Oracle：左連接非常大的表並將連接的行限制爲最大的值

Question

我有兩個表。第二個通過m_id引用第一個。

主表

M_ID | M_FIELD 
    1 | 'main1' 
    2 | 'main2' 
    3 | 'main3' 

子表

S_ID | S_FIELD | S_ORDER | M_ID 
    1 | 'sub1-1' | 1 | 1 
    2 | 'sub1-2' | 2 | 1 
    3 | 'sub1-3' | 3 | 1 
    4 | 'sub2-1' | 1 | 2 
    5 | 'sub2-2' | 2 | 2 
    6 | 'sub2-3' | 3 | 2 
    7 | 'sub3-1' | 1 | 3 
    8 | 'sub3-2' | 2 | 3 
    9 | 'sub3-3' | 3 | 3 

我需要連接這兩個表（由M_ID），但是從​​我只需要與最大的行值爲S_ORDER。

所以查詢的預期結果是：

M_ID | M_FIELD | S_FIELD 
    1 | 'main1' | 'sub1-3' 
    2 | 'main2' | 'sub2-3' 
    3 | 'main3' | 'sub3-3' 

有這個問題的答案與解析函數工作液：How do I limit the number of rows returned by this LEFT JOIN to one? （我將它張貼在底部） 但問題是那Sub-Table是非常大的（實際上是一些內部計算的視圖），這種子查詢的工作方式太長了。所以，我想我需要通過M_ID 第一後，才篩選出表找到最大S_ORDER

我需要像這樣簡單的東西（因爲第二級子查詢不看M.M_ID從而未能領域場之外）：

CREATE TABLE t_main (m_id NUMBER PRIMARY KEY, 
        m_field VARCHAR2(10)); 
CREATE TABLE t_sub (s_id NUMBER PRIMARY KEY, 
        s_field VARCHAR2(10), 
        s_order NUMBER, 
        m_id NUMBER); 
INSERT INTO t_main VALUES (1,'main1'); 
INSERT INTO t_main VALUES (2,'main2'); 
INSERT INTO t_main VALUES (3,'main3'); 
INSERT INTO t_sub VALUES (1,'sub1-1', 1, 1); 
INSERT INTO t_sub VALUES (2,'sub1-2', 2, 1); 
INSERT INTO t_sub VALUES (3,'sub1-3', 3, 1); 
INSERT INTO t_sub VALUES (4,'sub2-1', 1, 2); 
INSERT INTO t_sub VALUES (5,'sub2-2', 2, 2); 
INSERT INTO t_sub VALUES (6,'sub2-3', 3, 2); 
INSERT INTO t_sub VALUES (7,'sub3-1', 1, 3); 
INSERT INTO t_sub VALUES (8,'sub3-2', 2, 3); 
INSERT INTO t_sub VALUES (9,'sub3-3', 3, 3); 
COMMIT; 

上述（正與LARG太慢提到工作溶液：

SELECT m.*, 
     (SELECT s_field 
     FROM (SELECT s_field 
       FROM t_sub s 
       WHERE s.m_id = m.m_id 
       ORDER BY s_order DESC) 
     WHERE ROWNUM = 1) s_field 
FROM t_main m; 

創建和填充所述測試模式中的代碼ËT_SUB表）：

SELECT m.*, 
     s.s_field 
FROM t_main m 
LEFT JOIN 
    (SELECT * 
    FROM 
    (SELECT ts.*, 
      ROW_NUMBER() OVER (PARTITION BY m_id 
           ORDER BY s_order DESC) AS seq 
     FROM t_sub ts) 
    WHERE seq = 1) s ON s.m_id = m.m_id; 

我們使用的數據庫是Oracle 10g的

非常感謝您的幫助

Answer 1

試試這個

SELECT m.*, 
     (select s.s_field 
      from t_sub s 
     where s.m_id = m.m_id 
      and s.s_order = (select max(s_order) from t_sub where t_sub.m_id = s.m_id) 
      and rownum = 1) 
FROM t_main m 

或者你可以嘗試這（這是你的代碼，但一些修改）

SELECT m.*, 
     (select s.s_field from 
     (SELECT s_field, m_id 
      FROM t_sub 
     --where t_sub.m_id = m.m_id 
     order by s_order DESC) s 
     where s.m_id = m.m_id 
      and rownum = 1) 
FROM t_main m 

Answer 2

select t.*, s.s_field from t_main t 
left join (select m_id, min(s_field) keep(dense_rank first order by s_order desc) as s_field 
     from t_sub group by m_id) s on (s.m_id = t.m_id)