在MySQL中交叉/外部应用

Question

我需要在MySQL中使用CROSS APPLY(EC2 RDS MySQL实例).看起来MySQL无法识别CROSS APPLY语法.有谁可以帮助我吗？

这是查询.

SELECT ORD.ID
    ,ORD.NAME
    ,ORD.DATE
    ,ORD_HIST.VALUE
FROM ORD
CROSS APPLY (
    SELECT TOP 1 ORD_HISTORY.VALUE
    FROM ORD_HISTORY
    WHERE ORD.ID = ORD_HISTORY.ID
        AND ORD.DATE <= ORD_HISTORY.DATE
    ORDER BY ORD_HISTORY.DATE DESC
    ) ORD_HIST

Answer 1

最接近的直接近似是使用相关子查询作为谓词的连接.

SELECT
   ORD.ID
  ,ORD.NAME
  ,ORD.DATE
  ,ORD_HISTORY.VALUE
FROM
  ORD
INNER JOIN
  ORD_HISTORY
    ON  ORD_HISTORY.<PRIMARY_KEY>
        =
        (SELECT ORD_HISTORY.<PRIMARY_KEY>
           FROM ORD_HISTORY
          WHERE ORD.ID = ORD_HISTORY.ID
            AND ORD.DATE <= ORD_HISTORY.DATE
       ORDER BY ORD_HISTORY.DATE DESC
          LIMIT 1
        )

但是,在您的情况下,您只需要目标表中的一个字段.这意味着您可以直接在SELECT语句中使用相关子查询.

SELECT
   ORD.ID
  ,ORD.NAME
  ,ORD.DATE
  ,(SELECT ORD_HISTORY.VALUE
      FROM ORD_HISTORY
     WHERE ORD.ID = ORD_HISTORY.ID
       AND ORD.DATE <= ORD_HISTORY.DATE
  ORDER BY ORD_HISTORY.DATE DESC
     LIMIT 1
   )   AS VALUE
FROM
  ORD

Answer 2

基于@lujas szozda 的回答：

对于交叉申请：

SELECT ORD.ID
    ,ORD.NAME
    ,ORD.DATE
    ,ORD_HIST.VALUE
FROM ORD JOIN 
LATERAL (
    SELECT ORD_HISTORY.VALUE
    FROM ORD_HISTORY
    WHERE ORD.ID = ORD_HISTORY.ID
        AND ORD.DATE <= ORD_HISTORY.DATE
    ORDER BY ORD_HISTORY.DATE DESC
    LIMIT 1
    ) ORD_HIST ON 1=1
/* ON 1=1 is just a trick to join to whatever the LATERAL returns */

对于外部应用：

SELECT ORD.ID
    ,ORD.NAME
    ,ORD.DATE
    ,ORD_HIST.VALUE
FROM ORD LEFT JOIN 
LATERAL (
    SELECT ORD_HISTORY.VALUE
    FROM ORD_HISTORY
    WHERE ORD.ID = ORD_HISTORY.ID
        AND ORD.DATE <= ORD_HISTORY.DATE
    ORDER BY ORD_HISTORY.DATE DESC
    LIMIT 1
    ) ORD_HIST ON 1=1 
/* ON 1=1 is just a trick to join to whatever the LATERAL returns */

Answer 3

从MySQL 8.0.14开始，您可以使用LATERAL：

现在派生表之前可以使用LATERAL关键字，以指定允许它引用（取决于）同一FROM子句中的先前表的列。用LATERAL指定的派生表只能出现在FROM子句中，可以用逗号分隔的表列表中也可以出现在联接规范中（JOIN，INNER JOIN，CROSS JOIN，LEFT [OUTER] JOIN或RIGHT [OUTER] JOIN） 。横向派生表使某些SQL操作可能无法通过非横向派生表完成，或者需要效率较低的解决方法

CROSS APPLY () <=> ,LATERAL ()
OUTER APPLY () <=> LEFT JOIN LATERAL () ON 1=1

对添加到MySQL 8.0.14的LATERAL派生表的支持

在这种情况下：

SELECT ORD.ID
    ,ORD.NAME
    ,ORD.DATE
    ,ORD_HIST.VALUE
FROM ORD,
LATERAL (
    SELECT ORD_HISTORY.VALUE
    FROM ORD_HISTORY
    WHERE ORD.ID = ORD_HISTORY.ID
        AND ORD.DATE <= ORD_HISTORY.DATE
    ORDER BY ORD_HISTORY.DATE DESC
    LIMIT 1
    ) ORD_HIST