1.CASE WHEN. Shows up all the time.

2. Self joins. Common in product.

3. DISTINCT and GROUP BY

4. Left vs outer joins.

5. UNION. Rarely discussed but frequent.

6. SUM and COUNT

7. Date-time manipulation

8. String formatting, substring

9. Window functions like rank and row

10. Subqueries

11. HAVING vS WHERE

12. LAG and LEAD

13. Understanding indexing

14. Running totals

15. MAX and MIN

16. Using SUM CASE WHEN to count

17. COUNT DISTINCT

18. How to debug a query.

19. How to speed up a query.

20. Rank and dense rank

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