Note: this article is published on 06/12/2024.
This series of articles is about relationship between entity or datasets.
A - Introduction
A relationship in DBMS is the way in which two or more data sets are linked. This article will give a conceptual discussion about this topic. The content of this article:
- A - Introduction
- B - Types of Relationsip in DBMS
- C - Implementations in database
B - Types of Relationsip in DBMS
In a relational database, the data in one table is related to the data in other tables. In general, tables can be related in one of three different ways:
- One-to-One --- A row in a table is related to only one role in another table and vice versa.
- One-to-Many --- A row from one table can have multiple matching rows in another table.
- Many-to-Many --- A row from one table can have multiple matching rows in another table and vice versa.
Demo
One to One Relationship:
In restaurant, each customer (Table A) has one and only one order (Table B):
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One to Many Relationship:
Each Customer (Table A) could have multiple Orders (Table B)
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Many to Many Relationship:
Each employee could have multiple Project assigned,
Each Project chould be worked by multiple employees.
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C - Implementations in database
- One to One Relationship: Enforced by Primary key vs. Unique foreign key constraints
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- One to Many Relationship: Enforced by Primary key vs. Foreign key (not unique) relationship
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- Many to Mane Relationship: Enforced by a junction or bridging table as a connection for the two tables.
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Linked by a Middle Connection Table
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References