INSERT INTO if not exists SQL server

Question

I have a database structured as follows:

users

userid (Primary Key)
username

group

Answer 1

I would first create a stored proc on the db to do the check and insert if necessary:

CREATE PROCEDURE AddNewUserProc
(
@username       VarChar(50) -- replace with your datatype/size
)

AS

    IF NOT EXISTS (SELECT * FROM users WHERE username = @username)
    BEGIN
        INSERT INTO users
        VALUES (@username)
    END

Then a method on the app that will call this procedure

public void AddNewUserMethod(string userName)
{
    SqlConnection connection = new SqlConnection("connection string");
    SqlCommand command = new SqlCommand("AddNewUserProc", connection);

    command.CommandType = CommandType.StoredProcedure;
    command.Parameters.Add("username", SqlDbType.VarChar, 50).Value = userName;

    try
    {
        connection.Open();
        command.ExecuteNonQuery();
    }
    finally
    {
        if (connection.State == ConnectionState.Open) { connection.Close(); }
    }
}

Note leaving this as alternative/historical, but for purpose of correctness the correct way is using the Merge statement, see answer https://stackoverflow.com/a/9649040/167304 or checkout MS doc https://docs.microsoft.com/en-us/sql/t-sql/statements/merge-transact-sql?view=sql-server-ver15