**# ENTITY RELATIONSHIPS**

One - to - One
Many - to - One
Many - to - Many

**# PRIMARY / FOREIGN KEY**

PRIMARY KEY Meaning :
-- A PRIMARY KEY is the column or columns that contain values that 
-- uniquely identify each row in a table
-- A PRIMARY KEY can't be a Null

FOREIGN KEY Meaning :
-- A FOREIGN KEY is a field (or collection of fields) in one table, that 
-- refers to the PRIMARY KEY in another table.
-- It can be Null

**# PARENT TABLE (PRIMARY KEY) / CHILD TABLE (FOREIGN KEY)**

-- The referenced table is called the parent table while the table with the 
-- foreign key is called the child table.