Question

可以重写以下内容以提高效率吗？如果我不需要EXISTS中的字段，我会使用country，但我确实需要这些字段，并且不确定如何编写这些字段以提高效率。

SELECT      distinct 
        p.ProvinceID,
        p.Abbv as RegionCode,
        p.name as RegionName,
        cn.Code as CountryCode,
        cn.Name as CountryName
 FROM         dbo.provinces AS p 

                  INNER JOIN dbo.Countries AS cn ON p.CountryID = cn.CountryID
                  INNER JOIN dbo.Cities c on c.ProvinceID = p.ProvinceID
                  INNER JOIN dbo.Listings AS l ON l.CityID = c.CityID

WHERE l.IsActive = 1 AND l.IsApproved = 1

Answer 1

有两点需要注意：

您加入dbo.Listings会导致许多记录，因此您需要使用DISTINCT（通常是一个昂贵的运营商）
对于列不在选择中的任何表格，您可以移至EXISTS（但查询计划程序无论如何都会有效地为您执行此操作）

所以试试这个：

SELECT       
        p.ProvinceID,
        p.Abbv as RegionCode,
        p.name as RegionName,
        cn.Code as CountryCode,
        cn.Name as CountryName
 FROM  dbo.provinces AS p 
       INNER JOIN 
       dbo.Countries AS cn 
       ON p.CountryID = cn.CountryID
 WHERE EXISTS (SELECT 1 FROM 
               dbo.Listings l 
               INNER JOIN dbo.Cities c 
               on l.CityID = c.CityID
               WHERE  c.ProvinceID = p.ProvinceID
               AND l.IsActive = 1 AND l.IsApproved = 1 
              )

检查之前和之后的查询计划 - 无论如何，查询计划程序可能足够智能，但您已删除了distinct

Answer 2

通过为优化程序提供更多有用信息，以下内容通常会更好：

SELECT       
        p.ProvinceID,
        p.Abbv as RegionCode,
        p.name as RegionName,
        cn.Code as CountryCode,
        cn.Name as CountryName
 FROM  dbo.provinces AS p 
       INNER JOIN 
       dbo.Countries AS cn 
       ON p.CountryID = cn.CountryID
      INNER JOIN (
        SELECT
            p.ProvinceID
        FROM  
           dbo.Listings l 
           INNER JOIN dbo.Cities c 
           on l.CityID = c.CityID
           WHERE l.IsActive = 1 AND l.IsApproved = 1 
        GROUP BY
            p.ProvinceID
      ) list
    on list.ProvinceID = p.ProvinceID

TSQL效率 - INNER JOIN由EXISTS取代

2 个答案: