修改SQL MAX查询以查找对应字段的值

Question

I have the following query:

$imgDimensions_query = "SELECT MAX(imgWidth) maxWidth, MAX(imgHeight) maxHeight FROM (
    SELECT imgHeight, imgWidth FROM primary_images WHERE imgId=$imgId
    UNION 
    SELECT imgHeight, imgWidth FROM secondary_images WHERE primaryId=$imgId) as MaxHeight";

It's working fantastic, but I would like to know how I can find the value of the column imgId , as well as the table name, for both the maxWidth and maxHeight values?

The reason I want this is I need to know if the maxWidth and maxHeight values belong to the same item in the database.

I'm wondering if this is possible by amending the current SQL query?

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What would be perfect is if, along with querying the maxWidth and maxHeight values, a boolean could be set up to output true if both the maxWidth and maxHeight belong to the same entry (at least once).

I'm thinking, since the image data in primary_images is unique from the data in secondary_images (and vice versa), a boolean could be set up in each of the queries, and as long as one true exists, true is output. Does that make sense? Is that possible?

---

I have managed to put together a second query which uses the values of maxWidth and maxHeight from the first query to output the number of images in a specific set that hold both values. All I really care about is if there is or if there isn't one or more images that meet the above requirement, so again, a boolean would be better than the total number. If you have an idea of how to amend the following to show a boolean instead of the number of results, let me know!

I have been reassured that with a maximum number of entries in both tables being under 1000, using two queries instead of one shouldn't cause a hit to speed. If you think so as well, and if combining these queries into one is ridiculous, then let me know that as well.

The second query:

$haveDimensions_query = "SELECT sum(rows) AS total_rows FROM (
    SELECT count(*) AS rows FROM primary_images WHERE imgId = $imgId and imgWidth = $maxImageWidth and imgHeight = $maxImageHeight
    UNION ALL
    SELECT count(*) AS rows FROM secondary_images WHERE primaryId = $imgId and imgWidth = $maxImageWidth and imgHeight = $maxImageHeight
) as union_table";

Answer 1

[Original answer is below - this should be better]

I think I misunderstood your question. The following query should give a result set that includes one row for each image that has greatest height or greatest width compared to all images, and it will have the ID and table name in the row along with the height and width.

Since the maximum height and width may not be from the same image, there can't necessarily be just one row in the result with a single image ID and table source.

I hope this is at least closer to what you were looking for.

SELECT
  imgId,
  tName,
  imgWidth,
  imgHeight
FROM (
        SELECT imgId, 'Primary' tName, imgHeight, imgWidth
        FROM primary_images WHERE imgId=$imgId
        UNION 
        SELECT imgId, 'Secondary' tName, imgHeight, imgWidth
        FROM secondary_images WHERE primaryId=$imgId
      ) as T
WHERE 
 (T.imgHeight >= (SELECT MAX(imgHeight) FROM primary_images)
  AND
  T.imgHeight >= (SELECT MAX(imgHeight) FROM secondary_images)
 )
 OR
 (T.imgWidth >= (SELECT MAX(imgWidth) FROM primary_images)
  AND
  T.imgWidth >= (SELECT MAX(imgWidth) FROM secondary_images)
 )

-- ORIGINAL ANSWER BELOW --

Try something like this:

   SELECT
      imgId,
      tName,
      MAX(imgWidth) maxWidth,
      MAX(imgHeight) maxHeight FROM (
        SELECT imgId, 'Primary' tName, imgHeight, imgWidth
        FROM primary_images WHERE imgId=$imgId
        UNION 
        SELECT imgId, 'Secondary' tName, imgHeight, imgWidth
        FROM secondary_images WHERE primaryId=$imgId
      ) as T
   GROUP BY imgId, tName;

(MaxHeight wasn't a good alias name for the derived table, because it isn't a table of maximum heights. I changed it to T.)

Answer 2

Here's what I've got so far. It looks (and probably is) heavily suboptimal. Nevertheless, I'm posting it primarily to present the general idea. Hopefully, someone might develop it so as to make the result more efficient:

SELECT
  m.maxWidth,
  m.maxHeight,
  (u.imgWidth IS NOT NULL) AS OneImageHasBoth
FROM (
  SELECT
    MAX(imgWidth)  AS maxWidth,
    MAX(imgHeight) AS maxHeight
  FROM (
    SELECT imgWidth, imgHeight
    FROM primary_images
    UNION
    SELECT imgWidth, imgHeight
    FROM secondary_images
  ) u
) m
  LEFT JOIN  (
    SELECT imgWidth, imgHeight
    FROM primary_images
    UNION
    SELECT imgWidth, imgHeight
    FROM secondary_images
  ) u ON m.maxWidth = u.imgWidth AND m.maxHeight = u.imgHeight

Obviously, one way to optimise this could be to store the result of the repeating union into a temporary table. I'm not sure if your particular environment allows you to issue multi-statement queries, but if it does, that would definitely help to speed up the query.

Your idea of finding the results for each table separately and then combining them is actually not bad. But I think the logic behind combining the results should be a bit more complex. One obvious example would be, the result for one table contains true and the other false but the latter has bigger width and height, so returning true would be incorrect. Or consider this example:

Suppose, the result for primary_images is

maxWidth  maxHeight  OneImageHasBoth
--------  ---------  ---------------
1152      864        true

and for secondary_images it's

maxWidth  maxHeight  OneImageHasBoth
--------  ---------  ---------------
1280      800        true

Both tables have images with both attributes maximal, but it is clear that if the query was applied to the unioned set, OneImageHasBoth would be false .

So, as you can see, combining the two results should be more intricate than merely relying on the presence of true in one of them.

Here's my attempt at implementing the method:

SELECT
  CASE WHEN p.maxWidth  > s.maxWidth  THEN p.maxWidth  ELSE s.maxWidth  END AS maxWidth,
  CASE WHEN p.maxHeight > s.maxHeight THEN p.maxHeight ELSE s.maxHeight END AS maxHeight,
  (
    p.maxWidth >= s.maxWidth AND p.maxHeight >= s.maxHeight AND p.OneImageHasBoth OR
    p.maxWidth <= s.maxWidth AND p.maxHeight <= s.maxHeight AND s.OneImageHasBoth
  ) AS OneImageHasBoth
FROM (
  SELECT DISTINCT
    m.maxWidth,  m.maxHeight,
    (i.imgWidth IS NOT NULL) AS OneImageHasBoth
  FROM (
    SELECT
      MAX(imgWidth)  AS maxWidth,
      MAX(imgHeight) AS maxHeight
    FROM primary_images
  ) m
    LEFT JOIN primary_images i ON m.maxWidth = i.imgWidth AND m.maxHeight = i.imgHeight
) p
CROSS JOIN (
  SELECT DISTINCT
    m.maxWidth,  m.maxHeight,
    (i.imgWidth IS NOT NULL) AS OneImageHasBoth
  FROM (
    SELECT
      MAX(imgWidth)  AS maxWidth,
      MAX(imgHeight) AS maxHeight
    FROM secondary_images
  ) m
    LEFT JOIN secondary_images i ON m.maxWidth = i.imgWidth AND m.maxHeight = i.imgHeight
) s

Answer 3

You could do this in two queries, it's probably easier to read that way. First query is what you have to get to get the max height and width.

You then can issue the second query which looks like:

SElECT primaryId FROM (
   SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images
    UNION 
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images
) as union_table
WHERE imgWidth = [maxWidth] and imgHeight = [maxHeight];

Where [maxWidth] and [maxHeight] are the two values that you get from previous query. If they belong to the same image ID, you will have query result greater than zero, if not this query will have no result.

If you need to know which id is belong to which table, you could create artificial column (eg source) and your query would become:

SElECT primaryId, source FROM (
   SELECT imgHeight, imgWidth, imgId AS primaryId, 1 as source FROM primary_images
    UNION 
    SELECT imgHeight, imgWidth, primaryId, 2 as source FROM secondary_images
) as union_table
WHERE imgWidth = [maxWidth] and imgHeight = [maxHeight];

Note that there is now artificial column called source. So if your result from query is

primaryId     source
4             1 
4             2
5             2

You know that imgId 4 from primary_images as well as primaryId 4,5 from secondary_images match with the max height and max width of the previous query

And finally, if you just want to know whether there is image that is matching or not, per our comments and discussion below, you could do:

SElECT count(*) AS imgCount FROM (
   SELECT imgHeight, imgWidth, imgId AS primaryId FROM primary_images
    UNION ALL
    SELECT imgHeight, imgWidth, primaryId FROM secondary_images
) as union_table
WHERE primaryId = $imgId and imgWidth = [maxWidth] and imgHeight = [maxHeight];

Where imgCount will be zero if there is no matching image or greater than zero otherwise