If you very often need to calculate results such as counts based on information from a lot of rows, it's probably much better to introduce a new table and update the counter in real time. An update of the following form is very fast:

[SQL] pobierz, plaintext 
UPDATE tbl_name SET count_col=count_col+1 WHERE key_col=constant;
[SQL] pobierz, plaintext 

This is really important when you use MySQL storage engines such as MyISAM and ISAM that have only table-level locking (multiple readers / single writers). This will also give better performance with most databases, because the row locking manager in this case will have less to do.