What is the difference between ExecuteScalar, ExecuteReader and ExecuteNonQuery?

Each one is a different type execution.

• ExecuteScalar is going to be the type of query which will be returning a single value.

  An example would be returning a generated id after inserting.

  INSERT INTO my_profile (Address) VALUES ('123 Fake St.'); SELECT CAST(scope_identity() AS int)

• ExecuteReader gives you a data reader back which will allow you to read all of the columns of the results a row at a time.

  An example would be pulling profile information for one or more users.

  SELECT * FROM my_profile WHERE id = '123456'

• ExecuteNonQuery is any SQL which isn't returning values, but is actually performing some form of work like inserting deleting or modifying something.

  An example would be updating a user's profile in the database.

  UPDATE my_profile SET Address = '123 Fake St.' WHERE id = '123456'

• 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.).

ExecuteNonQuery():

1. will work with Action Queries only (Create,Alter,Drop,Insert,Update,Delete).
2. Returns the count of rows effected by the Query.
3. Return type is int
4. Return value is optional and can be assigned to an integer variable.

ExecuteReader():

1. will work with Action and Non-Action Queries (Select)
2. Returns the collection of rows selected by the Query.
3. Return type is DataReader.
4. Return value is compulsory and should be assigned to an another object DataReader.

ExecuteScalar():

1. will work with Non-Action Queries that contain aggregate functions.
2. Return the first row and first column value of the query result.
3. Return type is object.
4. Return value is compulsory and should be assigned to a variable of required type.

Reference URL:

http://nareshkamuni.blogspot.in/2012/05/what-is-difference-between.html