sqlite database combine multiple tables

Question

here i want know the process of combining the multiple tables in the context of username validation and search operation.
i have 2 two tables in my xamarin.froms shared application,

public class patient

public Patient()
{

 } [PrimaryKey, AutoIncrement] public int ID { get; set; } [MaxLength (40)] public string Name { get; set; } [MaxLength(40)] public string Username { get; set; } [MaxLength(40)] public string Password { get; set; } [MaxLength(40)] public string Confirmpassword { get; set; } [MaxLength(40)] public string Email { get; set; } [MaxLength(20)] public int Phoneno { get; set; } [MaxLength(20)] public string Address { get; set; }

////-------------another table --------------////

public class Doctors :IObject
{
public Doctors()
{

 } [PrimaryKey, AutoIncrement] public int ID { get; set; } [MaxLength(25)] public string Name { get; set; } [MaxLength(40)] public string Specialization { get; set; } [MaxLength(40)] public string Username { get; set; } [MaxLength(40)] public string Email { get; set; } [MaxLength(40)] public int Phoneno { get; set; } [MaxLength(20)] public string Password { get; set; } [MaxLength(30)] public string Confirmpassword { get; set; } [MaxLength(30)] public string Address { get; set; }

when iam performing validation it will check both the tables and return the result, if any username and password is already saved it's need to displayed.
and also performing search operation
guide me to write the query for required functionality......