具有子查询的多个GROUP BY

Question

I am trying to create a query that will provide me two separate COUNTs in two separate columns. A count for applications and a count for approved applications. I want these counts displayed for each month within a span of time. Like so:

date      applications  approvals
January   20            3    
February  25            1
March     20            4
April     10            2

When using the subqueries, I grouped the main query using "GROUP BY MONTH(APPL_CREATE_DT)". The problem with this though is that it only correctly grouped the first column while the next column contained the same count for each month. If I add the same GROUP BY to the other subquery section, then I get a 1242 error.

Any ideas on how to work this out would be helpful. I've seen other questions like this, but they weren't so dependent on other clauses for a solution.

SELECT
COUNT(DISTINCT APPL_ID) AS Applications,
(SELECT COUNT(DISTINCT APPL_ID)
FROM APPL_APP
JOIN APPU_APP_USER ON APPL_ID = APPU_APPL_ID
LEFT JOIN APPD_APP_DECISION ON APPD_APPL_ID = APPL_ID
 WHERE APPD_STATUS = 'LENDER_APPROVED'
  AND APPU_CUR_STATE = 'TX'
  AND APPL_CREATE_DT >= '2013-01-01 00:00:00'
  AND APPL_CREATE_DT <= '2014-01-31 23:59:59'
GROUP BY MONTH(APPL_CREATE_DT)) AS Approvals
FROM APPU_APP_USER
JOIN APPL_APP ON APPL_ID = APPU_APPL_ID
 AND APPL_CREATE_DT >= '2013-01-01 00:00:00'
 AND APPL_CREATE_DT <= '2014-01-31 23:59:59'
 AND APPU_CUR_STATE = 'TX'
GROUP BY MONTH(APPL_CREATE_DT);

Answer 1

To make things simple, I've deleted some where conditions.

As you mentioned

I had looked up doing something like MONTH(outerTable.APPL_CREATE_DATE) = MONTH(inner_table.APPL_CREATE_DATE)

You're right.

but I don't know where to place it to make it work

I hope following query would help you.

SELECT
    COUNT(DISTINCT APPL_ID) AS Applications,
    (
        SELECT COUNT(DISTINCT APPL_ID)
        FROM APPL_APP INNER JOIN APPU_APP_USER ON APPL_ID = APPU_APPL_ID
          LEFT JOIN APPD_APP_DECISION ON APPD_APPL_ID = APPL_ID
        WHERE APPD_STATUS = 'LENDER_APPROVED'
          AND MONTH(APPL_CREATE_DT) = MONTH(t1.APPL_CREATE_DT)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^= This condition added
        GROUP BY MONTH(APPL_CREATE_DT)
    ) AS Approvals
FROM APPL_APP t1 INNER JOIN APPU_APP_USER ON APPL_ID = APPU_APPL_ID
              ^^===== Alias added
GROUP BY MONTH(APPL_CREATE_DT);

Please note that MySQL's correlated sub-query has extreamly poor performance in my experience. Please consider converting your query to with just sub-query if possible.