How can sum using group by?

Question

I'm trying to count all clients that exist in the table policies

Here my tables:

|Policies|
  |id| |num_policy| |client_id| |state|
     1     1234          1         0
     2     5678          1         0
     3     9123          1         0
     4     4567          2         0
     5     8912          3         0
     6     3456          4         0
     7     7891          4         0
     8     2345          4         0
     9     6789          4         0

 |Clients|
   |id|  |name|
    1     ABC
    2     DEF
    3     GHI
    4     JKL
    5     MNO
    6     PQR
    7     STU
    8     VWX
    9     YZA

I'm trying to get the count all clients that exist in policies.

   |clients|  |state|
      5          0

I tried: http://sqlfiddle.com/#!2/eb479/2

select count(*) as total from policies p
INNER JOIN clients c ON c.id = p.client_id
WHERE p.state= 0
group by p.client_id

Please somebody can help me?

All can of help will be accepted.

Answer 1

You can just count the distinct client ids that appear in the policies table

select count(distinct client_id) as count 
from policies p
  inner JOIN clients c ON c.id = p.client_id
WHERE p.state= 0

You do not even need the join with the client table if you trust your referential integrity, ie if you are sure that all client_id field are filled with valid clients, just do

select count(distinct client_id) as count 
from policies p
WHERE p.state= 0

Answer 2

Are you trying to get the clients that have a minimum one policy ?

select p.client_id from policies p
INNER JOIN clients c ON c.id = p.client_id
WHERE p.state= 0
group by p.client_id

Or are you trying to get the count of how many clientes have a policy?

select count(distinct client_id) as count 
from policies p
   inner JOIN clients c ON c.id = p.client_id
WHERE p.state= 0

Or are you trying to get how many policy each client have?