Question

我需要创建动态INNER JOIN（我相信我还需要）。我有4桌梨，苹果，橘子，香蕉。我在这4个表中使用了所有相同的列名。

另外两张桌子'水果'和'水果评论'

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

对于Fruits表，typeid与其他表有关1 =梨，2 =苹果，3 =橙子，4 =香蕉。 Itemid与梨，苹果，橙子或香蕉的id相关。

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

我如何从水果表中选择前10名并动态获取相应表格中的匹配行

工作代码

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

旧的非工作代码：

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
)

类似的东西，但我不知道如何正确表达它。下面的预期结果是通过fruitsid DESC从'fruit'表顺序中选择top（3）。

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

使用UNION ALL使用较新代码

的4行上的IO统计信息

(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.

至少没有一个物理读物很棒

Answer 1

这样做的一种方法是使用UNION加入 -

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

如果您不想包含仅存在Fruits但不存在于PEAR|APPLES|ORACNGES|BANANAS中的记录，请改用<{1}}

如何创建动态INNER JOIN

1 个答案: