SQL 在 SP 中选择 maxDate

Question

USE [MyDatabase]
GO
Object:  StoredProcedure [dbo].[SP_MyProcedure]    
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO


CREATE PROCEDURE [dbo].[SP_MyProcedure] 
-- Add the parameters for the stored procedure here
@StartDate  NVARCHAR(19),
@EndDate    NVARCHAR(19)
AS
BEGIN

SET NOCOUNT ON;

 Insert statements for procedure here
DECLARE @FirstQuery nvarchar(1500);
DECLARE @SecondQuery nvarchar(1500);
DECLARE @TSQL nvarchar(4000);



SET @FirstQuery =
    '
    SELECT * FROM OPENQUERY(LinkedServer,
    ''
    SELECT * FROM Server.Table
    WHERE Name IN 
    (SELECT Tagname COLLATE DATABASE_DEFAULT FROM LocalServer.MyServer.dbo."NameList"
    WHERE LOCATION = ''''X'''' AND SOURCE = ''''Y'''') AND TIMESTAMP >= ''''' + @StartDate + ''''' AND TIMESTAMP < ''''' + @EndDate + ''''''')';

我的问题是:如何在代码中包含 max(date) ，以便将 @startDate 与数据库中列的最新可用日期而不是今天的日期进行比较。 您看到的代码工作正常，但是当我选择比数据库中最后一个现有日期晚的日期时，查询会运行一段时间，然后返回未找到列的错误。

预先感谢您的任何建议。

帮助。 以下部分代码不起作用，但我不明白为什么。有没有人有什么建议？谢谢

     -- Insert statements for procedure here
DECLARE @FirstQuery nvarchar(1500);
DECLARE @SecondQuery nvarchar(1500);
DECLARE @TSQL nvarchar(4000);
DECLARE @MaxTimeStamp nvarchar(19);


SET@MaxTimeStamp =
'SELECT MAX(TimeStamp) From OPENQUERY(LinkedServer)'


IF @StartDate <= @MaxTimeStamp
BEGIN

SET @FirstQuery =
    '
    SELECT * FROM OPENQUERY(LinkedServer,
    ''
    SELECT * FROM Server.Table
    WHERE Name IN 
    (SELECT Tagname COLLATE DATABASE_DEFAULT FROM LocalServer.MyServer.dbo."NameList"
    WHERE LOCATION = ''''X'''' AND SOURCE = ''''Y'''') AND TIMESTAMP >= ''''' + @StartDate + ''''' AND TIMESTAMP < ''''' + @EndDate + ''''''')';

END

Answer 1

那么，您可以用子查询替换它:

(select max(timestamp) from NameList)

如果这是一个存储过程，您可能希望将其放入变量中，例如:

declare @MaxTimestamp datetime;
select @MaxTimestamp = max(TimeStamp) from NameList

-- in your query, something like
      coalesce(@StartTime, maxTimeStamp)

如果性能是一个问题，请尝试在 NameList(Location, Source, Timestamp) 上添加索引。