Comparando dos conjuntos de datos en SQL

I am wishing to compare two sets of data using SQL wherever possible. Please consider the following data structures:

tbl_users:
+---------+----------+
| user_id |   avatar |
+---------+----------+
|    1    | test.jpg |
|    2    | 1234.jpg |
+---------+----------+

tbl_shortlists:
+------+--------+
|  id  |  owner |
+------+--------+
|   1  |    1   |
|   2  |    2   |
+------+--------+

tbl_shortlist_items:
+---------+--------------+
| item_id | shortlist_id |
+---------+--------------+
|    1    |       1      |
|    2    |       1      |
|    3    |       1      |
|    1    |       2      |
|    2    |       2      |
+---------+--------------+

Deseo seleccionar el tbl_users.user_id y tbl_users.avatar where the shortlist owned by any user contains two or more of the same item_id as the current owner. So, let's assume I am looking up the user_id 1, with the above data structure. Since the Shortlist with the ID shares two items with shortlist_id 1 (i.e. user 1's shortlist), I would like to return user_id = 2 y avatar = 1234.jpg.

I am at a loss as to how to do this in pure SQL. I was thinking that it might be possible to use IN(), but I don't know if that would work.

Here's some pseudo-code to hopefully explain a little better what I would like:

Select user_id and avatar for each shortlist that contains two or more item_ids that are in the shortlist that is owned by user_id = 1.

preguntado el 28 de agosto de 12 a las 14:08

I think that your question is not clear enough, can you explain it different? if you can use sqlfiddle would be great -

I have edited the question to include a brief pseudo-code that should make things a little clearer. -

1 Respuestas

As always with SQL it helps to break it down into small sections. i.e. the first thing you need is all the items in short lists that the current user owns:

SELECT  Item_ID
FROM    tbl_ShortList_Items
        INNER JOIN tbl_ShortLists
            ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
WHERE   Owner = 1 -- CURRENT USER

Next you need all users who have short lists with the same items in. For explanatory purposes I will build up as an IN statement, but an INNER JOIN may perform better.

SELECT  Owner
FROM    tbl_ShortList_Items
        INNER JOIN tbl_ShortLists
            ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
WHERE   Item_ID IN 
        -- RESULTS FROM LAST QUERY START
        (   SELECT  Item_ID
            FROM    tbl_ShortList_Items
                    INNER JOIN tbl_ShortLists
                        ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
            WHERE   Owner = 1 -- CURRENT USER
        )   
        -- RESULTS FROM LAST QUERY END
AND     Owner != 1 -- CURRENT USER

You then need to limit this to those that have 2 or more, by adding a GROUP BY y HAVING

SELECT  Owner
FROM    tbl_ShortList_Items
        INNER JOIN tbl_ShortLists
            ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
WHERE   Item_ID IN 
        -- RESULTS FROM LAST QUERY START
        (   SELECT  Item_ID
            FROM    tbl_ShortList_Items
                    INNER JOIN tbl_ShortLists
                        ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
            WHERE   Owner = 1 -- CURRENT USER
        )   
        -- RESULTS FROM LAST QUERY END
AND     Owner != 1 -- CURRENT USER
GROUP BY Owner
HAVING  COUNT(DISTINCT tbl_ShortList_Items.Item_ID) > 1

Entonces necesitas usar tbl_users to get the avatar for these owners 

SELECT  User_ID, Avatar
FROM    tbl_Users
WHERE   User_ID IN 
        -- RESULTS FROM LAST QUERY START
        (   SELECT  Owner
            FROM    tbl_ShortList_Items
                    INNER JOIN tbl_ShortLists
                        ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
            WHERE   Item_ID IN 
                    (   SELECT  Item_ID
                        FROM    tbl_ShortList_Items
                                INNER JOIN tbl_ShortLists
                                    ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
                        WHERE   Owner = 1 -- CURRENT USER
                    )
            AND     Owner != 1 -- CURRENT USER
            GROUP BY Owner
            HAVING  COUNT(DISTINCT tbl_ShortList_Items.Item_ID) > 1
        )
        -- RESULTS FROM LAST QUERY END

As I said, I think rearranging this to JOINs will be optimised better, but have not tested this theory.

SELECT  User_ID, Avatar
FROM    tbl_Users
        INNER JOIN
        (   SELECT  Owner
            FROM    tbl_ShortList_Items
                    INNER JOIN tbl_ShortLists
                        ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
                    INNER JOIN
                    (   SELECT  Item_ID
                        FROM    tbl_ShortList_Items
                                INNER JOIN tbl_ShortLists
                                    ON tbl_ShortLists.ID = tbl_ShortList_Items.ShortList_ID
                        WHERE   Owner = 1 -- CURRENT USER
                    ) CurrentUserItems
                        ON CurrentUserItems.Item_ID = tbl_ShortList_Items.Item_ID
            WHERE   Owner != 1
            GROUP BY Owner
            HAVING  COUNT(DISTINCT tbl_ShortList_Items.Item_ID) > 1
        ) MatchUsers
            ON MatchUsers.Owner = tbl_Users.User_ID

Respondido 28 ago 12, 15:08

¡Gracias por tu gran respuesta! - Ben M

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