Quiz for COUNT() Function

Q1: What does the COUNT() function return?

A. Sum of column values B. Average value C. Number of rows D. Max value

Q2: What does the COUNT() function do in SQL?

A. Counts non-null values B. Counts all rows including nulls C. Counts only nulls D. Counts distinct rows

Q3: Which of the following returns total number of rows in a table?

A. SELECT COUNT(*) FROM table; B. SELECT COUNT(id) FROM table; C. SELECT SUM(*) FROM table; D. SELECT TOTAL(*) FROM table;

Q4: What is the difference between COUNT(*) and COUNT(column_name)?

A. No difference B. COUNT(*) includes nulls, COUNT(column_name) doesn't C. COUNT(column_name) is faster D. COUNT(*) excludes nulls

Q5: Which function counts only unique values?

A. COUNT(*) B. COUNT(DISTINCT column) C. SUM(column) D. AVG(column)

Q6: What will COUNT(NULL) return?

A. 0 B. 1 C. Depends on database D. Error

Q7: To count all unique departments in "employees" table?

A. SELECT COUNT(*) FROM employees; B. SELECT COUNT(department) FROM employees; C. SELECT COUNT(DISTINCT department) FROM employees; D. SELECT SUM(department) FROM employees;

Q8: Which aggregate function can be used with COUNT()?

A. SUM() B. AVG() C. MAX() D. All of the above

Q9: What does COUNT(1) return?

A. Counts all rows B. Counts rows with 1s C. Same as COUNT(*) D. Error

Q10: Is COUNT() a group function?

A. Yes B. No C. Only in Oracle D. Only with HAVING clause

Q11: What is the result of COUNT(*) on an empty table?

A. 0 B. 1 C. NULL D. Error

Q12: What is the purpose of GROUP BY with COUNT()?

A. To sort rows B. To group rows and count in each group C. To eliminate nulls D. To find averages

Q13: Which clause can be used to filter results after COUNT()?

A. WHERE B. ORDER BY C. HAVING D. GROUP BY

Q14: Which SQL keyword is used to rename COUNT result?

A. AS B. BY C. TO D. RENAME

Q15: Which SQL returns number of orders per customer?

A. SELECT COUNT(*) FROM orders; B. SELECT customer, COUNT(*) FROM orders GROUP BY customer; C. SELECT * FROM orders; D. SELECT COUNT(customer) FROM orders;

Q16: Can COUNT() be used in subqueries?

A. Yes B. No C. Only in MySQL D. Only in PostgreSQL

Q17: What does COUNT(DISTINCT column_name) exclude?

A. Nulls B. Duplicates C. Both D. Neither

Q18: What happens if COUNT() is used without FROM clause?

A. Syntax error B. Returns 0 C. Returns 1 D. Depends on database

Q19: Which clause must be used with COUNT() for group analysis?

A. ORDER BY B. GROUP BY C. HAVING D. JOIN

Q20: Can COUNT() be used on text columns?

A. Yes B. No C. Only if numeric D. Only if not null

Q21: Which statement returns number of employees per department?

A. SELECT department, COUNT(*) FROM employees GROUP BY department; B. SELECT COUNT(*) FROM employees; C. SELECT COUNT(department) FROM employees; D. SELECT * FROM employees;