Insert values in second table using triggers - sql-server

Question

I have one table called [FridgeTemperture], when any record inserted it should add one value in the new table MpSensors. But records are not being inserted in the new table when a record is inserted.
Error
Explicit value must be specified for identity column in table
'MpSensors' either identity_insert is set to ON or when a replication
user is inserting into a not for replication identity column.
CREATE TRIGGER [dbo].[FridgeTemperature_INSERT]
ON [dbo].[FridgeTemperture]
AFTER INSERT
AS
BEGIN
SET IDENTITY_INSERT MpSensors ON;
SET NOCOUNT ON;
DECLARE #fridge_temp varchar(10)
INSERT INTO MpSensors(fridge_temp)
VALUES(#fridge_temp)
SET IDENTITY_INSERT MpSensors OFF;
END
GO
table schema
CREATE TABLE [dbo].[MpSensors](
[id] [int] IDENTITY(1,1) NOT NULL,
[fridge_temp] [varchar](10) NULL
) ON [PRIMARY]
CREATE TABLE [dbo].[FridgeTemperture](
[Id] [int] IDENTITY(1,1) NOT NULL,
[ShopId] [nvarchar](4) NULL,
[Fridgetemp] [decimal](4, 2) NOT NULL,
[UpdatedDate] [datetime2](7) NOT NULL
GO

Answer 1

You don't need the set identity_insert on if you are not attempting to insert values to the identity column. Also, your current insert statement, if you loose the set identity_insert, will simply inside a single null row for any insert statement completed successfully on the FridgeTemperture table.
When using triggers, you have access to the records effected by the statement that fired the trigger via the auto-generated tables called inserted and deleted.
I think you are after something like this:
CREATE TRIGGER [dbo].[FridgeTemperature_INSERT]
ON [dbo].[FridgeTemperture]
AFTER INSERT
AS
BEGIN
INSERT INTO MpSensors(fridge_temp)
SELECT CAST(Fridgetemp as varchar(10))
FROM inserted
END
Though I can't really see any benefit of storing the same value in two different places, and in two different data types.
Update
Following our conversation in the comments, you can simply use an update statement in the trigger instead of an insert statement:
UPDATE MpSensors
SET fridge_temp = (
SELECT TOP 1 CAST(Fridgetemp as varchar(10))
FROM inserted
ORDER BY Id DESC
)
This should give you the latest record in case you have an insert statement that inserts more than a single record into the FridgeTemperture table in a single statement.

Answer 2

create TRIGGER [dbo].[FridgeTemperature_INSERT]
ON [dbo].[FridgeTemperture]
AFTER INSERT
AS
BEGIN
UPDATE MpSensors
SET fridge_temp = CAST(Fridgetemp as varchar(10))
FROM inserted
END

Answer 3

You need to use Select statement with CAST as [fridge_temp] is varchar in MpSensors table in Trigger. Try like this:
CREATE trigger <table_name>
ON <table_name>
AFTER Insert
AS
BEGIN
INSERT INTO <table_name>(column_name)
Select CAST(column_name as varchar(10))
FROM inserted
END
The inserted table stores copies of the affected rows during INSERT and UPDATE statements. During an insert or update transaction, new rows are added to both the inserted table and the trigger table. The rows in the inserted table are copies of the new rows in the trigger table.