SQL50

50 SQL questions (SQL50 - https://leetcode.com/studyplan/top-sql-50/) - Leetcode (For Data and Analytics Engineering)

All Solutions in PostgreSQL

Recyclable and low fat products

Table: Products

Column Name	Type
product_id	int
recyclable	enum
low_fats	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.
Write a solution to find the ids of products that are both low fat and recyclable.
Solution: select product_id from products where low_fats='Y' and recyclable='Y';

Find Customer Referee

Table: Customer

Column Name	Type
id	int
name	varchar
referee_id	int

In SQL, id is the primary key column for this table.
Each row of this table 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 not referred by the customer with id = 2.
Return the result table in any order.
The result format is in the following example.
Example 1:
Input:
Customer table:

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:

select name from customer where referee_id != 2 or referee_id is null

Article Views

Table: Views

Column Name	Type
article_id	int
author_id	int
viewer_id	int
view_date	date

There is no primary key (column with unique values) for this table, the table may have duplicate rows. Each row of this table indicates that some viewer viewed an article (written by some author) on some date. Note that equal author_id and viewer_id indicate the same person.

Write a solution to find all the authors that viewed at least one of their own articles.
Return the result table sorted by id in ascending order.
Solution:

select distinct derived_views.author_id as id
from (select author_id, viewer_id, count(*) as view_count 
      from views where author_id = viewer_id 
      group by author_id, viewer_id) derived_views 
where derived_views.view_count >= 1 order by derived_views.author_id;

Invalid Tweets

Table: Tweets

Column Name	Type
tweet_id	int
content	varchar

tweet_id is the primary key (column with unique values) for this table.
This table contains all the tweets in a social media app.
Write a solution to find the IDs of the invalid tweets. The tweet is invalid if the number of characters used in the content of the tweet is strictly greater than 15.
Solution: Solution