Question 1

What is the difference between WHERE and HAVING in SQL?

Accepted Answer

WHERE filters individual rows before any grouping happens. HAVING filters groups after aggregation, so it can use functions such as COUNT or SUM. For example, WHERE can keep only orders from 2026. HAVING can then keep only customers whose total order value is above a threshold. A query can use both in the same statement.

Question 2

What is the difference between an INNER JOIN and a LEFT JOIN?

Accepted Answer

An INNER JOIN returns only the rows that match in both tables. A LEFT JOIN returns every row from the left table, and fills in NULL for the right-table columns where there is no match. Use an INNER JOIN when you only want matched records. Use a LEFT JOIN to find things with no match, such as customers who never placed an order, by filtering for NULL on the right side.