如何检查值是否已经存在

Question

我试图花很多时间检查值是否存在并且找不到，我需要检查两次：

1. 在注册
2. 在登录

这是我的代码块：

SqlConnection c = new SqlConnection(str);
        SqlCommand cmdUsername = new SqlCommand("SELECT 1 FROM Users WHERE UserName = @userName;", c);
        cmdUsername.Parameters.AddWithValue("userName", userName);
        cmdUsername.CommandType = System.Data.CommandType.Text;
        SqlCommand cmdEmail = new SqlCommand("SELECT 1 FROM Users WHERE Email = @email;", c);
        cmdUsername.Parameters.AddWithValue("email", email);

        c.Open();
        nameExists = (int)cmdUsername.ExecuteScalar();
        emailExists = (int)cmdEmail.ExecuteScalar();
        c.Close();

当我输入电子邮件时，它标记了该行

emailExists = (int)cmdEmail.ExecuteScalar();

并且在登录中一切正常。

请帮我！ 谢谢你们。

Answer 1

假设您要防止重复的用户名或电子邮件，则必须执行以下操作。 1.将ID列设置为INT，然后从列属性中将其设置为Identity列。 2.将“电子邮件”或“用户名”设置为主键，以防止重复发送“电子邮件”或“用户名”。最佳做法是在许多情况下，用户具有相同的名称，但具有唯一的电子邮件，将“电子邮件”列作为主键。

希望有帮助！

并在此处检查用户名或电子邮件是否已经存在！

SqlCommand check_User_Name = new SqlCommand("SELECT COUNT(*) FROM [Table] WHERE ([user] = @user || [email] = @email)" , c);
check_User_Name.Parameters.AddWithValue("@user", txtBox_UserName.Text);
check_User_Name.Parameters.AddWithValue("@email", txtBox_Email.Text);
int UserExist = (int)check_User_Name.ExecuteScalar();

if(UserExist > 0)
{
   //Username exist
}
else
{
   //Username doesn't exist.
}

Answer 2

使用您的代码，对其进行一些优化。

• 只需一次拨打数据库
• 即使发生异常也始终关闭连接
• 未找到或发生异常时返回false，否则返回true

如果这是登录检查，则还可以在密码中添加检查。

public bool ValidData(string username, string email, string connectionString)
{
    var c = new SqlConnection(connectionString);
    var cmdUsername = new SqlCommand("SELECT COUNT(*) FROM Users WHERE UserName = @userName AND email = @email;", c);
    cmdUsername.Parameters.AddWithValue("userName", username);
    cmdUsername.CommandType = System.Data.CommandType.Text;
    cmdUsername.Parameters.AddWithValue("email", email);

    c.Open();
    try
    {
        return (int) cmdUsername.ExecuteScalar() > 0;
    }
    catch (Exception ex)
    {
        //log exception

        return false;
    }
    finally
    {
        c.Close();
    }
}

如果您只需要用户名或电子邮件更改：

    var cmdUsername = new SqlCommand("SELECT COUNT(*) FROM Users WHERE UserName = @userName AND email = @email;", c);

至：

    var cmdUsername = new SqlCommand("SELECT COUNT(*) FROM Users WHERE UserName = @userName OR email = @email;", c);

Answer 3

尝试这个 -

string myConnection=str;//this is your connection string
string userName="";
string email="";
string parameteruserName = "";//your parameter should goes here
string parameterEmail = "";//your parameter should goes here
try
{
    SqlDataReader myReader = null;
    SqlCommand    myCommand = new SqlCommand(SELECT COUNT(*) as count FROM  Users WHERE UserName =" +parameteruserName+" or  Email ="+parameterEmail 
   )  "+  ";", 
                                             myConnection);
    myReader = myCommand.ExecuteReader();
    while(myReader.Read())
    {
      userName=  myReader["count"].ToString();

    }
myReader.close();
myConnection.Close();
}
catch (Exception e)
{
    Console.WriteLine(e.ToString());
}


if(Convert.ToInt32(userName)>0){
//your user name is exists
}