ExecuteScalar, ExecuteReader and ExecuteNonQuery不同点

• ExecuteScalar is typically used when your query returns a single value. If it returns more, then the result is the first column of the first row. An example might be SELECT @@IDENTITY AS 'Identity'.
• ExecuteReader is used for any result set with multiple rows/columns (e.g., SELECT col1, col2 from sometable).
• ExecuteNonQuery is typically used for SQL statements without results (e.g., UPDATE, INSERT, etc.).
• http://stackoverflow.com/questions/2974154/what-is-the-difference-between-executescalar-executereader-and-executenonquery