Reputation: 25
SQL: Taking Column From a Row Picked By Aggregate Function in View
I have three SQL tables: Release (which represents a release of a movie), Media (which represents the individual pieces of recordable media in those releases; i.e. for Blu-ray/DVD combos, there will be two rows in Media, one Blu-ray and one DVD, that point back to the same row in Release) and MediaType (which defines Blu-ray, DVD, VHS, etc.). There's a one-to-many relationship for Release/Media and MediaType/Media, with Media being on the "many" side of both relationships. I have a view for Release, vRelease, which contains aggregate functions, such as a COUNT that shows how many media are associated with that release. This is what I have for this view so far:
SELECT dbo.Release.ReleaseID
,dbo.Release.Name
,CASE WHEN Release.Compilation = 0 THEN 'No' WHEN Release.Compilation = 1 THEN 'Yes' END AS Compilation
,dbo.Release.Owner
,CASE WHEN Release.LentOut = 0 THEN 'No' WHEN Release.LentOut = 1 THEN 'Yes' END AS LentOut
,COUNT(dbo.Media.ReleaseID) AS NumberOfMedia
,MIN(dbo.Media.MediaID) AS FirstMediaID
,MIN(dbo.MediaType.Name) AS FirstMediaType
FROM dbo.MediaType INNER JOIN
dbo.Media ON dbo.MediaType.MediaTypeID = dbo.Media.MediaTypeID RIGHT OUTER JOIN
dbo.Release ON dbo.Media.ReleaseID = dbo.Release.ReleaseID
GROUP BY dbo.Release.ReleaseID, dbo.Release.Name, dbo.Release.Compilation, dbo.Release.Owner, dbo.Release.LentOut
You'll notice that I've also included two other aggregate columns: FirstMediaID grabs the ID of the media associated with that release that appears first in the Media table (i.e. if a release has two DVDs associated with it, it gets one with the lower ID value). This column on its own isn't useful; what I want to do is then, in turn, get the MediaType that that Media is associated with. In other words, I want a column that shows the MediaType of the first Media that is attached to each Release. The column after that, FirstMediaType, is supposed to do that, but it instead gets the MediaType among all of the Media associated with the Release and picks the one that is alphabetically first - which means that Blu-ray will always be prioritized over DVD (which is fine), but Audio CD will always be prioritized over everything else (which is not fine).
How do I get the FirstMediaType column in this view to get the MediaType of the Media identified in FirstMediaID?
UPDATE: Here are the tables, their columns and some sample rows.
A couple from Release:
+-----------+----------------------------------------+-------+-------------+---------+
| ReleaseID | Name | Owner | Compilation | LentOut |
+-----------+----------------------------------------+-------+-------------+---------+
| 2 | Alice in Wonderland | NULL | 0 | 0 |
| 6 | 4 Film Favorites - Family Comedies | NULL | 1 | 0 |
| 8 | Aladdin | NULL | 0 | 0 |
| 463 | Harry Potter and the Half-Blood Prince | NULL | 0 | 1 |
| 534 | Spirited Away | Ryan | 0 | 0 |
| 571 | The Original Christmas Classics | NULL | 1 | 0 |
+-----------+----------------------------------------+-------+-------------+---------+
Compilation indicates a release that has more than one movie in it.
Corresponding entries in Media:
+---------+-------------+-------------------------------------------------------------------------------------+-----------+
| MediaID | MediaTypeID | Name | ReleaseID |
+---------+-------------+-------------------------------------------------------------------------------------+-----------+
| 2 | 2 | Movie | 2 |
| 3 | 1 | Movie | 2 |
| 12 | 1 | Space Jam; Looney Tunes: Back in Action | 6 |
| 13 | 1 | Funky Monkey; Osmosis Jones | 6 |
| 17 | 3 | Movie | 8 |
| 620 | 1 | Movie | 463 |
| 726 | 1 | Movie | 534 |
| 807 | 1 | Rudolph the Red-Nosed Reindeer; Cricket on the Hearth | 571 |
| 808 | 1 | Frosty the Snowman; Frosty Returns | 571 |
| 809 | 1 | Santa Claus is Comin' to Town!; Mr. Magoo's Christmas Carol; The Little Drummer Boy | 571 |
| 810 | 4 | Tracks 1-7 | 571 |
+---------+-------------+-------------------------------------------------------------------------------------+-----------+
First few in MediaType:
+-------------+--------------+
| MediaTypeID | Name |
+-------------+--------------+
| 1 | DVD Disc |
| 2 | Blu-ray Disc |
| 3 | VHS |
| 4 | Audio CD |
+-------------+--------------+
The corresponding entries in vRelease SHOULD be this:
+-----------+----------------------------------------+-------------+-------+---------+---------------+--------------+----------------+
| ReleaseID | Name | Compilation | Owner | LentOut | NumberOfMedia | FirstMediaID | FirstMediaType |
+-----------+----------------------------------------+-------------+-------+---------+---------------+--------------+----------------+
| 2 | Alice in Wonderland | No | NULL | No | 2 | 2 | Blu-ray Disc |
| 6 | 4 Film Favorites - Family Comedies | Yes | NULL | No | 2 | 12 | DVD Disc |
| 8 | Aladdin | No | NULL | No | 1 | 17 | VHS |
| 463 | Harry Potter and the Half-Blood Prince | No | NULL | Yes | 1 | 620 | DVD Disc |
| 534 | Spirited Away | No | Ryan | No | 1 | 726 | DVD Disc |
| 571 | The Original Christmas Classics | Yes | NULL | No | 4 | 807 | DVD Disc |
+-----------+----------------------------------------+-------------+-------+---------+---------------+--------------+----------------+
But it's actually this:
+-----------+----------------------------------------+-------------+-------+---------+---------------+--------------+----------------+
| ReleaseID | Name | Compilation | Owner | LentOut | NumberOfMedia | FirstMediaID | FirstMediaType |
+-----------+----------------------------------------+-------------+-------+---------+---------------+--------------+----------------+
| 2 | Alice in Wonderland | No | NULL | No | 2 | 2 | Blu-ray Disc |
| 6 | 4 Film Favorites - Family Comedies | Yes | NULL | No | 2 | 12 | DVD Disc |
| 8 | Aladdin | No | NULL | No | 1 | 17 | VHS |
| 463 | Harry Potter and the Half-Blood Prince | No | NULL | Yes | 1 | 620 | DVD Disc |
| 534 | Spirited Away | No | Ryan | No | 1 | 726 | DVD Disc |
| 571 | The Original Christmas Classics | Yes | NULL | No | 4 | 807 | Audio CD |
+-----------+----------------------------------------+-------------+-------+---------+---------------+--------------+----------------+
It's that last one that's the problem.
Upvotes: 0
Views: 76
Answers (4)
Reputation: 25
I ended up finding a simple way to do what I wanted. It isn't as fancy as Used_By_Already's answer (which did end up working, as far as I could tell) and probably breaks a SQL Best Practices rule somewhere, but it's much easier to understand and maintain - at least for my newbie brain.
Since the problem was trying to get the view to use an aggregate column it calculated in a join, I just split the two-step action over two views. vReleasePre has all of the columns I outlined in my original query except for FirstMediaType. vRelease now simply takes all of the columns from vReleasePre and adds FirstMediaType, which takes its value from a join at the end: LEFT OUTER JOIN dbo.vMedia ON dbo.vReleasePre.FirstMediaID = dbo.vMedia.MediaID, where vMedia is a view with all the columns from Media, plus the MediaType column (I already had vMedia lying around).
Since this database is being used in an ASP.NET MVC web application via Entity Framework, and EF has been pretty strange about what it will and won't accept into the data model, I figure that a simple, if roundabout, solution is probably going to be my best option.
vReleasePre:
SELECT dbo.Release.ReleaseID
,dbo.Release.Name
,CASE WHEN Release.Compilation = 0 THEN 'No' WHEN Release.Compilation = 1 THEN 'Yes' END AS Compilation
,dbo.Release.Owner
,CASE WHEN Release.LentOut = 0 THEN 'No' WHEN Release.LentOut = 1 THEN 'Yes' END AS LentOut
,COUNT(dbo.Media.ReleaseID) AS NumberOfMedia
,MIN(dbo.Media.MediaID) AS FirstMediaID
FROM dbo.MediaType INNER JOIN
dbo.Media ON dbo.MediaType.MediaTypeID = dbo.Media.MediaTypeID RIGHT OUTER JOIN
dbo.Release ON dbo.Media.ReleaseID = dbo.Release.ReleaseID
GROUP BY dbo.Release.ReleaseID, dbo.Release.Name, dbo.Release.Compilation, dbo.Release.Owner, dbo.Release.LentOut
vRelease:
SELECT dbo.vReleasePre.ReleaseID
,dbo.vReleasePre.Name
,dbo.vReleasePre.Compilation
,dbo.vReleasePre.Owner
,dbo.vReleasePre.LentOut
,dbo.vReleasePre.NumberOfMedia
,dbo.vMedia.MediaType
FROM dbo.vReleasePre LEFT OUTER JOIN
dbo.vMedia ON dbo.vReleasePre.FirstMediaID = dbo.vMedia.MediaID
Upvotes: 1
Reputation: 35603
for the newbie brain, this is the subquery I used
SELECT
ROW_NUMBER() OVER(PARTITION BY ReleaseID
ORDER BY MediaID) AS rn
, Media.*
FROM dbo.Media
and this is what it does (see the rn column)
| rn | MediaID | MediaTypeID | Name | ReleaseID |
|----|---------|-------------|-------------------------------------------------------------------------------------|-----------|
| 1 | 2 | 2 | Movie | 2 |
| 2 | 3 | 1 | Movie | 2 |
| 1 | 12 | 1 | Space Jam; Looney Tunes: Back in Action | 6 |
| 2 | 13 | 1 | Funky Monkey; Osmosis Jones | 6 |
| 1 | 17 | 3 | Movie | 8 |
| 1 | 620 | 1 | Movie | 463 |
| 1 | 726 | 1 | Movie | 534 |
| 1 | 807 | 1 | Rudolph the Red-Nosed Reindeer; Cricket on the Hearth | 571 |
| 2 | 808 | 1 | Frosty the Snowman; Frosty Returns | 571 |
| 3 | 809 | 1 | Santa Claus is Comin' to Town!; Mr. Magoo's Christmas Carol; The Little Drummer Boy | 571 |
| 4 | 810 | 4 | Tracks 1-7 | 571 |
Now keep only those rows with 1 in the rn column:
| rn | MediaID | MediaTypeID | Name | ReleaseID |
|----|---------|-------------|-------------------------------------------------------|-----------|
| 1 | 2 | 2 | Movie | 2 |
| 1 | 12 | 1 | Space Jam; Looney Tunes: Back in Action | 6 |
| 1 | 17 | 3 | Movie | 8 |
| 1 | 620 | 1 | Movie | 463 |
| 1 | 726 | 1 | Movie | 534 |
| 1 | 807 | 1 | Rudolph the Red-Nosed Reindeer; Cricket on the Hearth | 571 |
Then join just those rows to Releases and MediaType
Bingo
= the wanted result.
Not hard, really not hard. You really will want to learn about those window functions because they can solve heaps of problems.
Upvotes: 0
Reputation: 35603
A very convenient technique that returns whole rows associated with needs such as "First", "Last", "Earliest", "Latest" is to use row_number() over(). Here you want the "first media type", so it is relevant here.
As you will see in the following query joining the [Media] table is replaced with a subquery that includes a row number calculation. Here we partition by ReleaseID and order by MediaID, so, for each ReleaseID the first row will be the one with the lowest MediaID value. Then in the join to this derived table an extra condition is added to only consider rows with a row number of 1.
Proposed Query
SELECT
r.ReleaseID
, m.MediaID
, mt.MediaTypeID
, mt.name MediaName
, r.Name
, CASE
WHEN r.Compilation = 0 THEN 'No'
WHEN r.Compilation = 1 THEN 'Yes'
END AS compilation
, r.Owner
, CASE
WHEN r.LentOut = 0 THEN 'No'
WHEN r.LentOut = 1 THEN 'Yes'
END AS lentout
FROM dbo.Release r
INNER JOIN (
SELECT
Media.*
, ROW_NUMBER() OVER(PARTITION BY ReleaseID
ORDER BY MediaID) AS rn
FROM dbo.Media
) m ON r.ReleaseID = m.ReleaseID and rn = 1
INNER JOIN dbo.MediaType mt ON m.MediaTypeID = mt.MediaTypeID
Result
| ReleaseID | MediaID | MediaTypeID | MediaName | Name | compilation | Owner | lentout |
|-----------|---------|-------------|--------------|----------------------------------------|-------------|--------|---------|
| 2 | 2 | 2 | Blu-ray Disc | Alice in Wonderland | No | (null) | No |
| 6 | 12 | 1 | DVD Disc | 4 Film Favorites - Family Comedies | Yes | (null) | No |
| 8 | 17 | 3 | VHS | Aladdin | No | (null) | No |
| 463 | 620 | 1 | DVD Disc | Harry Potter and the Half-Blood Prince | No | (null) | Yes |
| 534 | 726 | 1 | DVD Disc | Spirited Away | No | Ryan | No |
| 571 | 807 | 1 | DVD Disc | The Original Christmas Classics | Yes | (null) | No |
Upvotes: 0
Reputation: 964
The easiest way would be to add another join to your MediaType table on FirstMediaId = MediaType.MediaId
;WITH data AS (
SELECT dbo.Release.ReleaseID
,dbo.Release.Name
,CASE WHEN Release.Compilation = 0 THEN 'No' WHEN Release.Compilation = 1 THEN 'Yes' END AS Compilation
,dbo.Release.Owner
,CASE WHEN Release.LentOut = 0 THEN 'No' WHEN Release.LentOut = 1 THEN 'Yes' END AS LentOut
,COUNT(dbo.Media.ReleaseID) AS NumberOfMedia
,MIN(dbo.Media.MediaID) AS FirstMediaID
FROM dbo.MediaType
INNER JOIN dbo.Media
ON dbo.MediaType.MediaTypeID = dbo.Media.MediaTypeID
RIGHT OUTER JOIN dbo.Release
ON dbo.Media.ReleaseID = dbo.Release.ReleaseID
GROUP BY dbo.Release.ReleaseID, dbo.Release.Name, dbo.Release.Compilation, dbo.Release.Owner, dbo.Release.LentOut
)
SELECT data.ReleaseId
,data.Name
,data.Compilation
,data.Owner
,data.LentOut
,data.NumberOfMedia
,data.FirstMediaId
,MediaType.Name as FirstMediaName
FROM data
LEFT OUTER JOIN dbo.MediaType
ON data.FirstMediaId = MediaType.MediaTypeId
Upvotes: 0
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