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content/notes/13-databases-1.md
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@ -4,6 +4,9 @@ aliases:
tags:
- cosc203
- lecture
sr-due: 2022-09-25
sr-interval: 3
sr-ease: 250
---
# What is a database
@ -118,9 +121,20 @@ relationship types can have attributes
- marked by single line
- relationsihp is not required
## weak entities
- regular tyupes
![cardinality examples|300](https://i.imgur.com/DmHQaVc.png)
![participation example|300](https://i.imgur.com/hH0v4bR.png)
## weak entities
- regular types have key attributes
- weak types do not have their own key attributes
- they are identified by their relationship with another entity
- this creates and identifying relationship
- must have a total participation constraint
- the other entity type is called the identifying or owner entity type
- denoted by double lined rectable and double lined diamond
- have a partial key
- a set of attributes that uniquiely identify the weak entities that are related to the same owner entity
![weak entity example|300](https://i.imgur.com/4JMDcN7.png)