asp.netに次のWebサービスがあります
//setup profile
    [WebMethod]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
    public void SetProfile(string userName, string firstName, string lastName, string imageUrl)
    {
        //create and open connection
        NpgsqlConnection profileConnection = new NpgsqlConnection(ConfigurationManager.ConnectionStrings["PrevueConnString"].ToString());
        profileConnection.Open();
        //create query and command
        string query = "INSERT into \"Users\" (\"FirstName\", \"LastName\", \"ImageUrl\") values(:fname, :lname, :imageUrl) where \"UserName\" = :user";
        NpgsqlCommand profileCommand = new NpgsqlCommand(query, profileConnection);
        profileCommand.Parameters.Add(new NpgsqlParameter("user", DbType.String));
        profileCommand.Parameters.Add(new NpgsqlParameter("fname", DbType.String));
        profileCommand.Parameters.Add(new NpgsqlParameter("lname", DbType.String));
        profileCommand.Parameters.Add(new NpgsqlParameter("imageUrl", DbType.String));
        profileCommand.Parameters[0].Value = userName;
        profileCommand.Parameters[1].Value = firstName;
        profileCommand.Parameters[2].Value = lastName;
        profileCommand.Parameters[3].Value = imageUrl;
        int result = profileCommand.ExecuteNonQuery();
        profileCommand.Dispose();
        profileConnection.Close();
        string json = new JavaScriptSerializer().Serialize(result);
        Context.Response.Clear();
        Context.Response.ContentType = "application/json";
        Context.Response.Flush();
        Context.Response.Write(json);
    }
Web サービスを呼び出すと、次のエラーが発生します。
Npgsql.NpgsqlException: エラー: 42601: "where" またはその近くで構文エラーが発生しました