Aggregation & Grouping interview questions

1. A.COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
2. B.groups rows sharing a value so aggregates are computed per group
3. C.every selected non-aggregated column must also appear in the GROUP BY clause
4. D.filters groups after aggregation, whereas WHERE filters rows before it

Answer + AI explanation with Pro

1. A.COUNT / SUM / AVG
2. B.GROUP BY column rule
3. C.GROUP BY
4. D.COUNT(*) vs COUNT(col)

Answer + AI explanation with Pro

1. A.GROUP BY — filters groups after aggregation, whereas WHERE filters rows before it
2. B.GROUP BY — groups rows sharing a value so aggregates are computed per group
3. C.GROUP BY — COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
4. D.GROUP BY — returns the first non-NULL value from its arguments

Answer + AI explanation with Pro

1. A.groups rows sharing a value so aggregates are computed per group
2. B.filters groups after aggregation, whereas WHERE filters rows before it
3. C.aggregate functions returning row count, total, and mean over a group
4. D.COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column

Answer + AI explanation with Pro

1. A.COALESCE
2. B.COUNT(*) vs COUNT(col)
3. C.COUNT / SUM / AVG
4. D.HAVING

Answer + AI explanation with Pro

1. A.HAVING — aggregate functions returning row count, total, and mean over a group
2. B.HAVING — COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
3. C.HAVING — every selected non-aggregated column must also appear in the GROUP BY clause
4. D.HAVING — filters groups after aggregation, whereas WHERE filters rows before it

Answer + AI explanation with Pro

1. A.returns the first non-NULL value from its arguments
2. B.aggregate functions returning row count, total, and mean over a group
3. C.every selected non-aggregated column must also appear in the GROUP BY clause
4. D.groups rows sharing a value so aggregates are computed per group

Answer + AI explanation with Pro

1. A.GROUP BY column rule
2. B.COUNT(*) vs COUNT(col)
3. C.COUNT / SUM / AVG
4. D.HAVING

Answer + AI explanation with Pro

1. A.COUNT / SUM / AVG — aggregate functions returning row count, total, and mean over a group
2. B.COUNT / SUM / AVG — every selected non-aggregated column must also appear in the GROUP BY clause
3. C.COUNT / SUM / AVG — returns the first non-NULL value from its arguments
4. D.COUNT / SUM / AVG — COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column

Answer + AI explanation with Pro

1. A.aggregate functions returning row count, total, and mean over a group
2. B.groups rows sharing a value so aggregates are computed per group
3. C.COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
4. D.every selected non-aggregated column must also appear in the GROUP BY clause

Answer + AI explanation with Pro

1. A.COALESCE
2. B.GROUP BY
3. C.GROUP BY column rule
4. D.COUNT(*) vs COUNT(col)

Answer + AI explanation with Pro

1. A.GROUP BY column rule — aggregate functions returning row count, total, and mean over a group
2. B.GROUP BY column rule — COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
3. C.GROUP BY column rule — returns the first non-NULL value from its arguments
4. D.GROUP BY column rule — every selected non-aggregated column must also appear in the GROUP BY clause

Answer + AI explanation with Pro

1. A.aggregate functions returning row count, total, and mean over a group
2. B.groups rows sharing a value so aggregates are computed per group
3. C.COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
4. D.returns the first non-NULL value from its arguments

Answer + AI explanation with Pro

1. A.COALESCE
2. B.GROUP BY column rule
3. C.HAVING
4. D.COUNT(*) vs COUNT(col)

Answer + AI explanation with Pro

1. A.COUNT(*) vs COUNT(col) — returns the first non-NULL value from its arguments
2. B.COUNT(*) vs COUNT(col) — COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
3. C.COUNT(*) vs COUNT(col) — every selected non-aggregated column must also appear in the GROUP BY clause
4. D.COUNT(*) vs COUNT(col) — groups rows sharing a value so aggregates are computed per group

Answer + AI explanation with Pro

1. A.every selected non-aggregated column must also appear in the GROUP BY clause
2. B.COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
3. C.groups rows sharing a value so aggregates are computed per group
4. D.returns the first non-NULL value from its arguments

Answer + AI explanation with Pro

1. A.COUNT / SUM / AVG
2. B.GROUP BY
3. C.GROUP BY column rule
4. D.COALESCE

Answer + AI explanation with Pro

1. A.COALESCE — COUNT(*) counts all rows; COUNT(col) counts only non-NULL values of that column
2. B.COALESCE — returns the first non-NULL value from its arguments
3. C.COALESCE — groups rows sharing a value so aggregates are computed per group
4. D.COALESCE — every selected non-aggregated column must also appear in the GROUP BY clause

Answer + AI explanation with Pro