ExecuteNonQuery() Executes non-SELECT commands, such as SQL commands that insert, delete, or update records. The returned value indicates the number of rows affected by the command. You can also use ExecuteNonQuery() to execute data-definition commands that create, alter, or delete database objects (such as tables, indexes, constraints, and so on).

ExecuteScalar() Executes a SELECT query and returns the value of the first field of the first row from the rowset generated by the command. This method is usually used when executing an aggregate SELECT command that uses functions such as COUNT() or SUM() to calculate a single value.

ExecuteReader() Executes a SELECT query and returns a DataReader object that wraps a read-only, forward-only cursor.