1. Learn how to join tables
2. Filtering data using the where clause
3. Computational data using aggregation function
4. use subqueries to handle more complex scenarios
5. Simplify complex queries by breaking them into smaller logical steps using CTE
6. Handle null values
7. Learn some window functions (ROW_NUMBER() to avoid duplication
8. know some SQL built-in function like declare, date, etc
9. The most important is to understand the business
10. etc
https://stackoverflow.com/questions/379062/best-way-to-understand-complex-sql-statements
learn from this if you want
