MySQL-带嵌套分组的嵌套选择

Question

I have two tables, and I am building some statistics from it, so I am counting multiple columns and tables, my issue is when I try to join another table and group it in the nested statement, and I keep getting the following error when I run my code.

General error: 2014 Cannot execute queries while other unbuffered queries are active...

This is the scenario:

++============================================================================++
||                               CLIENTS TABLE                                ||
++=======+===============+==============+==============+======================++
|   id   |   firstname   |   lastname   |     email    |      created_at       |
+--------+---------------+--------------+--------------+-----------------------+
|    1   |     JOHN      |      DOE     | john@doe.com |  2014-08-22 20:10:30  |
+--------+---------------+--------------+--------------+-----------------------+
|    2   |     JUNE      |      DAE     | june@dae.com |  2014-07-28 18:12:08  |
+--------+---------------+--------------+--------------+-----------------------+

++============================================================================++
||                               PURCHASES TABLE                              ||
++=======+===============+=============================+======================++
|   id   |   client_id   |     transaction_status      |      created_at       |
+--------+---------------+-----------------------------+-----------------------+
|    1   |        1      |          COMPLETED          |  2014-08-22 20:10:30  |
+--------+---------------+-----------------------------+-----------------------+
|    2   |        2      |          INCOMPLETE         |  2014-08-22 20:10:30  |
+--------+---------------+-----------------------------+-----------------------+
|    1   |        2      |          COMPLETED          |  2014-08-22 20:10:30  |
+--------+---------------+-----------------------------+-----------------------+
|    2   |        1      |          COMPLETED          |  2014-08-22 20:10:30  |
+--------+---------------+-----------------------------+-----------------------+

This are some of the things I'm counting:

1. Total clients globally
2. Total clients current month
3. Total clients past month
4. Last registered client
5. Client with completed purchases (This fails)

And finally this is my failing query:

SELECT 
  ( SELECT 
      COUNT(*) 
    FROM
      clients
  ) AS 
      total_registered_clients,

  ( SELECT 
      COUNT(*) 
    FROM 
      clients 
    AND 
      (YEAR(created_at) = YEAR(CURRENT_DATE)) 
    AND 
      (MONTH(created_at) = MONTH(CURRENT_DATE)) 
  ) AS 
      current_month_registered_clients,

  ( SELECT 
      COUNT(*) 
    FROM 
      clients 
    AND 
      created_at 
    BETWEEN 
      (CURRENT_DATE - INTERVAL 1 MONTH) 
    AND 
      CURRENT_DATE 
  ) AS 
      last_month_registered_clients,

  -- This part fails
  ( SELECT
      COUNT(*)
    FROM
      clients
    INNER JOIN
      purchases
    WHERE
      purchases.client_id = clients.id
    AND
      purchases.transaction_status = 'completed'
    GROUP BY
      purchases.client_id
   ) AS
       clients_with_purchases

EDIT: My expected result with var_dump is:

[0] =>
object(stdClass)#60 (10) {
  ["total_registered_clients"]=>
    string(1) "2"
  ["current_month_registered_clients"]=>
    string(1) "1"
  ["last_month_registered_clients"]=>
    string(1) "1"
  ["clients_with_purchases"]=>
    string(1) "2"

}

Answer 1

To remove that error:

INNER JOIN
      purchases
    ON    -- not WHERE
      purchases.client_id = clients.id

and remove the GROUP BY

But in that third query are trying to count the number of clients who have purchased or the number of purchases? (you are counting purchases)

2 methods for counting number of clients who have purchased

SELECT
      COUNT(DISTINCT clients.id)
FROM clients
      INNER JOIN purchases
                  ON purchases.client_id = clients.id
                        AND purchases.transaction_status = 'completed'

SELECT
      COUNT(*)
FROM clients
WHERE EXISTS (
            SELECT
                  1
            FROM purchases
            WHERE transaction_status = 'completed'
                  AND clients.id = purchases.client_id
      )

Answer 2

SELECT 
  ( SELECT 
      COUNT(*) 
    FROM
      clients
  ) AS 
      total_registered_clients,

  ( SELECT 
      COUNT(*) 
    FROM 
      clients 
    where
      (YEAR(created_at) = YEAR(GETDATE())) 
    AND 
      (MONTH(created_at) = MONTH(GETDATE())) 
  ) AS 
      current_month_registered_clients,

  ( SELECT 
      COUNT(*) 
    FROM 
      clients 
    where 
      created_at 
    BETWEEN 
      (dateadd(m,-1,GETDATE())  ) 
    AND 
      GETDATE() 
  ) AS 
      last_month_registered_clients,

  -- This part fails
  ( SELECT
      COUNT(*)
    FROM
      clients
    INNER JOIN
      purchases
    on
      purchases.client_id = clients.id
    where
      purchases.transaction_status = 'completed'

   ) AS
       clients_with_purchases