TSQL Update Issue - sql-server

Question

Ok SQL Server fans I have an issue with a legacy stored procedure that sits inside of a SQL Server 2008 R2 Instance that I have inherited also with the PROD data which to say the least is horrible. Also, I can NOT make any changes to the data nor the table structures.
So here is my problem, the stored procedure in question runs daily and is used to update the employee table. As you can see from my example the incoming data (#New_Employees) contains the updated data and I need to use it to update the data in the Employee data is stored in the #Existing_Employees table. Throughout the years different formatting of the EMP_ID value has been used and must be maintained as is (I fought and lost that battle). Thankfully, I have been successfully in changing the format of the EMP_ID column in the #New_Employees table (Yeah!) and any new records will use this format thankfully!
So now you may see my problem, I need to update the ID column in the #New_Employees table with the corresponding ID from the #Existing_Employees table by matching (that's right you guessed it) by the EMP_ID columns. So I came up with an extremely hacky way to handle the disparate formats of the EMP_ID columns but it is very slow considering the number of rows that I need to process (1M+).
I thought of creating a staging table where I could simply cast the EMP_ID columns to an INT and then back to a NVARCHAR in each table to remove the leading zeros and I am sort of leaning that way but I wanted to see if there was another way to handle this dysfunctional data. Any constructive comments are welcome.
IF OBJECT_ID(N'TempDB..#NEW_EMPLOYEES') IS NOT NULL
DROP TABLE #NEW_EMPLOYEES
CREATE TABLE #NEW_EMPLOYEES(
ID INT
,EMP_ID NVARCHAR(50)
,NAME NVARCHAR(50))
GO
IF OBJECT_ID(N'TempDB..#EXISTING_EMPLOYEES') IS NOT NULL
DROP TABLE #EXISTING_EMPLOYEES
CREATE TABLE #EXISTING_EMPLOYEES(
ID INT PRIMARY KEY
,EMP_ID NVARCHAR(50)
,NAME NVARCHAR(50))
GO
INSERT INTO #NEW_EMPLOYEES
VALUES(NULL, '00123', 'Adam Arkin')
,(NULL, '00345', 'Bob Baker')
,(NULL, '00526', 'Charles Nelson O''Reilly')
,(NULL, '04321', 'David Numberman')
,(NULL, '44321', 'Ida Falcone')
INSERT INTO #EXISTING_EMPLOYEES
VALUES(1, '123', 'Adam Arkin')
,(2, '000345', 'Bob Baker')
,(3, '526', 'Charles Nelson O''Reilly')
,(4, '0004321', 'Ed Sullivan')
,(5, '02143', 'Frank Sinatra')
,(6, '5567', 'George Thorogood')
,(7, '0000123-1', 'Adam Arkin')
,(8, '7', 'Harry Hamilton')
-- First Method - Not Successful
UPDATE NE
SET ID = EE.ID
FROM
#NEW_EMPLOYEES NE
LEFT OUTER JOIN #EXISTING_EMPLOYEES EE
ON EE.EMP_ID = NE.EMP_ID
SELECT * FROM #NEW_EMPLOYEES
-- Second Method - Successful but Slow
UPDATE NE
SET ID = EE.ID
FROM
dbo.#NEW_EMPLOYEES NE
LEFT OUTER JOIN dbo.#EXISTING_EMPLOYEES EE
ON CAST(CASE WHEN NE.EMP_ID LIKE N'%[^0-9]%'
THEN NE.EMP_ID
ELSE LTRIM(STR(CAST(NE.EMP_ID AS INT))) END AS NVARCHAR(50)) =
CAST(CASE WHEN EE.EMP_ID LIKE N'%[^0-9]%'
THEN EE.EMP_ID
ELSE LTRIM(STR(CAST(EE.EMP_ID AS INT))) END AS NVARCHAR(50))
SELECT * FROM #NEW_EMPLOYEES

Answer 1

the number of rows that I need to process (1M+).
A million employees? Per day?
I think I would add a 3rd table:
create table #ids ( id INT not NULL PRIMARY KEY
, emp_id not NULL NVARCHAR(50) unique );
Populate that table using your LTRIM(STR(CAST, ahem, algorithm, and update Employees directly from a join of those three tables.
I recommend using ANSI update, not Microsoft's nonstandard update ... from because the ANSI version prevents nondeterministic results in cases where the FROM produces more than one row.