COUNT function behavior in SQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count all rows in the table named 'employees'.

SQL

SELECT COUNT([1]) FROM employees;

Drag options to blanks, or click blank then click option'

Aname

Bsalary

Cid

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(column_name) when you want to count all rows including NULLs.

Leaving the parentheses empty inside COUNT.

The COUNT(*) counts all rows, including those with NULLs in any column.

2fill in blank

medium

Complete the code to count only the non-NULL values in the 'salary' column from 'employees'.

SQL

SELECT COUNT([1]) FROM employees;

Drag options to blanks, or click blank then click option'

A*

Bsalary

CNULL

DDISTINCT salary

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(*) when only non-NULL values are needed.

Using COUNT(NULL) which always returns 0.

COUNT(column_name) counts only rows where the column is NOT NULL.

3fill in blank

hard

Fix the error in the code to count distinct non-NULL 'department' values in 'employees'.

SQL

SELECT COUNT([1]) FROM employees;

Drag options to blanks, or click blank then click option'

ADISTINCT department

Bdepartment

C*

DALL department

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(department) which counts all non-NULL but not distinct values.

Using COUNT(*) which counts all rows regardless of department.

Use COUNT(DISTINCT column) to count unique non-NULL values.

4fill in blank

hard

Fill both blanks to count how many employees have a non-NULL 'email' and a salary greater than 50000.

SQL

SELECT COUNT([1]) FROM employees WHERE [2] > 50000;

Drag options to blanks, or click blank then click option'

Aemail

Bsalary

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(*) which counts all rows regardless of email NULLs.

Using WHERE email > 50000 which is invalid.

COUNT(email) counts non-NULL emails. The WHERE clause filters salaries greater than 50000.

5fill in blank

hard

Fill all three blanks to count distinct non-NULL 'manager_id' values for employees with 'status' = 'active'.

SQL

SELECT COUNT([1]) FROM employees WHERE [2] = [3];

Drag options to blanks, or click blank then click option'

ADISTINCT manager_id

Bstatus

C'active'

Dmanager_id

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(manager_id) which counts duplicates.

Using WHERE status = active without quotes causing syntax error.

COUNT(DISTINCT manager_id) counts unique non-NULL managers. The WHERE clause filters active employees.