MySQL sum case value 等于该组的最大值

Question

I'm trying to solve this query to return the total count of items in a group and the total of items in that group which value equals the max value of one field of that group and also which is the max(value).

So far:

SELECT
    limid, COUNT(*), SUM(CASE WHEN cotavertical=MAX(cotavertical) THEN 1 ELSE 0 END), MAX(cotavertical)
FROM limites
LEFT JOIN tbparentchild ON parent=limid
LEFT JOIN tbspatialbi ON child=rgi
WHERE limtipo=4 AND x=1
GROUP BY limid

So far MySQL returns

"Invalid use of group function."

Is it too complex to solve in MySQL only? Better to use algorithm?

Answer 1

You are trying to use the max value for each group in an aggregate function ( SUM ) before the aggregation has finished, and hence it is not available. The query below uses the strategy of joining a subquery which contains the max value of cotavertical for each limid group. In this case, the max value per group which you want to use will now be available from another source, and you can sum using it.

SELECT l.limid,
       COUNT(*),
       SUM(CASE WHEN cotavertical = t.cotamax THEN 1 ELSE 0 END),
       MAX(cotavertical)
FROM limites l
LEFT JOIN tbparentchild pc
    ON pc.parent = l.limid
LEFT JOIN tbspatialbi s
    ON pc.child = s.rgi
LEFT JOIN
(
    SELECT limid, MAX(cotavertical) AS cotamax
    FROM limites
    LEFT JOIN tbparentchild
        ON parent = limid
    LEFT JOIN tbspatialbi
        ON child = rgi
    WHERE limtipo = 4 AND x = 1
    GROUP BY limid
) t
    ON l.limid = t.limid
WHERE limtipo = 4 AND l.x = 1
GROUP BY l.limid

Another option for solving your problem would be to use a subquery directly in the CASE statement. But, given the size and number of joins in your original query, this would be way uglier than the query above. MySQL does not support common table expressions, which would have helped with both these solutions.