sql SOLO CHALLENGE的解决方案：建模一对多数据库

Posted 2021-05-14

*Solution for Challenge: SOLO CHALLENGE: Modeling a One to Many Database. Started 2014-02-10T06:32:23+00:00*<br />
![schema](http://goo.gl/oMLubf)

###Object Model
####One to Many Relationship: Users & Tweets
Each *user* posts **1 to many** *tweets* (technically, *0* to many, right? I'll go with 1 for consistency throughout)<br />
Each *tweet* originates from **one to one** *user*<br />

Since only one direction has a many potentential, this is a 1 to many relationship between users and tweets.<br />  


####Many to Many Relationship: Users & Followers
Each *user* has **one two many** *followers*<br />
Each *follower* follows **one to many** *users*<br />

Since a user follows many users and is followed by many users, this is a many to many relationship. However, I think this is a 
special case of a many to many relationship because it is between two of the same object type (that is, a follower *is a* user). Because of this, it requires only two tables and separate records invert the `user_id`/`follower_id` foreign keys when there is a bidirectional follow relationship between two users.  

###Reflection
I have experience with relational databases and the relational model. While that was helpful, I think that experience actually tripped me up a bit in trying to mentally map my instinctive relational ER diagramming perspective to the object model diagram we depict in SQL designer. (I think that makes sense..?) I'm still working on a better understanding of how the two intersect and their role. 

I tried to further model the twitter **User** - **List** relationship but couldn't quite wrap my head around how to depict the complex subtype relationships between users and lists (**creator** *1..1*; **subscriber** *0..many*; **member** *1..many*). I started an ER diagram thinking that if I could successfully create the relational model then I could work on understanding the map to the object model. ![ER diagram](http://goo.gl/9ESPJo) Unfortunately, I got stuck on splitting the multiple many to manys between the same entities without creating traps and aborted for the time being. Hopefully these things will become clearer as we move forward!

-- all the tweets for a certain user id
SELECT tweet_id, tweet_text
FROM tweets
WHERE user_id = 123456678923402;

-- the tweets for a certain user id that were made after last Wednesday
SELECT tweet_id, tweet_text
FROM tweets
WHERE created_at >= date('2014-02-07') AND
      (user_id = 123456678923402);

-- all the tweets associated with a given user's twitter handle
SELECT t.tweet_id, t.tweet_text
FROM tweets t JOIN users u ON (t.user_id = u.user_id)
WHERE u.user_handle = 'carolineartz';

-- the username associated with a given tweet id
SELECT u.username
FROM users u JOIN tweets t ON (u.user_id = t.user_id)
WHERE t.tweet_id = 345673123456;

-- all followers and the followed date for a given user's twitter handle
SELECT f.follower_id, date(f.followed_at)
FROM user_followers f, users u
WHERE (f.user_id = u.user_id) AND
      (u.user_handle = 'carolineartz');