Question

我们在Sql Server 2005上有两个相关的表（inventory和inventoryLocalization）。第一个表的脚本。

CREATE TABLE [dbo].[inventory](
    [code] [varchar](35) NOT NULL,
    [salePrice1] [decimal](22, 2) NULL,
 CONSTRAINT [PK_inventory] PRIMARY KEY CLUSTERED 
(
    [code] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

，第二个是;

CREATE TABLE [dbo].[inventoryLocalization](
    [code] [varchar](35) NOT NULL,
    [language] [varchar](2) NOT NULL,
    [name] [nvarchar](100) NOT NULL,
    [description] [nvarchar](max) NULL,
 CONSTRAINT [PK_inventoryLocalization] PRIMARY KEY CLUSTERED 
(
    [code] ASC,
    [language] ASC,
    [name] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

我们有一个查询取决于这两个，如下：

select  i.[code],
        iL.[language], 
        case  
        when iL.[name] is null then (select iL2.name  from dbo.inventoryLocalization iL2 where iL2.[language] = 'de' and iL2.code = i.[code])
        else iL.[name]
        end as [name]
from dbo.inventory i
left join dbo.inventoryLocalization iL on i.code = iL.code and iL.[language] =  'en';

正如您猜测的那样，我们选择[language] ='en'的所有记录，但如果没有'en'记录，则查询采用'de'（默认）。但该查询需要更长的时间（大约13秒）。

你认为任何其他优雅的方法可以减少相同的结果吗？

提前感谢您提供的任何帮助。

Answer 1

可能很慢，因为当case语句评估null并运行子查询时，它必须按需执行查询。

也许这会更快，总是连接到'de'，而不是每次找到null时都进行查询。

select  i.[code],
        iL.[language], 
        case  
        when iL.[name] is null then iL2.name
        else iL.[name]
        end as [name]
from dbo.inventory i
left join dbo.inventoryLocalization iL on i.code = iL.code and iL.[language] =  'en'
left join dbo.inventoryLocalization iL2 on i.code = iL2.[code] and iL2.[language] = 'de'

Answer 2

select i.code
      ,coalesce(en.language, de.language) as language
      ,coalesce(en.name, de.name) as name
  from inventory i
  left join inventoryLocalization en on(i.code = en.code and en.language = 'en')
  left join inventoryLocalization de on(i.code = de.code and de.language = 'de');

Answer 3

试试这个：

select  i.[code],
        iL.[language], 
        coalesce(il.[name],il2.[name],'Unknown') as [name]
from dbo.inventory i
left join dbo.inventoryLocalization iL on i.code = iL.code and iL.[language] =  'en';
left join  dbo.inventoryLocalization iL2 on i.code=il2.code and il2.[language] = 'de'

Sql Server 2005上的查询优化

3 个答案: