Question

I’m building a MySQL query with subqueries. The query requires, as described in Getting the number of rows with a GROUP BY query, the number of records returned by a group-by query, because I want the number of days with records in the database. So I’m using the following:

SELECT
  COUNT(*)
FROM
(
  SELECT
    cvdbs2.dateDone
  FROM
    cvdbStatistics cvdbs2
  WHERE
    cvdbs2.mediatorId = 123
  GROUP BY
    DATE_FORMAT( cvdbs2.dateDone, "%Y-%d-%m" )
) AS activityTempTable

Now, I want this as a subquery, because I need some more data with different WHERE statements. So my query becomes:

SELECT
  x,
  y, 
  z,
  (
    SELECT
      COUNT(*)
    FROM
    (
      SELECT
        cvdbs2.dateDone
      FROM
        cvdbStatistics cvdbs2
      WHERE
        cvdbs2.mediatorId = mediators.id
      GROUP BY
        DATE_FORMAT( cvdbs2.dateDone, "%Y-%d-%m" )
    ) AS activityTempTable
  ) AS activeDays
FROM
  mediators
LEFT JOIN
  cvdbStatistics
ON
  mediators.id = cvdbStatistics.mediatorId
WHERE
  mediators.recruiterId = 409
GROUP BY
  mediators.email

(I left out some irrelevant WHERE-statements from my queries. 409 is just an example id, this is inserted by PHP).

Now, I’m getting the following error:

#1054 - Unknown column 'mediators.id' in 'where clause' 

MySQL forgot about the mediators.id in the deepest subquery. How can I build a query, with the number of results of a GROUP-BY query, which requires a value from the main query, as one of the results? Why isn’t the deepest query aware of ‘mediators.id’?

Answer 1

Try the following:

SELECT
  x,
  y, 
  z,
  (
    SELECT
      COUNT(distinct DATE_FORMAT( cvdbs2.dateDone, "%Y-%d-%m" ))
    FROM
      cvdbStatistics cvdbs2
    WHERE
      cvdbs2.mediatorId = mediators.id
  ) AS activeDays
FROM
  mediators
LEFT JOIN
  cvdbStatistics
ON
  mediators.id = cvdbStatistics.mediatorId
WHERE
  mediators.recruiterId = 409
GROUP BY
  mediators.email