SELECT count(*) FROM search LEFT JOIN click ON search.id = click.id WHERE click.id IS NOT NULL;
or
SELECT count(*) FROM search, click WHERE search.id = click.id AND click.id IS NOT NULL;
or
SELECT count(*) FROM search, click WHERE search.id = click.id;
Count the rows in search that have a corresponding matching id row in click.
A piggy bank of commands, fixes, succinct reviews, some mini articles and technical opinions from a (mostly) Perl developer.