How to tell if a value exists in a database in C #

Select a column, such as a user number column

User number to insert
String ll_userid= "xxxxxxxx";

Query whether this number exists
SqlCommand mycmd = new SqlCommand ("Select User number from table where user number = '" +ll_userid+ "'", mycon);

SqlDataReader MYSDR = myCMD. ExecuteReader ();

if (MYSDR. HasRows)
{
There are already records using this number
}
Else
{
This number is not being used
}

MYSDR. Close ();
Mycon. Close ();

----------If the user number column is an integer type, you can also write this

int Ll_userid;

Remove the maximum number value from the current record
SqlCommand mycmd = new SqlCommand ("Select max (user number) from table", mycon);

SqlDataReader MYSDR = myCMD. ExecuteReader ();

It also detects if a record is intended to determine if the table is empty
if (MYSDR. HasRows)
{
Table is not empty, based on the maximum number of queries obtained +1, and then insert a new record
}
Else
{
Table is empty, a default minimum number record is inserted
}

MYSDR. Close ();
Mycon. Close ();

How to tell if a value exists in a database in C #