MYSQL：检索每组的最后一条记录并结合另一个表的数据

Question

有一个购买表，其中包含有关产品的信息。 这将为购买的每个单独产品在 tStockMP 表中生成一行或多行。

现在，我需要显示每个库存产品的表格信息。 由于 purchase 表包含更改的历史记录，因此在 tPurchases 表中按 purchase_id 分组时，该信息位于最高的 keyid 中。

我提供了一个完整的脚本，这里有描述我的问题的示例数据。

DROP TABLE IF EXISTS tPurchases;
DROP TABLE IF EXISTS tStockMP;

-- The purchase table
CREATE TABLE tPurchases (
                            keyid                        INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
                            brand                        VARCHAR(255),
                            model                        VARCHAR(255),
                            purchase_id                  INT
                           );

INSERT INTO tPurchases (brand,model,purchase_id) VALUES ("Hp","note1",23);
INSERT INTO tPurchases (brand,model,purchase_id) VALUES ("Lg","IPSLED",45);
INSERT INTO tPurchases (brand,model,purchase_id) VALUES ("Hp","notE1",23);
INSERT INTO tPurchases (brand,model,purchase_id) VALUES ("Bx","BOX",56);
INSERT INTO tPurchases (brand,model,purchase_id) VALUES ("LG","IPSLED",45);
INSERT INTO tPurchases (brand,model,purchase_id) VALUES ("HP","NOTE1",23);


-- The Stock MP Table
CREATE TABLE tStockMP (
                       keyid                        INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
                       purchase_id                  INT,
                       status                       TINYINT
);

INSERT INTO tStockMP (purchase_id,status) VALUES (23,1);
INSERT INTO tStockMP (purchase_id,status) VALUES (23,1);
INSERT INTO tStockMP (purchase_id,status) VALUES (23,0);
INSERT INTO tStockMP (purchase_id,status) VALUES (45,0);
INSERT INTO tStockMP (purchase_id,status) VALUES (56,1);
INSERT INTO tStockMP (purchase_id,status) VALUES (56,1);
INSERT INTO tStockMP (purchase_id,status) VALUES (56,0);

-- Example data
-- 
-- tPurchases table
-- keyid brand model   purchase_id
-- 0     Hp    note1   23
-- 1     Lg    IPSLED  45
-- 2     Hp    notE1   23
-- 3     Bx    BOX     56
-- 4     LG    IPSLED  45
-- 5     HP    NOTE1   23
-- 
-- 
-- tStockMP   table.
-- purchase_id  status
-- 23           1 
-- 23           1
-- 23           0
-- 45           0
-- 56           1
-- 56           1
-- 56           0
-- 
-- 
-- Expected result
-- 
-- purchase_id  status  brand  model  
-- 23           1       HP     NOTE1   
-- 23           1       HP     NOTE1
-- 23           0       HP     NOTE1
-- 45           0       LG     IPSLED
-- 56           1       Bx     BOX
-- 56           1       Bx     BOX
-- 56           0       Bx     BOX


SELECT s.keyid, s.purchase_id, s.status, p.brand, p.model, p.keyid AS purkeyid 
FROM tStockMP AS s, tPurchases AS p 
WHERE s.purchase_id = p.purchase_id;

-- +-------+-------------+--------+-------+--------+----------+
-- | keyid | purchase_id | status | brand | model  | purkeyid |
-- +-------+-------------+--------+-------+--------+----------+
-- |     1 |          23 |      1 | Hp    | note1  |        1 |
-- |     1 |          23 |      1 | Hp    | notE1  |        3 |
-- |     1 |          23 |      1 | HP    | NOTE1  |        6 |-> *
-- |     2 |          23 |      1 | Hp    | note1  |        1 |
-- |     2 |          23 |      1 | Hp    | notE1  |        3 |
-- |     2 |          23 |      1 | HP    | NOTE1  |        6 |-> *
-- |     3 |          23 |      0 | Hp    | note1  |        1 |
-- |     3 |          23 |      0 | Hp    | notE1  |        3 |
-- |     3 |          23 |      0 | HP    | NOTE1  |        6 |-> *
-- |     4 |          45 |      0 | Lg    | IPSLED |        2 |
-- |     4 |          45 |      0 | LG    | IPSLED |        5 |-> *
-- |     5 |          56 |      1 | Bx    | BOX    |        4 |-> *
-- |     6 |          56 |      1 | Bx    | BOX    |        4 |-> *
-- |     7 |          56 |      0 | Bx    | BOX    |        4 |-> *
-- +-------+-------------+--------+-------+--------+----------+

我需要“过滤”结果，以便只有 * 行保留在最终查询中，以免我需要手动完成。 但我不知道如何修改我的查询并实现这一点。

Answer 1

如果您的数据库支持 window 函数，您可以使用ROW_NUMBER()来识别每个组的“最新”记录，并使用该信息来过滤数据集：

SELECT *
FROM (
    SELECT 
        s.keyid, 
        s.purchase_id, 
        s.status, 
        p.brand, 
        p.model, 
        p.keyid AS purkeyid,
        ROW_NUMBER() OVER(PARTITION BY s.keyid ORDER BY p.keyid DESC) rn
    FROM tStockMP AS s
    INNER JOIN tPurchases AS p ON p.purchase_id = s.purchase_id
) t
WHERE rn = 1

Answer 2

切勿在FROM子句中使用逗号。 我典型的解决方案是使用相关子查询：

SELECT s.keyid, s.purchase_id, s.status, p.brand, p.model, p.keyid AS purkeyid 
FROM tStockMP s JOIN
     tPurchases p 
     ON s.purchase_id = p.purchase_id
WHERE p.keyid = (SELECT MAX(p2.keyid)
                 FROM tPurchases p2
                 WHERE p2.purchase_id = p.purchase_id
                );

使用tPurchases(purchase_id, keyid)上的索引，这通常具有最佳性能。

如果我使用 window 函数来解决这个问题，我会将其表述为：

SELECT s.keyid, s.purchase_id, s.status, p.brand, p.model, p.keyid AS purkeyid 
FROM tStockMP s JOIN
     (SELECT p.*,
             ROW_NUMBER() OVER (PARTITION BY purchase_id ORDER BY keyid DESC) as seqnum
      FROM tPurchases p 
     ) p
     ON s.purchase_id = p.purchase_id
WHERE seqnum = 1;

GMB 有另一种方法。 如果您有大量数据，比较这两种方法的性能会很有趣。 对于所有比较，我建议使用与上述相同的索引。

Answer 3

首先在tPurchases中使用NOT EXISTS以仅获取具有最大keyid的行，然后加入tStockMP ：

SELECT s.keyid, s.purchase_id, s.status, t.brand, t.model, t.keyid AS purkeyid 
FROM tStockMP AS s 
INNER JOIN (
  SELECT p.* FROM tPurchases AS p
  WHERE NOT EXISTS (
    SELECT 1 FROM tPurchases
    WHERE purchase_id = p.purchase_id AND keyid > p.keyid
  )
) AS t ON t.purchase_id = s.purchase_id

请参阅演示。
结果：

| keyid | purchase_id | status | brand | model  | purkeyid |
| ----- | ----------- | ------ | ----- | ------ | -------- |
| 1     | 23          | 1      | HP    | NOTE1  | 6        |
| 2     | 23          | 1      | HP    | NOTE1  | 6        |
| 3     | 23          | 0      | HP    | NOTE1  | 6        |
| 4     | 45          | 0      | LG    | IPSLED | 5        |
| 5     | 56          | 1      | Bx    | BOX    | 4        |
| 6     | 56          | 1      | Bx    | BOX    | 4        |
| 7     | 56          | 0      | Bx    | BOX    | 4        |