eliminates the many-to-many relationship for use by more casual users.
  • Eliminate the many-to-many relationship by collapsing/aggregating multiple rows. Add a row to the many-to-many dimension table: “Multiple rows”. The fact table can then link directly with the dimension. As with all design decisions, the IT organization cannot choose this approach without consulting with the user community. For something which has limited information value, this approach may be quite acceptable.
  • Identify a single primary row (The top 1) It may be possible to identify a primary row, either based on some logic in the transaction system or by way of business rules.
  • Pivot out the many-to-many dimension (see below the Boolean Column Method). If the domain of the multi-choice space is small, you can eliminate the bridge table by creating a dimension table with one column for each choice.


Discover More
Bobj Chasm Trap
BOBJ - Chasm Traps (converging many to one joins)

Chasm Traps is see as a Join Path problem or data modeling issue (convergence type ) which returns more data than expected by returning a cartesian product. A chasm trap is a type of join path...
Star Schema
Dimensional Data Modeling - Fact Table

A fact table is a table that stores fact where a fact can be seen as the representation of an event. It's also known as: Transactional history Unalterable fact. (The content of this kind of table...
Star Schema
Dimensional Data Modeling - Relationship

In a dimensional data model, you have mostly a one-to-many relationship between the dimension and the fact table between each level in a hierarchy man-to-many relationship are modeled with a bridge...
Dimensional Data Modeling - Snowflake schema

A snowflake schema is a star schema with fully normalised (3NF) dimensions. It gets its name from that it has a similar shape than a snowflake. A snowflake is a dimensional model : in which a central...
Card Puncher Data Processing

is a suite of tools for web and mobile applications. Data is saved and queried in a tree structure. When you: fetch data at a location in your database, you also retrieve all of its child nodes....
Java Conceptuel Diagram
How to break a reference chain error in Jackson? (StackOverflowException error)

When trying to serialize a POJO with Jackson's ObjectMapper, you may get this StackOverflowException error: This article explains to you how you can resolve it. This error happens when you have...
Hello Data Model Many To Many
JPA - How to model a @Many-to-Many Relationship ?

How to model a many-to-many relationship in JPA. One Hello can have several Category and One Category can have several Hello. The Ddl File: [Ddl File] One sequence: The HelloWorld entity...
Bi Server Architecture With Client
OBIEE - How to model the different relationships ?

The relationships modeling in OBIEE. Star schemas and snowflake schemas work well for modeling a particular part of a business where there are one-to-many relationships between the dimension tables...
Many To Many Relationship
OBIEE 10G - How to define a Many-to-Many relationship with a Bridge table ?

Dimensional Schemas (Star and snowflake schemas) work well for modeling a particular part of a business where there are one-to-many relationships between the dimension tables and the fact tables. However,...
Obiee Bridge Schema
OBIEE 10G/11G - How to model a bridge table (Many-to-Many relationship) with the joins property of a Logical Table ?

Dimensional Schemas (Star and snowflake schemas) work well for modeling a particular part of a business where there are one-to-many relationships between the dimension tables and the fact tables. However,...

Share this page:
Follow us:
Task Runner