SQL Server (T-SQL) interview questions

Table sales(id, region, amount).

For every region that appears in the table, return how many sales it has and the sum of their amounts. Columns: region, sale_count,

Table page_views(id, page, visitor). The same visitor can view the same page many times, and visitor is NULL when the view came from an anonymous session.

Table exam_results(id, student, subject, score). Every student registered for a subject has a row; score is NULL when the student did not actually sit the exam.

Table tickets(id, team, status, minutes_spent) where status is either 'open' or 'closed'.

Produce one row per team with three numbers side by side: how many

What is the difference between WHERE and HAVING? Can a query need both at the same time?

Explain the difference between COUNT(*), COUNT(column) and COUNT(DISTINCT column). Which of them can return a smaller number than the others, and why?