Difference between Equal() and ==

• Equals() method compares the state of an object i.e. it compares the contents of the two objects.
• == compares the instance of the object i.e. it comparing the identifier(references).

Example :
 

string s1 = "test";
string s2 = "test";
string s3 = new string(new char[] { 't', 'e', 's', 't' });
object s4 = s3;
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s2), s1 == s2, s1.Equals(s2));
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s3), s1 == s3, s1.Equals(s3));
Console.WriteLine("{0} {1} {2}", object.ReferenceEquals(s1, s4), s1 == s4, s1.Equals(s4));

Output :

True True True
False True True
False False True

Ref: here, here

Difference between Abstract classes and Interface

• Abstract Class
• A class may inherit only one abstract class.
• An abstract class can contain access modifiers for the subs, functions, properties.
• An Abstract class has abstract or concrete subs, functions, properties etc.
• An abstract class can have fields and constants defined.

 

• Interface
• A class may inherit several interfaces, which help achieving Multiple Inheritance.
• An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public
• An interface can only have abstract subs, functions, properties etc
• No fields can be defined in interfaces 

Ref- here, here

Types of sql statements

• DML
• DML is abbreviation of Data Manipulation Language.
• Examples: UPDATE, INSERT, DELETE statements
• DDL
• DDL is abbreviation of Data Definition Language.
• Examples: CREATE, ALTER, DROP statements
• DCL
• DCL is abbreviation of Data Control Language.
• Examples: GRANT, REVOKE statements
• TCL
• TCL is abbreviation of Transactional Control Language.
• Examples: COMMIT, ROLLBACK statements
• DQL
• DQL is abbreviation of Data Query Language.
• Examples: SELECT statements

Ref: here