SQL：3個自我聯接，然后將它們聯接在一起

Question

我有2個表以特定方式加入。 我認為我的查詢是正確的，但不確定。

select t1.userID, t3.Answer Unit, t5.Answer Demo  
FROM    
    table1 t1  
    inner join (select * from table2) t3 ON t1.userID = t3.userID   
    inner join (select * from table2) t5 ON t1.userID = t5.userID  
where  
    NOT EXISTS (SELECT * FROM table1 t2 WHERE t2.userID = t1.userID AND t2.date > t1.date)  
    and NOT EXISTS (SELECT * FROM table2 t4 WHERE t4.userID = t3.userID and t4.counter > t3.counter)  
    and NOT EXISTS (SELECT * FROM table2 t6 WHERE t6.userID = t5.userID and t6.counter > t5.counter)  
    and t1.date_submitted >'1/1/2009'  
    and t3.question = Unit  
    and t5.question = Demo  
order by  
    t1.userID  

從表1我想要日期> 1/1/2009的不同的userID

table1      
userID    Date
1         1/2/2009  
1         1/2/2009  
2         1/2/2009  
3         1/2/2009  
4         1/1/2008  

所以我想要的結果來自table1應該是這樣的：

userID  
1  
2  
3  

然后，我想在userID和table2上加入它，如下所示：

table2 
userID    question   answer   counter
1         Unit       A        1
1         Demo       x        1
1         Prod       100      1
2         Unit       B        1
2         Demo       Y        1
3         Prod       100      1
4         Unit       A        1
1         Unit       B        2
1         Demo       x        2
1         Prod       100      2
2         Unit       B        2
2         Demo       Z        2
3         Prod       100      2
4         Unit       A        2

我想將table1與table2結合在一起，結果如下：

userID    Unit    Demo
1         B       X
2         B       Z

換一種說法，
從表2中選擇不同的用戶ID，其中問題=最高計數器的單位
接着
從table2中選擇不同的用戶ID，其中問題=最高計數器的演示。

我認為我要做的是3個自我聯接，然后將這3個聯接在一起。

你認為對嗎？

Answer 1

SELECT  du.userID, unit.answer, demo.answer
FROM    (
        SELECT  DISTINCT userID
        FROM    table1
        WHERE   date > '1/1/2009'
        ) du
LEFT JOIN
        table2 unit
ON      (userID, question, counter) IN
        (
        SELECT  du.userID, 'Unit', MAX(counter)
        FROM    table2 td
        WHERE   userID = du.userID
                AND question = 'Unit'
        )
LEFT JOIN
        table2 demo
ON      (userID, question, counter) IN
        (
        SELECT  du.userID, 'Demo', MAX(counter)
        FROM    table2 td
        WHERE   userID = du.userID
                AND question = 'Demo'
        )

在table2 (userID, question, counter)上有一個索引table2 (userID, question, counter)將大大改善此查詢。

自從您提到SQL Server 2005以來，以下內容將變得更加容易和高效：

SELECT  du.userID,
        (
        SELECT  TOP 1 answer
        FROM    table2 ti
        WHERE   ti.user = du.userID
                AND ti.question = 'Unit'
        ORDER BY
                counter DESC
        ) AS unit_answer,
        (
        SELECT  TOP 1 answer
        FROM    table2 ti
        WHERE   ti.user = du.userID
                AND ti.question = 'Demo'
        ORDER BY
                counter DESC
        ) AS demo_answer
FROM    (
        SELECT  DISTINCT userID
        WHERE   date > '1/1/2009'
        FROM    table1
        ) du

匯總：

SELECT  answer, COUNT(*)
FROM    (
        SELECT  DISTINCT userID
        FROM    table1
        WHERE   date > '1/1/2009'
        ) du
JOIN    table2 t2
ON      t2.userID = du.userID
        AND t2.question = 'Unit'
GROUP BY
        answer