php - Why use letters infront of each value in mysqli query? -

so have been wondering while now. why use letters infront of each value in query this? in database, each of these values without letter infront.

$sql = "     select c.client_id, c.client_name, c.contactperson, c.internal_comment,      if null(r.region, 'alle byer') region, c.phone, c.email,     uu.fullname changed_by,      (select count(p.project_id)          projects p          p.client_id = c.client_id , (p.is_deleted != 1 or p.is_deleted null)     ) numprojects      clients c left join users uu on c.db_changed_by = uu.id      left join regions r on c.region_id = r.region_id      (c.is_deleted != 1 or c.is_deleted null)     "; 

i have tried looking up, can't find anywhere.

when in sql need use more 1 table query, can this:

select person.name, vehicle.id person, vehicle; 

or can smaller, , put this

select p.name, v.id person p, vehicle v; 

its reduce query lenght, , useful you