Add all my notes

imperfect_notes/pragy/LLD 6.md

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+LLD 6
+-----
+
+BookMyShow
+----------
+
+```python
+class CinemaBrand:
+    name: str
+
+class CinemaCenter:
+    address: str
+
+'Brand 1 <>---- * Center'
+
+class CinemaHall:
+    hall_number: int
+    number_of_seats: int
+    
+'Center 1 <>---- * Hall'
+
+# show seat is different from phycial Hall seat
+class HallSeat:
+    location: Location
+    type: str # delux / normal / ..
+    # can you book a physical seat?
+    # note: don't have the price of chair here
+    # note: don't have is_reserved here
+
+'Hall 1 <>---- * HallSeat'
+    
+class Movie:
+    name: str
+    director: str
+    description: str
+    language: str
+    
+class Show:
+    created_on: Date
+    start_time: Time
+    end_time: Time
+    
+'Movie 1 <>----- * Show'
+'Hall 1 ---- * Show'
+
+class Booking:
+    number: str
+    number_of_seats: int
+    
+    def cancel():
+
+'Booking * ----<> 1 Show'
+
+class ShowSeat:
+    seat_number: int
+    is_reserved: bool
+
+'Booking 1 ----- * ShowSeat'
+'HallSeat 1 <>----- * ShowSeat'
+'Show 1 <>---- * ShowSeat '  # can have. not strictly needed. Depends on usecase
+
+
+class Payment:
+    amount: float
+    trans_id: str
+
+class Coupon:
+    id: int
+    amount: float
+    expiry: Date
+    
+'Payment 1 ----- 0-1 Coupon'
+'Payment 1 ----- Booking'
+    
+class Cash(Payment):
+class Card(Payment):
+'''
+Cash <--ext-- Payment
+Card <--ext-- Payment
+'''
+
+# first show without inheritance
+    
+class Customer(Person):
+    make_booking()
+
+'Customer 1 ----> Booking'
+
+class Admin(Person):
+    add_movie()
+    add_show()
+    block_show()
+
+'Admin 1 ---- * Show'
+'Admin 1 ---- * Movie'
+
+class FrontDesk(Person):
+'FrontDesk 1 ---> * Booking'
+
+class Person:
+    name: str
+    address: str
+    email: str
+    
+'''
+FrontDesk <--ext-- Person
+Admin <--ext-- Person
+Customer <--ext-- Person
+'''
+
+class Account:
+    id
+    password
+
+'Account 1 ----<> 1 Person'
+
+class Guest:
+    # note: doesn't extend person
+    register()
+
+<<interface>> interface Search:
+    by_name()
+    by_language()
+    
+'Person ---uses--> search'
+'Guest ---uses--> search'
+
+class Catalogue <implements> Search:
+    movie_titles: {name -> list of movies}
+    language_titles: {lang -> list of movies}
+    by_name()
+    by_language()
+```
+
+-- --
+
+Schema Design
+-------------
+
+
+- store everything in one table
+- yay \o/
+- is this good design?
+why store into multiple tables?
+-------------------------------
+
+Anomalies:
+- college name, rank. Student name. Branch.
+- **insert anomaly:** null values when data is not known
+- **delete anomaly:** deleting a student causes loss of info about a particular college's rank
+- **update anomaly:** updating the rank of a college needs updation of thousands of rows
+
+
+- Map each entity to a table
+    - Attributes of entity become the columns of the table
+    - Define a primary key (could be a single column or a group of column)
+- Create a table for each relationship
+    - Think about if the relationship needs to have some properties or not:
+        - entities: students, course
+        - relationship: enrolled
+        - relationship attributes: enrollment date, marks, type (elective/audit/core)
+        - add these attribs to the relationship table
+    - The primary key of the relationship table will be the group of primary key of each of the participants in the relationship. (ex, (user_id, course_id))
+    - Create foreign key constraints: user_id, course_id
+
+
+Normalizations
+--------------
+
+- 1NF: data should be atomic. Don't store lists of things. Instead, make multiple rows.
+    - example: languages a student knows.
+    - now, I need to decide an id, because the previous id might not be able to identify a unique column
+- 2NF, 3NF, BCNF: we won't go over them. More in DBMS class
+    - follow logical thinking, and you will automatically get proper normalization
+
+Over Normalization?
+-------------------
+
+- n+1 problem
+- need to join the tables to get back the data anyway
+
+
+-- --
+
+- How to do composition in DB?
+    - depends. Cascading deleted turned on or not?
+    - depends on the DB. Different DBs require different things
+        - some will automatically delete references for you
+        - some will not
+    - safe way: setup your DB to not do it automatically. Do it yourself in code
+        - delete references first
+        - then delete row
+- How to do auto increment in DB?
+- null / unique / primary / foreign
+    ```sql
+    CREATE TABLE Persons (
+    ID int NOT NULL AUTO_INCREMENT,
+    UNIQUE (ID),
+    PRIMARY KEY (ID)
+    );
+    
+    CREATE TABLE Objects (
+    ID int NOT NULL AUTO_INCREMENT,
+    UNIQUE(ID),
+    PRIMARY KEY(ID)
+    );
+    
+    CREATE TABLE PersonObjects (
+        PersonID int,
+        ObjectID int,
+        FOREIGN KEY (PersonID) REFERENCES Persons(ID),
+        FOREIGN KEY (ObjectID) REFERENCES Objects(ID),
+        PRIMARY KEY (PersonID, ObjectID)
+    )
+
+    ```
+    
+
+How to do extension? Just create another table
+Usually not enforced in DB
+