在多對多行中選擇與輸入完全匹配的行

Question

使用3個表（線程，標記，ThreadTag），我試圖選擇具有查詢中提供的所有標記的線程。在這個例子中，我需要有「聯邦快遞」和「運費」標籤的線程（S），所以預期的結果是2行：線程ID 1和4

線程表

+----------+----------------------------------+ 
| ThreadID |   ThreadTitle   | 
+----------+----------------------------------+ 
|  1 | Can I choose Fedex for shipping? | 
|  2 | Is Shipping Free     | 
|  3 | Can I use a credit card?   | 
|  4 | Does Fedex ship next day?  | 
|  5 | Is Fedex reliable?    | 
+----------+----------------------------------+ 

標籤表

+-------+-------------+ 
| TagID | TagName | 
+-------+-------------+ 
|  1 | shipping | 
|  2 | fedex  | 
|  3 | ups   | 
|  4 | price  | 
|  5 | free  | 
|  6 | credit card | 
+-------+-------------+ 

ThreadTag表

+----------+-------+ 
| ThreadID | TagID | 
+----------+-------+ 
|  1 |  1 | 
|  1 |  2 | 
|  1 |  3 | 
|  2 |  1 | 
|  2 |  5 | 
|  3 |  6 | 
|  4 |  1 | 
|  4 |  2 | 
|  5 |  2 | 
+----------+-------+ 

這裏是我想出了代碼：

WITH ThreadsWithMatchingTags AS (
    SELECT ThreadID 
    FROM ThreadTag tt 
    INNER JOIN Tag t on tt.TagID = t.TagID 
    WHERE TagName IN ('shipping', 'fedex') 
    GROUP BY ThreadID 
    HAVING COUNT(ThreadID) = 2 
) 
SELECT * 
FROM Thread 
WHERE ThreadID IN (SELECT ThreadID FROM ThreadsWithMatchingTags) 

OUTPUT：

+----------+----------------------------------+ 
| ThreadID |   ThreadTitle   | 
+----------+----------------------------------+ 
|  1 | Can I choose Fedex for shipping? | 
|  4 | Does Fedex ship next day?  | 
+----------+----------------------------------+ 

其作品，但有一定有一個更好的辦法。

Answer 1

「更好」是主觀的，我總是比較優化器的輸出，因爲您的解決方案沒有任何問題。

話雖如此，我會做這樣的事情： -

select 
    a.* 
from 
    Thread a 
where 
    (
    select 
     count(*) 
    from 
     Tag  b, 
     ThreadTag c 
    where 
     c.ThreadID = a.ThreadID and 
     b.TagID = c.TagID and 
     b.TagName in ('shipping', 'fedex') 
    ) = 2; 

Answer 2

下面是寫你查詢的其他方式。

SELECT * from Thread th 
WHERE(SELECT COUNT(*) from Tag ta, ThreadTag tt 
WHERE ta.TagID = tt.TagID 
AND tt.ThreadID = th.ThreadID 
AND ta.TagName in ('fedex', 'shipping')) = 2;