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.).
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