COUNT(*) vs COUNT(column) difference in SQL - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to count all rows in the table.

SQL

SELECT [1] FROM employees;

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ACOUNT(1)

BCOUNT(employee_id)

CCOUNT(*)

DCOUNT(DISTINCT employee_id)

Attempts:

3 left

💡 Hint

Common Mistakes

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

Confusing COUNT(*) with COUNT(DISTINCT column).

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

2fill in blank

medium

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

SQL

SELECT [1] FROM employees;

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

ACOUNT(*)

BCOUNT(salary)

CCOUNT(1)

DCOUNT(DISTINCT salary)

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(*) which counts all rows including NULLs.

Using COUNT(DISTINCT salary) which counts unique non-NULL salaries.

COUNT(salary) counts only rows where salary is not NULL.

3fill in blank

hard

Fix the error in the query to count all rows including those with NULLs in department_id.

SQL

SELECT [1] FROM employees WHERE department_id IS NOT NULL;

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ACOUNT(*)

BCOUNT(department_id)

CCOUNT(1)

DCOUNT(DISTINCT department_id)

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(column) which ignores NULLs.

Filtering rows with WHERE department_id IS NOT NULL.

To count all rows including those with NULLs, use COUNT(*) and remove the WHERE filter.

4fill in blank

hard

Fill both blanks to count unique non-NULL values in the email column.

SQL

SELECT [1]([2]) FROM employees;

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ACOUNT

Bemail

CDISTINCT

Dsalary

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT(email) which counts all non-NULL emails including duplicates.

Using COUNT(DISTINCT salary) instead of email.

Use COUNT(DISTINCT email) to count unique non-NULL emails.

5fill in blank

hard

Fill all three blanks to count all rows and count non-NULL phone numbers separately.

SQL

SELECT [1](*) AS total_rows, [2](phone_number) AS phone_count, [3](phone_number) AS unique_phones FROM employees;

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ACOUNT

CCOUNT(DISTINCT)

DSUM

Attempts:

3 left

💡 Hint

Common Mistakes

Using SUM instead of COUNT.

Using COUNT(*) for phone_number counts which includes NULLs.

COUNT(*) counts all rows.
COUNT(phone_number) counts non-NULL phone numbers.
COUNT(DISTINCT phone_number) counts unique non-NULL phone numbers.