SQL Aggregation problems

11 problems, easiest first.

Dates 16 Window Functions 13 Null Handling 11 Aggregation 11 Group By 8 Joins 7 Case When 6 Sorting 6 Strings 5 Subqueries 5 Select Filter 4 Running Totals 4 Ctes 3 Recursive Cte 3 Set Operations 2 Ranking 2 Deduplication 2 Self Join 2 Gaps And Islands 2