sql中类型转换涉及的性能差异之convert和cast

五六个表多次连接，数据仅有200多条时

1.用convert把datetime转换成date

WITH Virtual_T AS
                        (
                            SELECT 
                                    a.[ID]
                                    ,a.[CityID]
                                    ,(select CityName from SlCity where ID=a.[CityID]) as CityName
                                    ,(select CityDistrictID from SlCity where ID=a.[CityID]) as CityDistrictID
                                    ,a.[ContractNo]
                                    ,a.[ContractSignedDate]
                                    ,a.[TransactionDate]
                                    ,a.[HousesProcedureDate]
                                    ,a.[Customer]
                                    ,a.[BuildingName]
                                    ,a.[HouseProperties]
                                    ,a.[HousingContractAmount]
                                    ,a.[DealPrice]
                                    ,a.[MarketingPrice]
                                    ,a.[PayType]
                                    ,a.[ExecuteScheduleRatio]
                                    ,a.[PropertyAddress]
                                    ,a.[RoomNo]
                                    ,a.[PropertyType]
                                    ,a.[BuidingType]
                                    ,a.[Decoration]
                                    ,a.[BuidingArea]
                                    ,a.[UnitPrice]
                                    ,a.[ApartmentLayout]
                                    ,a.[Seller]
                                    ,a.[ContactInfo]
                                    ,a.[CreatorID]
                                    ,b.[CreateDate]
                                    ,a.[EditDate]
                                    ,a.[IsDelete]
                                    ,b.PaymentOfWeek
                                    ,b.PaymentSummary
                                    ,b.ExtraHouseAmount
                                    ,b.PaymentOfNextWeek
                                    ,b.ProgressOfWeek
                                    ,b.Remark
                                    ,b.EditDate AS WeekEditDate
                                    ,(select  min(StatDate)  from [dbo].[SlReportWeekDefine] where LiSuMeiWeek =(select lisumeiweek from [SlReportWeekDefine] where StatDate=CONVERT(char(10),b.createDate,120))) as 'start'
                                    ,(select  max(StatDate) from [dbo].[SlReportWeekDefine] where LiSuMeiWeek = (select lisumeiweek from [SlReportWeekDefine] where StatDate=CONVERT(char(10),b.createDate,120)))  as 'end'
                                    ,(select LiSuMeiWeek from [SlReportWeekDefine] where StatDate=CONVERT(char(10),b.createDate,120)) as LiSuMeiWeek
                            FROM [SlReportMortgageDetail] a WITH (NOLOCK)
                            JOIN [SlReportMortgageDetailOfWeek] b ON a.ID=b.DetailID
                            WHERE a.IsDelete=0 AND b.IsDelete=0 
                        ),T AS
                        (
                            SELECT a.*,b.CityDistrictName,ROW_NUMBER() OVER (ORDER BY a.CreateDate desc) AS [RowNumber],(Case LiSuMeiWeek when '200001'then '1' else '0' end) as IsCurrentWeek FROM Virtual_T a 
                            JOIN SlCityDistrict b ON a.CityDistrictID=b.ID
                            WHERE 1=1 
                        )
                        SELECT * FROM T 

用时：

2.用cast把datetime转换成date

    WITH Virtual_T AS
                        (
                            SELECT 
                                    a.[ID]
                                    ,a.[CityID]
                                    ,(select CityName from SlCity where ID=a.[CityID]) as CityName
                                    ,(select CityDistrictID from SlCity where ID=a.[CityID]) as CityDistrictID
                                    ,a.[ContractNo]
                                    ,a.[ContractSignedDate]
                                    ,a.[TransactionDate]
                                    ,a.[HousesProcedureDate]
                                    ,a.[Customer]
                                    ,a.[BuildingName]
                                    ,a.[HouseProperties]
                                    ,a.[HousingContractAmount]
                                    ,a.[DealPrice]
                                    ,a.[MarketingPrice]
                                    ,a.[PayType]
                                    ,a.[ExecuteScheduleRatio]
                                    ,a.[PropertyAddress]
                                    ,a.[RoomNo]
                                    ,a.[PropertyType]
                                    ,a.[BuidingType]
                                    ,a.[Decoration]
                                    ,a.[BuidingArea]
                                    ,a.[UnitPrice]
                                    ,a.[ApartmentLayout]
                                    ,a.[Seller]
                                    ,a.[ContactInfo]
                                    ,a.[CreatorID]
                                    ,b.[CreateDate]
                                    ,a.[EditDate]
                                    ,a.[IsDelete]
                                    ,b.PaymentOfWeek
                                    ,b.PaymentSummary
                                    ,b.ExtraHouseAmount
                                    ,b.PaymentOfNextWeek
                                    ,b.ProgressOfWeek
                                    ,b.Remark
                                    ,b.EditDate AS WeekEditDate
                                    ,(select  min(StatDate)  from [dbo].[SlReportWeekDefine] where LiSuMeiWeek =(select lisumeiweek from [SlReportWeekDefine] where StatDate=cast(b.CreateDate as date))) as 'start'
                                    ,(select  max(StatDate) from [dbo].[SlReportWeekDefine] where LiSuMeiWeek = (select lisumeiweek from [SlReportWeekDefine] where StatDate=cast(b.CreateDate as date)))  as 'end'
                                    ,(select LiSuMeiWeek from [SlReportWeekDefine] where StatDate=cast(b.CreateDate as date)) as LiSuMeiWeek
                            FROM [SlReportMortgageDetail] a WITH (NOLOCK)
                            JOIN [SlReportMortgageDetailOfWeek] b ON a.ID=b.DetailID
                            WHERE a.IsDelete=0 AND b.IsDelete=0 
                        ),T AS
                        (
                            SELECT a.*,b.CityDistrictName,ROW_NUMBER() OVER (ORDER BY a.CreateDate desc) AS [RowNumber],(Case LiSuMeiWeek when '200001'then '1' else '0' end) as IsCurrentWeek FROM Virtual_T a 
                            JOIN SlCityDistrict b ON a.CityDistrictID=b.ID
                            WHERE 1=1 
                        )
                        SELECT * FROM T 

结果用时：

结论：cast性能在转换date类型完爆convert，convert灵活性、类型适用性更广，更强大，所以性能损耗大；cast能满足使用的情况下，不用convert；程序能执行的情况下，避免在sql中使用内置函数