Question 1

What are Aggregate Functions in SQL?

Accepted Answer

Aggregate functions take many rows of data and collapse them into a single summary value — COUNT counts rows, SUM adds values, AVG averages them, MAX finds the highest value, and MIN finds the lowest. They turn raw row-by-row data into business-ready summaries.

Question 2

What is COUNT(*) in SQL?

Accepted Answer

COUNT(*) counts every row returned by a query, regardless of whether any column in that row contains NULL. It is the standard way to answer 'how many rows/records are there?'

Question 3

What is the difference between COUNT(*) and COUNT(column)?

Accepted Answer

COUNT(*) counts all rows including ones with NULLs. COUNT(column) only counts rows where that specific column is NOT NULL, so it can return a smaller number than COUNT(*) if that column has missing values.

Question 4

Does AVG ignore NULL values?

Accepted Answer

Yes. AVG (like SUM, MAX, MIN, and COUNT(column)) ignores NULL values entirely — both in the total and in the row count used to divide, so NULLs do not get silently treated as zero.

Question 5

Can Aggregate Functions be used without GROUP BY?

Accepted Answer

Yes. Without GROUP BY, an aggregate function treats the entire result set as one single group and returns one summary row, such as SELECT COUNT(*) FROM employees; returning the total employee count in a single row.

Question 6

What is the difference between WHERE and HAVING?

Accepted Answer

WHERE filters individual rows before grouping and aggregation happen, and cannot reference an aggregate function. HAVING filters groups after aggregation, and is specifically designed to filter on aggregate results like AVG(salary) > 50000.

Question 7

Which Aggregate Function is most commonly used?

Accepted Answer

COUNT is generally the most commonly used aggregate function in real-world SQL, since counting rows, records, customers, or orders is one of the most frequent reporting needs, closely followed by SUM and AVG for revenue and performance metrics.

Question 8

Are Aggregate Functions important in SQL interviews?

Accepted Answer

Extremely — aggregate functions, especially COUNT(*) vs COUNT(column), GROUP BY vs HAVING, and NULL handling, are among the most frequently asked SQL interview topics for data analyst, BI, and backend roles.

Question 9

Can you use multiple Aggregate Functions in one query?

Accepted Answer

Yes — a single SELECT can combine COUNT, SUM, AVG, MAX, and MIN together, for example SELECT COUNT(*), SUM(salary), AVG(salary), MAX(salary), MIN(salary) FROM employees; returning one summary row with five aggregate values.

Question 10

Do Aggregate Functions work on text or string columns?

Accepted Answer

COUNT, MAX, and MIN work on text columns (MAX/MIN compare alphabetically), but SUM and AVG are numeric-only and will raise an error or behave unpredictably if applied to a non-numeric column.