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
