PHP MySQL : Count number of elements that are not present in another table

Question

我现在有这种表

table a
==============
pid |   block
--------------
1   |   1
2   |   1
3   |   2
4   |   3
5   |   2
6   |   3
--------------

table b
==============
cid |   pid
--------------
1   |   1
2   |   3
3   |   5
--------------

output (count each block who no exist on table b)
===================
block   |   total
-------------------
1       |   1 <--- because on table b exist 1 from 2
2       |   0 <--- because on table b exist 2 from 2
3       |   2 <--- because on table b not exist
-------------------

所以基本上我需要计算表 b 中不存在但存在于表 a 中的 block 数，并根据 block 名称对其进行排序，这可能吗？因为我需要将计数结果显示为图表

Answer 1

select a.block
, 2 - count(distinct b.cid) 
from a
left join b on a.pid = b.pid
group by 1