Exercises from Leetcode - Top SQL 50

Exercise # 1

Table: Products

Column Name	Type
product_id	int
low_fats	enum
recyclable	enum

product_id is the primary key (column with unique values) for this table. low_fats is an ENUM (category) of type ('Y', 'N') where 'Y' means this product is low fat and 'N' means it is not. recyclable is an ENUM (category) of types ('Y', 'N') where 'Y' means this product is recyclable and 'N' means it is not.

Question

Write a solution to find the ids of products that are both low fat and recyclable. Return the result table in any order. The result format is in the following example.

Example 1:

Input: Products table:

product_id	low_fats	recyclable
0	Y	N
1	Y	Y
2	N	Y
3	Y	Y
4	N	N

Output:

product_id
1
3

Solution # 1

SELECT product_id 
FROM Products 
WHERE
low_fats ='Y' 
AND
recyclable ='Y'

Explanation: Only products 1 and 3 are both low fat and recyclable.

Exercise # 2

Table: Customer

column	Data type
id	int
name	string
referee_id	int

In table, id is the primary key column for this table. Each document of this collection indicates the id of a customer, their name, and the id of the customer who referred them.

Find the names of the customer that are either:

1. referred by any customer with id != 2.
2. not referred by any customer.

Return the result in any order.

The result format is in the following example.

Input:

Table: Customer

id	name	referee_id
1	Will	null
2	Jane	null
3	Alex	2
4	Bill	null
5	Zack	1
6	Mark	2

Output:

name
Will
Jane
Bill
Zack

Solution # 2

SELECT name 
FROM Customer
WHERE referee_id != 2
OR
referee_id is null