What are the different cases when we use these three? Where should I use one and where should I not?
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.).