Question

I need to create dynamic INNER JOIN (I believe I need to anyway). I have 4 tables pears, apples, oranges, bananas. I used all the same column names in those 4 tables.

Another 2 tables ‘fruit’ and ‘fruitcomments’

pears    apples    oranges   bananas
-------------------------------------
id
user
photo
comment
date

For the Fruits table typeid relates to other tables 1=pear, 2=apples, 3=oranges, 4=banana.
Itemid correlates to the id of either pears, apples, oranges or bananas.

Fruit 
-----------------------------------------------------------------------------
fruitsid
typeid
itemid

How would I go about selecting the top 10 from the fruit table and dynamically get the matching row from the corresponding table

Working Code

SELECT TOP (10) fruitId, id, user, photo, comment FROM 
(
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN PEARS T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=1
UNION ALL
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN APPLES T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=2
UNION ALL
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN ORANGES T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=3
UNION ALL 
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN BANANAS T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=4
)a
ORDER BY fruitId DESC

OLD NON-Working Code:

SELECT TOP (10) id, user, photo, comment FROM
(

SELECT f.typeid, f.itemid, a.id, a.user, a.photo, a.comment FROM Fruit f INNER JOIN a on f.item=a.id

--excuse the gibberish. I never used CASE before
case when f.typeid=1 then
(SELECT id, user, photo, comment FROM Pears WHERE id=f.itemid)a
case when f.typeid=2 then
(SELECT id, user, photo, comment FROM Apples WHERE id=f.itemid)a
case when f.typeid=3 then
(SELECT id, user, photo, comment FROM Oranges WHERE id=f.itemid)a
case when f.typeid=4 then
(SELECT id, user, photo, comment FROM Bananas WHERE id=f.itemid)a
)

Something like that, but I dont know how to express it correctly. Expected results below by selecting top (3) from following ‘fruit’ table order by fruitsid DESC.

fruits
-----------
fruitsid   typeid     itemid
22         1          19
23         3          73
24         2          46

pears
--------------
id     user     photo    comment    date
19     tom      1.jpg    hi         2012-06-01 12:00:00.000
22     bill     5.jpg    hello      2012-06-01 13:00:00.000

apples
--------------
id     user     photo    comment
46     sam      78.jpg   howdy
22     bill     5.jpg    hello

bananas
--------------
id     user     photo    comment
32     tom      1.jpg    hi
73     bill     5.jpg    hello

oranges
--------------
id     user     photo    comment
73     jane     55.jpg   wave
22     bill     5.jpg    hello

results
-------------
19     tom      1.jpg    hi 
73     bill     5.jpg    hello
46     sam      78.jpg   howdy

IO Stats on 4 rows using UNION ALL with newer code

(4 row(s) affected)
Table 'pears'. Scan count 1, logical reads 1, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'fruit'. Scan count 3, logical reads 3, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'apples'. Scan count 1, logical reads 16, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'oranges'. Scan count 1, logical reads 115, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

At least there wasn’t one physical read which is awesome

Answer 1

One way of doing it is by using UNION to join each –

SELECT TOP (10) fruitId, id, user, photo, comment FROM 
(
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN PEARS T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=1
UNION ALL
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN APPLES T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=2
UNION ALL
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN ORANGES T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=3
UNION ALL
    SELECT T1.fruitsId, T2.id, T2.user, T2.photo, T2.comment 
    FROM FRUITS T1 LEFT JOIN BANANAS T2 ON (T1.itemId=T2.id)
    WHERE T1.typeId=4
)
ORDER BY fruitId DESC

If you dont want to include records which exists only in Fruits but not in PEAR|APPLES|ORACNGES|BANANAS then use INNER JOIN instead