Exactly one row? At least one row? (Part 2)

To summarize, if you want at least some number of rows, use an inequality in the HAVING clause of the IN?(subquery). If you want at least one row, the HAVING clause is redundant, and you should use an EXISTS?(subquery):

select id
     , loc_numb
     , loc_prec
  from item_loc  xx
 where id in 
       ( select id
           from item_loc
          where loc_prec = 'S'
         group by id
         having count(*) = 1 )
   and exists
       ( select 1
           from item_loc
          where id = xx.id
            and loc_prec = 'P' )

Note that the EXISTS subquery is correlated. The EXISTS syntax gives the database optimizer a chance to improve performance, since the EXISTS subquery will be true as soon as the first row is found. As soon as the row is found, grouping is not necessary.

In summary, use EXISTS if you are looking for one or more rows. If you want exactly some number, including one, or at least some number where the number is two or more, then grouping is necessary, and you can't use EXISTS.