1. English for star schema, snowflake schema, and constellation schema
star schema
snowflake schema snowflake schema
constellation schema
2. Three modes of image memory
↓Take Pai Daxing as a fact table
. The star schema is like Pai Daxing in SpongeBob SquarePants. It has multiple hands, and each hand holds a person. Notice! Only one Patrick (There is only one fact table.)
Snowflake mode, a Patrick, it can only use its right hand, its right hand is holding SpongeBob SquarePants, SpongeBob's right hand is holding Squidward, and Squidward's right is holding Boss Krabs.
Query the fields of a certain dimension table through the fact table in snowflake mode: similar: one day, you want to find a book, and this book is actually in the hands of Mr. Crab, but you don’t know it, you only know a password, and use The password is connected to Pai Daxing, and Pai Daxing will tell you how to find it. So you took the code to find Pai Daxing, and Pai Daxing gave you another code to find SpongeBob SquarePants. SpongeBob saw Pai Daxing's code and gave you a new code to find him. His friend Octopus, Brother Octopus saw SpongeBob SquarePants' password, and gave you another password, asking you to take it to Boss Crab, and finally you took Brother Octopus's password to go to Boss Crab, and Boss Crab gave you the book . During this process, you can't directly go to Boss Crab with the code given by Pai Daxing, but you can only find his friend SpongeBob SquarePants, because only the two of them can understand that code.
Constellation mode, there are two or more Patricks. (Extended from the star schema.) (Attention! There are two or more Paida stars, that is, there are two or more fact tables) (What does the constellation schema look like a constellation, not at all )
Imagine, if it is Pai Daxing mode, Xiao Ming wants to find someone, Xiao Ming can just ask Pai Daxing, so the search efficiency is high. If it is the snowflake mode, Xiao Ming wants to find someone, but he can only find someone through you, he needs to find your friend through you first, then your friend finds her friend, and her friend finds her friend again. . Search efficiency is not that high.
Non-human language version:
star schema: a fact table is connected to multiple dimension tables
Snowflake schema: a fact table is connected to a dimension table, and the dimension table is connected to other dimension tables . (The fact table can be connected with multiple dimension tables, the point is that the dimension table can be connected with other dimension tables)
3. Introductions written by other people who feel good
https://www.databricks.com/glossary/star-schema
4. CDA level 1 related topics
