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.

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?

Tables customers(id, name) and orders(id, customer_id, total).

Compute each customer's total spend, then return only the customers whose spend is strictly greater than the average spend across all customers who