update transaction table data with new table id - sql

Question

I have 3 tables with following structure and data.
CREATE TABLE [dbo].[Department]
(
[Id] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](50) NOT NULL
)
CREATE TABLE [dbo].[Department_New]
(
[Id] [int] IDENTITY(1,1) NOT NULL,
[Name] [nvarchar](50) NOT NULL
)
CREATE TABLE [dbo].[Employee]
(
[Id] [int] IDENTITY(1,1) NOT NULL,
[DepId] [int] NOT NULL,
[Name] [nvarchar](50) NOT NULL
)
SET IDENTITY_INSERT [dbo].[Department] ON
INSERT [dbo].[Department] ([Id], [Name])
VALUES (1, N'HR'), (2, N'IT'), (3, N'Account'), (4, N'Finance'), (5, N'Software')
SET IDENTITY_INSERT [dbo].[Department] OFF
SET IDENTITY_INSERT [dbo].[Department_New] ON
INSERT [dbo].[Department_New] ([Id], [Name])
VALUES (1, N'IT'), (2, N'HR'), (3, N'Software'), (4, N'Account'),
(5, N'iDontKnow'), (6, N'Finance')
SET IDENTITY_INSERT [dbo].[Department_New] OFF
SET IDENTITY_INSERT [dbo].[Employee] ON
INSERT [dbo].[Employee] ([Id], [DepId], [Name])
VALUES (1, 1, N'abc'), (2, 2, N'zxc'), (3, 1, N'xcv'),
(4, 3, N'cvb'), (5, 4, N'vbn'), (6, 1, N'bnm'),
(7, 3, N'asd'), (8, 3, N'sdf'), (9, 2, N'dfg'),
(10, 4, N'fgh'), (11, 5, N'ghj'), (12, 5, N'hjk'),
(13, 6, N'jkl'), (14, 6, N'qwe'), (15, 2, N'wre'),
(16, 3, N'ert'), (17, 6, N'rty'), (18, 1, N'tyu')
SET IDENTITY_INSERT [dbo].[Employee] OFF
As of now Employee table has old depId. I just want to update with new depId. I can not write for loop because employee table contains more than 1,000,000 rows of data.
What I have tried
CREATE PROCEDURE UpdateDepId
AS
BEGIN
DECLARE #totalRecords INT
DECLARE #I INT
SELECT #I = 1
SELECT #totalRecords = COUNT(DISTINCT DepId) FROM Employee
DECLARE #DPID INT
DECLARE #Id INT
--drop table #Temp
IF OBJECT_ID('tempdb.dbo.#Temp', 'U') IS NOT NULL
DROP TABLE #Temp;
CREATE TABLE #Temp
(
Id INT IDENTITY(1,1),
DepId INT
)
INSERT INTO #Temp
SELECT DISTINCT DepId
FROM Employee
ORDER BY DepId
WHILE (#I <= #totalRecords)
BEGIN
SET #DPID = (SELECT DepId FROM #Temp WHERE Id = #I)
EXEC UpdateDepByDepId #DPID
SELECT #I = #I + 1
END
DROP TABLE #Temp
END
CREATE PROCEDURE UpdateDepByDepId
#DepId INT
AS
BEGIN
DECLARE #DepIdNew INT
SET #DepIdNew = (SELECT Id FROM Department_New
WHERE Name IN (SELECT Name FROM Department WHERE id = #DepId))
SELECT #DepIdNew AS NewDepId, #DepId AS OldDepId
IF ISNULL(#DepIdNew, 0) <> 0
BEGIN
UPDATE Employee
SET DepId = #DepIdNew
WHERE DepId = #DepId
END
END
But it is not working for me. Please help

Answer 1

Use an update and from:
update e
set depid = dn.id
from employee e join
department d
on e.depid = d.id join
department_new dn
on dn.name = d.name;