Insert new records only into SQL Table Using VBA - sql-server

Question

I have an Excel workbook with the below code -
Sub Button1_Click()
Dim conn As New ADODB.Connection
Dim iRowNo As Integer
Dim sFirstName, sLastName As String
With Sheets("Sheet1")
'Open a connection to SQL Server
conn.Open "Provider=SQLOLEDB;" & _
"Data Source=server1;" & _
"Initial Catalog=table1;" & _
"User ID=user1; Password=pass1"
'Skip the header row
iRowNo = 2
'Loop until empty cell in CustomerId
Do Until .Cells(iRowNo, 1) = ""
sFirstName = .Cells(iRowNo, 1)
sLastName = .Cells(iRowNo, 2)
'Generate and execute sql statement
' to import the excel rows to SQL Server table
conn.Execute "Insert into dbo.Customers (FirstName, LastName) " & _
"values ('" & sFirstName & "', '" & sLastName & "')"
iRowNo = iRowNo + 1
Loop
MsgBox "Customers imported."
conn.Close
Set conn = Nothing
End With
End Sub
This opens up a connection to my database and inputs the values from the stated columns.
The primary key is an incremental key on the database. The problem is it will copy ALL values.
I'd like to add new rows of data into the Excel Sheet and only insert those rows that don't already exist.
I've tried different methods ('merge', 'if exist', if not exist', etc.) but I can't get it right.
The solution has to be through VBA. Setting up a link using SSMS is not an option.
I understand that it may be possible to use temporary tables and then trigger a procedure which performs the merge but I want to look into that as a last resort. Haven't read up on it yet (making my way through my MS SQL bible book) but I'm hoping it won't be necessary.
---Update from #Kannan's answer---
New portion of VBA -
conn.Execute "IF EXISTS (SELECT 1 FROM dbo.Customers WHERE FirstName = '" & sFirstName & "' and LastName = '" & sLastName & "') " & _
"THEN UPDATE dbo.customers SET WHERE Firstname = '" & sFirstName & "' and LastName = '" & sLastName & "' " & _
"ELSE INSERT INTO dbo.Customers (FirstName, LastName) " & _
"VALUES ('" & sFirstName & "', '" & sLastName & "')"
This returns error 'Incorrect syntax near the keyword 'THEN'.

Answer 1

Your SQL query isn't quite right - there is no THEN in a SQL IF.
Also, you don't need to do anything if it does exist, so just use if not exists.
"IF NOT EXISTS (SELECT 1 FROM dbo.Customers WHERE FirstName = '" & sFirstName & "' and LastName = '" & sLastName & "') " & _
"INSERT INTO dbo.Customers (FirstName, LastName) " & _
"VALUES ('" & sFirstName & "', '" & sLastName & "')"

Answer 2

This should do it for you.
Sub Button_Click()
'TRUSTED CONNECTION
On Error GoTo errH
Dim con As New ADODB.Connection
Dim rs As New ADODB.Recordset
Dim strPath As String
Dim intImportRow As Integer
Dim strFirstName, strLastName As String
Dim server, username, password, table, database As String
With Sheets("Sheet1")
server = .TextBox1.Text
table = .TextBox4.Text
database = .TextBox5.Text
If con.State <> 1 Then
con.Open "Provider=SQLOLEDB;Data Source=" & server & ";Initial Catalog=" & database & ";Integrated Security=SSPI;"
'con.Open
End If
'this is the TRUSTED connection string
Set rs.ActiveConnection = con
'delete all records first if checkbox checked
If .CheckBox1 Then
con.Execute "delete from tbl_demo"
End If
'set first row with records to import
'you could also just loop thru a range if you want.
intImportRow = 10
Do Until .Cells(intImportRow, 1) = ""
strFirstName = .Cells(intImportRow, 1)
strLastName = .Cells(intImportRow, 2)
'insert row into database
con.Execute "insert into tbl_demo (firstname, lastname) values ('" & strFirstName & "', '" & strLastName & "')"
intImportRow = intImportRow + 1
Loop
MsgBox "Done importing", vbInformation
con.Close
Set con = Nothing
End With
Exit Sub
errH:
MsgBox Err.Description
End Sub
Add a reference to:
Microsoft ActiveX Data Objects 2.8 Library
FYI, my sheet looks like this.

Answer 3

You can use sql query like this:
IF Exists ( Select 1 from dbo.customers where Firstname = '" & sFirstName & "' and LastName = '" & sLastName & "' THEN Update dbo.customers set --other columns where Firstname = '" & sFirstName & "' and LastName = '" & sLastName & "'
ELSE Insert into dbo.Customers (FirstName, LastName) " & _
"values ('" & sFirstName & "', '" & sLastName & "')"
Not sure about the syntax for excel and C#, you can correct that similar to this query