COUNT(*) vs COUNT(column) difference in SQL - Quick Revision & Key Differences

Recall & Review

beginner

What does COUNT(*) do in SQL?

COUNT(*) counts all rows in a table, including rows with NULL values in any column.

beginner

What does COUNT(column) do in SQL?

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

intermediate

How does COUNT(*) differ from COUNT(column) when the column contains NULL values?

COUNT(*) counts every row regardless of NULL values, but COUNT(column) skips rows where the column is NULL.

beginner

If a table has 10 rows and 3 rows have NULL in column age, what will COUNT(*) and COUNT(age) return?

COUNT(*) returns 10 because it counts all rows.<br>COUNT(age) returns 7 because it counts only rows where age is NOT NULL.

beginner

Why might you choose COUNT(column) over COUNT(*)?

Use COUNT(column) when you want to count only rows with meaningful (non-NULL) data in that column.

What does COUNT(*) count in a SQL table?

A Only rows with NULL values

B Only rows where a specific column is NOT NULL

C All rows including those with NULL values

D Only distinct rows

COUNT(*) counts every row in the table, regardless of NULL values.

What will COUNT(column) ignore when counting?

A All rows

B Rows where the column is NULL

C Rows where the column is NOT NULL

D Only duplicate rows

COUNT(column) counts only rows where the column is NOT NULL, so it ignores NULLs.

If a column has no NULL values, how do COUNT(*) and COUNT(column) compare?

A They return zero

B <code>COUNT(*)</code> returns more

C <code>COUNT(column)</code> returns more

D They return the same count

When there are no NULLs, both counts return the total number of rows.

Which COUNT function should you use to count all rows regardless of NULLs?

A COUNT(*)

B COUNT(column)

C COUNT(DISTINCT column)

D COUNT(NULL)

COUNT(*) counts all rows including those with NULLs.

What does COUNT(column) return if all values in the column are NULL?

A 0

B Number of rows

C NULL

D Error

COUNT(column) returns 0 if all values are NULL because it counts only non-NULL values.

Explain the difference between COUNT(*) and COUNT(column) in SQL.

Think about how NULL values affect counting.

When would you use COUNT(column) instead of COUNT(*)?

Consider what NULL means in your data.