From 9fba6d236d28e79560f8c30d68bedbaf71af63fe Mon Sep 17 00:00:00 2001 From: Jet Hughes Date: Thu, 22 Sep 2022 11:09:01 +1200 Subject: [PATCH] vault backup: 2022-09-22 11:09:01 --- content/notes/13-databases-1.md | 20 +++++++++++++++++--- 1 file changed, 17 insertions(+), 3 deletions(-) diff --git a/content/notes/13-databases-1.md b/content/notes/13-databases-1.md index 0541401b7..85cf88c38 100644 --- a/content/notes/13-databases-1.md +++ b/content/notes/13-databases-1.md @@ -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) +