在 SELECT 语句中使用参数化函数调用。 SQL 服务器

Question

我从以前的开发人员那里接手了一些代码，并且遇到了这个调用多个 SQL 函数的 SQL 语句。如您所见，select 语句中的函数调用将参数传递给函数。 SQL 语句如何知道用什么值替换变量？对于下面的示例，查询引擎如何知道在调用时用什么替换 nDeptID，fn_SelDeptName_DeptID(nDeptID)

nDeptID是表Note中的一列。

选择语句：

SELECT nCustomerID AS [Customer ID], 
 nJobID AS [Job ID], 
 dbo.fn_SelDeptName_DeptID(nDeptID) AS Department, 
 nJobTaskID AS JobTaskID, 
 dbo.fn_SelDeptTaskDesc_OpenTask(nJobID, nJobTaskID) AS Task, 
 nStandardNoteID AS StandardNoteID, 
 dbo.fn_SelNoteTypeDesc(nNoteID) AS [Note Type], 
 dbo.fn_SelGPAStandardNote(nStandardNoteID) AS [Standard Note], 
 nEntryDate AS [Entry Date], 
 nUserName as [Added By], 
 nType AS Type, 
 nNote AS Note FROM Note 
 WHERE nJobID = 844261 
 ORDER BY nJobID, Task, [Entry Date]

=======================

Function fn_SelDeptName_DeptID:

ALTER FUNCTION [dbo].[fn_SelDeptName_DeptID] (@iDeptID int)
RETURNS varchar(25)

-- Used by DataCollection for Job Tracking
-- if the Deptartment isnt found return an empty string

BEGIN 
 -- Return the Department name for the given DeptID.
 DECLARE @strDeptName varchar(25)

 IF @iDeptID = 0 
  SET @strDeptName = ''

 ELSE
 BEGIN
  SET @strDeptName = (SELECT dName FROM Department WHERE dDeptID = @iDeptID)
  IF (@strDeptName IS NULL) SET @strDeptName = ''
 END

 RETURN @strDeptName

END

===========================

提前致谢。

Answer 1

当你说SELECT nDeptID FROM Note 时，它知道如何返回列值。查询编译，名称绑定元数据，执行计划，获取结果。

但我想指出，在外连接上使用标量函数而不是 COALESCE 会让人难以置信。这是 SQL，而不是程序命令式的普通 C 派生语言。

为了性能，请改为这样做：

SELECT nCustomerID AS [Customer ID], 
 nJobID AS [Job ID], 
 COALESCE(d.Name, "") AS Department, 
 ...
 FROM Note 
 LEFT JOIN Departments d ON Note.nDeptID = d.nDeptID
 WHERE nJobID = 844261 
 xORDER BY nJobID, Task, [Entry Date]

我很确定这同样适用于您查询中的所有其他标量函数。