table

id name
1 a
2 b
3 c
4 c
5 b

select *, count(distinct name) from table group by name

id name count(distinct name)
1 a 1
2 b 1
3 c 1

SELECT cat_id, COUNT(*) AS goods_num FROM talble GROUP BY cat_id

$M = M('table');
$rs = $M->field(array('cat_id','count(*)'=>'goods_num'))->group('cat_id')->select();

echo $M->getLastSQL();
print_r($rs);