beginner

What does the SQL COUNT(*) function do?

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

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beginner

How does COUNT(column_name) differ from COUNT(*)?

COUNT(column_name) counts only rows where the specified column is NOT NULL, ignoring rows with NULL in that column.

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intermediate

What will COUNT(1) count in a SQL query?

COUNT(1) counts all rows just like COUNT(*), because 1 is a constant and not NULL, so it counts every row.

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beginner

If a column has NULL values, will COUNT(column) include those rows?

No, COUNT(column) excludes rows where the column value is NULL. Only non-NULL values are counted.

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intermediate

Can COUNT be used to count distinct values? How?

Yes, use COUNT(DISTINCT column_name) to count unique non-NULL values in that column.

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What does COUNT(*) return in a SQL query?

ANumber of NULL values in a column

BNumber of rows with non-NULL values in a specific column

CNumber of distinct values in a column

DNumber of all rows including those with NULLs

Which COUNT function counts only non-NULL values in a column?

ACOUNT(*)

BCOUNT(1)

CCOUNT(column_name)

DCOUNT(DISTINCT *)

What will COUNT(1) return compared to COUNT(*)?

ACounts only rows where column 1 is not NULL

BCounts all rows, same as COUNT(*)

CCounts distinct rows

DCounts only NULL rows

How to count unique non-NULL values in a column?

ACOUNT(DISTINCT column_name)

BCOUNT(column_name)

CCOUNT(*)

DCOUNT(NULL)

If a column has NULL values, what does COUNT(column) do with those rows?

AIgnores them

BCounts them

CCounts them as zero

DReturns an error

Explain how COUNT(*) and COUNT(column_name) behave differently when counting rows.

Describe how to count unique values in a column using COUNT.