MySQL vs SQLSERVER语法，用于选择声明的变量

我需要验证尚未输入客户，在mysql中，我可以通过在下面的变量（@V_CustomerID）中选择客户ID来做到这一点。但是，ssms不喜欢该语法，还有另一种方法可以实现此目的吗？

Create procedure AddCustomer(
    @CustomerFirstName as varchar(50),
    @CustomerLastName as varchar(50),
    @SiteName as varchar(50),
    @CustomerPhone1 as varchar(20),
    @CustomerPhone2 as varchar(20),
    @CustomerAddress1 as varchar(20),
    @CustomerAddress2 as varchar(20),
    @CustomerCity as varchar(50),
    @CustomerState as varchar(5),
    @CustomerZip as char(5)
    )
    as
    begin

        declare @V_CustomerID as int,--use this to store the returned value
                @V_CustomerExists as bit --use this to store the result of the if else condition

        --First we should check if the customer data already exists in the table--
            select dbo.customer.CustomerID 
                into @V_CustomerID
            from 
                dbo.Customer
            where 
                dbo.customer.FirstName = @CustomerFirstName and dbo.customer.LastName = @CustomerLastName and dbo.customer.sitename = @SiteName;

        if @V_CustomerID is null

        begin
                --Yay!  we can insert the customer--
                        insert into Customer
                        values(@CustomerFirstName,@CustomerLastName,@SiteName);
        end
        else
        begin
                set @V_CustomerExists = 1 
                print 'Customer already exists';

        end

    end