How much time does it take to learn SQL since it is required everywhere for data science jobs.

1-2 complete weeks to learn SQL.

10 places to start your SQL journey :

1. Zachary Thomas' SQL Questions 
2. Select * SQL: https://selectstarsql.com/
3. Leetcode
4. LinkedIn Learning
5. Window Functions
6. HackerRank
7. W3 Schools
8. CodeAcademy
9. SQLZoo: https://sqlzoo.net/
10. SQL Bolt: https://sqlbolt.com/

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Datalemur by far the best

Which is the comprehensive source for leaning?

Hardly 1 month