COUNT(*) vs COUNT(email)?

Options

1. They are identical and always return the same number
2. COUNT(*) counts all rows; COUNT(email) skips NULL emails (Correct option)
3. COUNT(email) counts only the distinct emails
4. COUNT(*) skips rows that contain any NULL

Answer

COUNT(*) counts all rows; COUNT(email) skips rows where email is NULL

Why

COUNT(*) counts rows, no matter what they contain.

COUNT(email) counts only rows where email has a value (not NULL). Most aggregate functions (SUM, AVG, COUNT(column)) ignore NULLs.

For unique values: COUNT(DISTINCT email).

Remember

COUNT(column) ignores NULLs.

Key terms

• Aggregate function
• NULL
• COUNT(*) vs COUNT(column)
• SQL
• Table
• GROUP BY