What to count when counting all rows MySQL

Question

Is there any difference in the performance, or the background execution behavior when counting * , any custom field ( n ), or the primary id in MySQL?

What exactly does * refer to in the query, and how does it differ from those two other ways?

SELECT COUNT( * ) FROM t;
SELECT COUNT( id ) FROM t;
SELECT COUNT( n ) FROM t;

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UPDATE:

Assume, that neither id nor n is null at any record.

Answer 1

COUNT(*) will include NULLS
COUNT(column_or_expression) won't.

This means COUNT(any_non_null_column) will give the same as COUNT(*) of course because there are no NULL values to cause differences.

Generally, COUNT(*) should be better because any index can be used because COUNT(column_or_expression) may not be indexed or SARGable

From ANSI-92 (look for "Scalar expressions 125")

Case:

a) If COUNT(*) is specified, then the result is the cardinality of T.

b) Otherwise, let TX be the single-column table that is the result of applying the to each row of T and eliminating null values. If one or more null values are eliminated, then a completion condition is raised: warning- null value eliminated in set function.

The same rules apply to SQL Server and Sybase too at least

Note: COUNT(1) is the same as COUNT(*) because 1 is a non-nullable expression.