时间戳

首页 > 解决方案 > 避免用于派生选择中的列的多个重复子查询

sql-server - 避免用于派生选择中的列的多个重复子查询

问题描述

我有一个由多个子查询组成的视图，这些子查询用于派生选择列表中的列（为了简单起见，我没有指定所有子查询）。我的问题是，编写这样一个包含这么多子查询的查询是完全可以的，还是有更好的方法来重写它以避免它们……任何可以遵循的最佳实践。我尝试查看执行派生查询或 cte 的选项，但由于某种原因，我无法将这部分放在一起。如果可能，我想消除那些重复的子查询。

  SELECT a.id,
   (
     SELECT TOP 1
      name
     FROM x.dbo.Info l
     WHERE orderno = a.orderno
       AND releaseno = a.releaseno
       AND stamp =
       (
       SELECT MIN(stamp)
       FROM x.dbo.Info
       WHERE orderno = l.orderno
         AND releaseno = l.releaseno
         AND status = 'Released'
       )
     ORDER BY stamp DESC
   ) [shop_name],
   c.line_no,
   a.status,
   d.family,
   (
     SELECT TOP 1
      name
     FROM x.dbo.Info
     WHERE orderno = a.orderno
       AND releaseno = a.releaseno
       AND status NOT LIKE 'backflus%'
       AND status NOT LIKE 'so%'
     ORDER BY stamp DESC
   ) AS [lastworkplace],
   (
     SELECT TOP 1
      lstatus
     FROM x.dbo.Info
     WHERE orderno = a.orderno
       AND releaseno = a.releaseno
       AND status NOT LIKE 'backflus%'
       AND status NOT LIKE 'so%'
     ORDER BY stamp DESC
   ) AS [laststatus]
FROM BI.dbo.tblz a -- this is a view (not sure if that matters)
  LEFT JOIN X.dbo.tblx b
    ON b.id = a.salesorder
  LEFT JOIN X.dbo.tbls c
    ON c.tranid = a.salesorder
     AND c.itemid = a.assemblyid
     AND c.serialnum = a.ordercode
  LEFT JOIN Z.dbo.tbli d
    ON d.prodline = LEFT(COALESCE(STUFF(a.assemblyid, CHARINDEX('+', a.assemblyid), 1, ''), a.assemblyid), 2)
WHERE a.id = 'p'
  AND
  (
    LEFT(a.prun, 8) >= '20120101'
    OR a.prun IS NULL
  )
UNION ALL
SELECT a.id,
   (
     SELECT TOP 1
      name
     FROM x.dbo.Info l
     WHERE orderno = a.orderno
       AND releaseno = a.releaseno
       AND stamp =
       (
       SELECT MIN(stamp)
       FROM x.dbo.Info
       WHERE orderno = l.orderno
         AND releaseno = l.releaseno
         AND status = 'Released'
       )
     ORDER BY stamp DESC
   ) [shop_name],
   c.line_no,
   a.status,
   d.family,
   (
     SELECT TOP 1
      name
     FROM x.dbo.Info
     WHERE orderno = a.orderno
       AND releaseno = a.releaseno
       AND status NOT LIKE 'backflus%'
       AND status NOT LIKE 'so%'
     ORDER BY stamp DESC
   ) AS [lastworkplace],
   (
     SELECT TOP 1
      lstatus
     FROM x.dbo.Info
     WHERE orderno = a.orderno
       AND releaseno = a.releaseno
       AND status NOT LIKE 'backflus%'
       AND status NOT LIKE 'so%'
     ORDER BY stamp DESC
   ) AS [laststatus]
FROM BI.dbo.tblz a -- this is a view (not sure if that matters)
  LEFT JOIN X.dbo.tblx b
    ON b.id = a.salesorder
  LEFT JOIN X.dbo.tbls c
    ON c.tranid = a.salesorder
     AND c.itemid = a.assemblyid
     AND c.serialnum = a.ordercode
  LEFT JOIN Z.dbo.tbli d
    ON d.prodline = LEFT(COALESCE(STUFF(a.assemblyid, CHARINDEX('+', a.assemblyid), 1, ''), a.assemblyid), 2)
WHERE a.id = 'm'
  AND
  (
    LEFT(a.prun, 8) >= '20120101'
    OR a.prun IS NULL
  );

标签： sql-servertsqlsql-server-2005sql-tuning

解决方案

您可以使用CTE重写您的选择。它更具可读性。引用文档：

指定一个临时命名结果集，称为公用表表达式 (CTE)。这源自一个简单的查询，并在单个 SELECT、INSERT、UPDATE 或 DELETE 语句的执行范围内定义。该子句也可以在 CREATE VIEW 语句中用作其定义的 SELECT 语句的一部分。公共表表达式可以包含对自身的引用。这称为递归公用表表达式。

只是一个简单的示例：

WITH 
 step1 as 
  ( select a+1 as x, b-1 as y
    from t
  ),
 step2 as
  ( select x*2 as i, y/2 as j
    from step1
  )
select i+j as r
from step2;

您可以以这种方式链接多个句子。

推荐阅读