Using sub-query to SELECT MAX value along with LEFT JOIN

问题

I have a query for getting search results, which works fine.

Example of successful query:

 SELECT
    individuals.individual_id,
    individuals.unique_id,
    TIMESTAMPDIFF(YEAR,individuals.day_of_birth,CURDATE()) AS age,
    individuals_dynamics.id,
    individuals_achievements.degree
  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  LEFT JOIN
    individuals_achievements AS individuals_achievements ON individuals.unique_id = individuals_achievements.individual_id
  WHERE
    $uuid_access_status $display_type $detailed_search_query
  ORDER BY 
    $search_sort $search_order

From now on, I have more than one record in individuals_achievements per each individual and this is the where I would like to get the MAX value (latest id).

I tried the many different queries but always was getting an error Call to a member function rowCount() on a non-object.

I understand what that error means but I can't figure out where I'm making that mistake and what is wrong in general.

Example of my unsuccessful attempt:

  SELECT
    individuals.individual_id,
    individuals.unique_id,
    TIMESTAMPDIFF(YEAR,individuals.day_of_birth,CURDATE()) AS age,
    individuals_dynamics.id,
    individuals_achievements.degree
  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements ON individuals.unique_id = individuals_achievements.individual_id
  WHERE
    $uuid_access_status $display_type $detailed_search_query
  ORDER BY 
    $search_sort $search_order

What am I missing here? Any help please?

回答1:

This is your from clause:

  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements ON individuals.unique_id = individuals_achievements.individual_id;

I can spot at least three problems. The first is individuals_achievements AS individuals_achievements; the second is the reference to individuals_achievements.individual_id which isn't in the subquery. The third is the group by latest_record.

  FROM individuals LEFT JOIN
       individuals_dynamics
       ON individuals.unique_id = individuals_dynamics.individual_id LEFT JOIN
       individuals_achievements
       ON individuals.unique_id = individuals_achievements.individual_id JOIN
       (SELECT ia.individual_id, MAX(ia.id) AS latest_record
        FROM individuals_achievements ia
        GROUP BY ia.individual_id
       ) iamax
       ON individuals.unique_id = iamax.individual_id and
          individuals_achievements.id = iamax.latest_record

This adds an additional subquery, with the id of the latest record.

By the way, it is redundant to have a table alias be the same name as the table name. That just clutters up the query. Also, it is a good idea to use table abbreviations for the aliases, such as ia for individuals_achievements. Because this answer focuses only on the from clause, I have not made that change.

回答2:

PDO::query returns a FALSE if there's an error while processing your query, and applying rowCount to FALSE as a member function will result in the error you're seeing.

Check your query for syntax errors and parse any error messages PDO would return you.

回答3:

You have to select individual_id in your INNER JOIN because that part you are using in your ON clause and you have selected degree, MAX(id) AS latest_record only and trying to join with individual_id which is undefined in the newly generated table by inner join

individuals_achievements AS individuals_achievements 
ON individuals.unique_id = individuals_achievements.individual_id

try this one

SELECT
    individuals.individual_id,
    individuals.unique_id,
    TIMESTAMPDIFF(YEAR,individuals.day_of_birth,CURDATE()) AS age,
    individuals_dynamics.id,
    individuals_achievements.degree
  FROM
    individuals as individuals
  LEFT JOIN
    individuals_dynamics AS individuals_dynamics ON individuals.unique_id = individuals_dynamics.individual_id
  INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record,individual_id
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements_1 ON individuals.unique_id = individuals_achievements_1.individual_id
  WHERE
    $uuid_access_status $display_type $detailed_search_query
  ORDER BY 
    $search_sort $search_order

Make sure you do have a column individual_id in table individuals_achievements

回答4:

First off you're not really using the AS keyword in SQL to make your query more readable, which would help. And I'm wondering what is in the (php?) variables of the WHERE clause.

Also, your subquery isn't returning a field individual_id yet you try to JOIN on it. I think you might want this:

SELECT
  ind.individual_id,
  ind.unique_id,
  TIMESTAMPDIFF(YEAR,ind.day_of_birth,CURDATE()) AS age,
  ind_dyn.id,
  ind_ach.degree
FROM
  individuals as ind
LEFT JOIN
  individuals_dynamics AS ind_dyn ON ind.unique_id = ind_dyn.individual_id
INNER JOIN
  individuals_achievements AS ind_ach 
     ON ind_ach.individual_id = ind.unique_id
    AND ind_ach.id =
    (
      SELECT MAX(id) FROM individuals_achievements
      WHERE individuals_achievements.individual_id = ind.unique_id
    )
WHERE
  $uuid_access_status $display_type $detailed_search_query
ORDER BY 
  $search_sort $search_order

回答5:

INNER JOIN
  (
       SELECT
            degree, MAX(id) AS latest_record,individual_id
       FROM
            individuals_achievements
       GROUP BY
            latest_record
  ) individuals_achievements AS individuals_achievements_1 ON individuals.unique_id = individuals_achievements_1.individual_id

It should be

INNER JOIN (SELECT blah) AS something ON individuals.unique_id=something.individual_id

来源：https://stackoverflow.com/questions/18902790/using-sub-query-to-select-max-value-along-with-left-join

标签

mysql

sql

join

subquery