Closing SQLite with C# in Unity

I'm making an app for Android with Unity now.

But I have very very basic questions about SQLite because I've never used database yet.

dbcmd.CommandText = "some select command here";

IDataReader reader = dbcmd.ExecuteReader();

// some jobs with the reader..


dbcmd.CommandText = "another select command here";

reader = dbcmd.ExecuteReader();

// some jobs with the reader..


reader.Close();
dbcmd.Dispose();
dbconn.Close();

reader = null;
dbcmd = null;
dbconn = null;

I need to call ExecuteReader() method several times like that.

Here are my questions.

1. Do I have to close the reader before I call ExecuteReader() method AGAIN?

2. Do I have to assign null to the variables after closing the reader and database like above?

Thanks in advance!