SQL联接和排除

假设T-SQL中的查询也是这样

   SELECT *  
     FROM Stock S    
LEFT JOIN StockBarcode SB ON SB.StockID = S.StockID
                         AND SB.ShopID = @ShopID 
                         AND SB.Inactive = 0    
LEFT JOIN StockBarcode SB1 ON SB1.StockID = S.StockID
                          AND SB1.ShopID = 0 
                          AND SB1.Inactive = 0    
    WHERE S.StockID = @StockID

我的理解是，我可以重写查询本身

   SELECT * 
     FROM Stock S    
LEFT JOIN StockBarcode SB ON S.StockID = SB.StockID 
                         AND SB.ShopID = @ShopID    
LEFT JOIN StockBarcode SB1 ON S.StockID = SB1.StockID 
                          AND SB1.ShopID = 0     
    WHERE S.StockID = @StockID 
      AND ISNULL(SB.Inactive, 0) = 0 
      AND ISNULL(SB1.Inactive, 0) = 0

…结果是一样的。（如果我错了，请纠正我） 哪个是最佳查询？为什么？如果我使用另一个数据库引擎，比如MySql，情况会有所不同吗

提前感谢您的回答：-）

编辑： 为了澄清，这里是目前的整个问题，如果这有帮助的话

SELECT 
    SSCLRU.SupplierCode,  
    S.[Description],
    S.TaxRate AS GSTRate,
    ISNULL(ISNULL(SB.PackPrice, SB1.PackPrice), S.RRP) AS Price,
    ISNULL(SB.PackSize, SB1.PackSize) AS Quantity,
    ISNULL(SB.SalePrice, SB1.SalePrice) AS SalePrice,
    ISNULL(SB.SaleDateFrom, SB1.SaleDateFrom) AS SalePriceStartDate,
    ISNULL(SB.SaleDateTo, SB1.SaleDateTo) AS SalePriceEndDate
FROM Stock S

LEFT JOIN StockSupplierCodePreferredLastReceivedUnique SSCLRU ON
S.StockID = SSCLRU.StockID

LEFT JOIN StockBarcode SB ON
S.StockID = SB.StockID AND
SB.ShopID = @ShopID AND
SB.Inactive = 0

LEFT JOIN StockBarcode SB1 ON
S.StockID = SB1.StockID AND
SB1.ShopID = 0 AND
SB1.Inactive = 0

WHERE S.StockID = @StockID 

哪个是最佳查询？为什么

查看查询计划

但我打赌第一个应该更有效，因为索引可以覆盖

SB.Inactive=0

条件

如果我使用另一个数据库引擎，比如MySql，情况会有所不同吗

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当然，执行计划和性能严格取决于供应商。

第一个更清楚，因为您不必担心WHERE中不匹配的行

然而，我可能会使用这个构造来完全分离连接和过滤条件

FROM
   Stock S
   LEFT JOIN
   StockSupplierCodePreferredLastReceivedUnique SSCLRU ON S.StockID = SSCLRU.StockID

   LEFT JOIN
   (
    SELECT StockID, ...
    FROM StockBarcode
    WHERE ShopID = @ShopID AND Inactive = 0
   ) SB ON S.StockID = SB.StockID

   LEFT JOIN
   (
    SELECT StockID, ...
    FROM StockBarcode
    WHERE ShopID = 0 AND Inactive = 0
   ) SB1 ON S.StockID = SB1.StockID
WHERE
   S.StockID = @StockID 

派生表也可以推送到CTE（或2）中。

它们不完全相同，除非StockBarcode.Inactive不可为空

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如果StockBarcode.Inactive可为空，则第一次查询将不会返回Inactive为空的StockBarcode的任何详细信息（因为它们未通过联接条件），而第二个查询将包括它们，如果它们匹配其他连接条件-它们将匹配

where

条件。

第一个查询看起来会快一点，但是如果您想要真实的统计信息。。。查看探查器跟踪和执行计划