Using parameterized function calls in SELECT statements. SQL Server

Question

I have taken over some code from a previous developer and have come across this SQL statement that calls several SQL functions. As you can see, the function calls in the select statement pass a parameter to the function. How does the SQL statement know what value to replace the variable with? For the below sample, how does the query engine know what to replace nDeptID with when it calls, fn_SelDeptName_DeptID(nDeptID)

nDeptID IS a column in table Note.

SELECT STATEMENT:

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

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

Thanks in advance.

Answer 1

Same way it knows how to return a column value when you say SELECT nDeptID FROM Note. Query is compiled, names are bound to metadata, plan is executed, results are obtained.

But I would like to address that using a scalar function instead of a COALESCE on ana outer join is going to suck beyond believe. This is SQL, not a procedural imperative run-of-the-mill-C-derivate language.

Do this instead, for sake of performance:

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]

I'm pretty sure same applies to every other scalar function in your query.