SQL SERVER – Interview Questions and Answers

1) General Questions of SQL SERVER

What is RDBMS?

Relational Data Base Management Systems (RDBMS) are database management systems that maintain data records and indices in tables. Relationships may be created and maintained across and among the data and tables. In a relational database, relationships between data items are expressed by means of tables. Interdependencies among these tables are expressed by data values rather than by pointers. This allows a high degree of data independence. An RDBMS has the capability to recombine the data items from different files, providing powerful tools for data usage. 

What are the properties of the Relational tables?

Relational tables have six properties:

·         Values are atomic.

·         Column values are of the same kind.

·         Each row is unique.

·         The sequence of columns is insignificant.

·         The sequence of rows is insignificant.

·         Each column must have a unique name.

What is Normalization?

Database normalization is a data design and organization process applied to data structures based on rules that help building relational databases. In relational database design, the process of organizing data to minimize redundancy is called normalization. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data so that additions, deletions, and modifications of a field can be made in just one table and then propagated through the rest of the database via the defined relationships.

What are different normalization forms?

1NF: Eliminate Repeating Groups

Make a separate table for each set of related attributes, and give each table a primary key. Each field contains at most one value from its attribute domain.

2NF: Eliminate Redundant Data

If an attribute depends on only part of a multi-valued key, remove it to a separate table.

3NF: Eliminate Columns Not Dependent On Key

If attributes do not contribute to a description of the key, remove them to a separate table. All attributes must be directly dependent on the primary key.

BCNF: Boyce-Codd Normal Form

If there are non-trivial dependencies between candidate key attributes, separate them out into distinct tables.

4NF: Isolate Independent Multiple Relationships

No table may contain two or more 1:n or n:m relationships that are not directly related.

5NF: Isolate Semantically Related Multiple Relationships

There may be practical constrains on information that justify separating logically related many-to-many relationships.

ONF: Optimal Normal Form

A model limited to only simple (elemental) facts, as expressed in Object Role Model notation.

DKNF: Domain-Key Normal Form

A model free from all modification anomalies is said to be in DKNF.

Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first fulfill all the criteria of a 2NF and 1NF database.

What is De-normalization?

De-normalization is the process of attempting to optimize the performance of a database by adding redundant data. It is sometimes necessary because current DBMSs implement the relational model poorly. A true relational DBMS would allow for a fully normalized database at the logical level, while providing physical storage of data that is tuned for high performance. De-normalization is a technique to move from higher to lower normal forms of database modeling in order to speed up database access.

What is Stored Procedure?

A stored procedure is a named group of SQL statements that have been previously created and stored in the server database. Stored procedures accept input parameters so that a single procedure can be used over the network by several clients using different input data. And when the procedure is modified, all clients automatically get the new version. Stored procedures reduce network traffic and improve performance. Stored procedures can be used to help ensure the integrity of the database.

e.g. sp_helpdb, sp_renamedb, sp_depends etc.

What is Trigger?

A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE) occurs. Triggers are stored in and managed by the DBMS. Triggers are used to maintain the referential integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed; DBMS automatically fires the trigger as a result of a data modification to the associated table. Triggers can be viewed as similar to stored procedures in that both consist of procedural logic that is stored at the database level. Stored procedures, however, are not event-drive and are not attached to a specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the procedure while triggers are implicitly executed. In addition, triggers can also execute stored procedures.

Nested Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within itself, so when the trigger is fired because of data modification it can also cause another data modification, thereby firing another trigger. A trigger that contains data modification logic within itself is called a nested trigger. 

What is View?

A simple view can be thought of as a subset of a table. It can be used for retrieving data, as well as updating or deleting rows. Rows updated or deleted in the view are updated or deleted in the table the view was created with. It should also be noted that as data in the original table changes, so does data in the view, as views are the way to look at part of the original table. The results of using a view are not permanently stored in the database. The data accessed through a view is actually constructed using standard T-SQL select command and can come from one to many different base tables or even other views.

What is Index?

An index is a physical structure containing pointers to the data. Indices are created in an existing table to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of a table, and each index is given a name. The users cannot see the indexes; they are just used to speed up queries. Effective indexes are one of the best ways to improve performance in a database application. A table scan happens when there is no index available to help a query. In a table scan SQL Server examines every row in the table to satisfy the query results. Table scans are sometimes unavoidable, but on large tables, scans have a terrific impact on performance.

What is a Linked Server?

Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query both the SQL Server dbs using T-SQL Statements. With a linked server, you can create very clean, easy to follow, SQL statements that allow remote data to be retrieved, joined and combined with local data. Stored Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked Server

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ADO.NET and Database Questions

1.    What is the role of the DataReader class in ADO.NET connections? 
It returns a read-only, forward-only rowset from the data source.  A DataReader provides fast access when a forward-only sequential read is needed.
 

2.    What are advantages and disadvantages of Microsoft-provided data provider classes in ADO.NET? 
SQLServer.NET data provider is high-speed and robust, but requires SQL Server license purchased from Microsoft. OLE-DB.NET is universal for accessing other sources, like Oracle, DB2, Microsoft Access and Informix.  OLE-DB.NET is a .NET layer on top of the OLE layer, so it’s not as fastest and efficient as SqlServer.NET.
 

3.    What is the wildcard character in SQL? 
Let’s say you want to query database with LIKE for all employees whose name starts with La. The wildcard character is %, the proper query with LIKE would involve ‘La%’.
 

4.    Explain ACID rule of thumb for transactions.
A transaction must be:
1.       Atomic - it is one unit of work and does not dependent on previous and following transactions.
2.       Consistent - data is either committed or roll back, no “in-between” case where something has been updated and something hasn’t.
3.       Isolated - no transaction sees the intermediate results of the current transaction).
4.       Durable - the values persist if the data had been committed even if the system crashes right after.
 

5.    What connections does Microsoft SQL Server support? 
Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and password).
 

6.    Between Windows Authentication and SQL Server Authentication, which one is trusted and which one is untrusted? 
Windows Authentication is trusted because the username and password are checked with the Active Directory, the SQL Server authentication is untrusted, since SQL Server is the only verifier participating in the transaction.
 

7.    What does the Initial Catalog parameter define in the connection string? 
The database name to connect to.
  

8.    What does the Dispose method do with the connection object? 
Deletes it from the memory.
To Do: answer better.  The current answer is not entirely correct.
 

9.    What is a pre-requisite for connection pooling? 
Multiple processes must agree that they will share the same connection, where every parameter is the same, including the security settings.  The connection string must be identical.
 

Assembly Questions

1.    How is the DLL Hell problem solved in .NET? 
Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly.
 

2.    What are the ways to deploy an assembly? 
An MSI installer, a CAB archive, and XCOPY command.
 

3.    What is a satellite assembly? 
When you write a multilingual or multi-cultural application in .NET, and want to distribute the core application separately from the localized modules, the localized assemblies that modify the core application are called satellite assemblies.
 

4.    What namespaces are necessary to create a localized application? 
System.Globalization and System.Resources.
 

5.    What is the smallest unit of execution in .NET?
an Assembly.
 

6.    When should you call the garbage collector in .NET?
As a good rule, you should not call the garbage collector.  However, you could call the garbage collector when you are done using a large object (or set of objects) to force the garbage collector to dispose of those very large objects from memory.  However, this is usually not a good practice.
 

7.    How do you convert a value-type to a reference-type?
Use Boxing.
 

8.    What happens in memory when you Box and Unbox a value-type?
Boxing converts a value-type to a reference-type, thus storing the object on the heap.  Unboxing converts a reference-type to a value-type, thus storing the value on the stack.

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Events and Delegates and Debugging and Testing and XML Documentation Questions

1.    What’s a delegate? 
A delegate object encapsulates a reference to a method.
 

2.    What’s a multicast delegate? 
A delegate that has multiple handlers assigned to it.  Each assigned handler (method) is called.
 

XML Documentation Questions

1.    Is XML case-sensitive? 
Yes.
 

2.    What’s the difference between // comments, /* */ comments and /// comments? 
Single-line comments, multi-line comments, and XML documentation comments.
 

3.    How do you generate documentation from the C# file commented properly with a command-line compiler? 
Compile it with the /doc switch.
 

Debugging and Testing Questions

1.    What debugging tools come with the .NET SDK?
1.   CorDBG – command-line debugger.  To use CorDbg, you must compile the original C# file using the /debug switch.
2.   DbgCLR – graphic debugger.  Visual Studio .NET uses the DbgCLR.
 

2.    What does assert() method do? 
In debug compilation, assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is false.  The program proceeds without any interruption if the condition is true.
 

3.    What’s the difference between the Debug class and Trace class? 
Documentation looks the same.  Use Debug class for debug builds, use Trace class for both debug and release builds.
 

4.    Why are there five tracing levels in System.Diagnostics.TraceSwitcher? 
The tracing dumps can be quite verbose.  For applications that are constantly running you run the risk of overloading the machine and the hard drive.  Five levels range from None to Verbose, allowing you to fine-tune the tracing activities.
 

5.    Where is the output of TextWriterTraceListener redirected? 
To the Console or a text file depending on the parameter passed to the constructor.
 

6.    How do you debug an ASP.NET Web application? 
Attach the aspnet_wp.exe process to the DbgClr debugger.
 

7.    What are three test cases you should go through in unit testing? 
1.       Positive test cases (correct data, correct output).
2.       Negative test cases (broken or missing data, proper handling).
3.       Exception test cases (exceptions are thrown and caught properly).
 

Can you change the value of a variable while debugging a C# application? 
Yes.  If you are debugging via Visual Studio.NET, just go to Immediate window. 

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Method and Property Questions

1.    What’s the implicit name of the parameter that gets passed into the set method/property of a class? 
Value.  The data type of the value parameter is defined by whatever data type the property is declared as.
 

2.    What does the keyword “virtual” declare for a method or property? 
The method or property can be overridden.
 

3.    How is method overriding different from method overloading? 
When overriding a method, you change the behavior of the method for the derived class.  Overloading a method simply involves having another method with the same name within the class.
 

4.    Can you declare an override method to be static if the original method is not static? 
No.  The signature of the virtual method must remain the same.  (Note: Only the keyword virtual is changed to keyword override)
 

5.    What are the different ways a method can be overloaded? 
Different parameter data types, different number of parameters, different order of parameters.
 

If a base class has a number of overloaded constructors, and an inheriting class has a number of overloaded constructors; can you enforce a call from an inherited constructor to a specific base constructor?
Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.

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Class Questions

1.    What is the syntax to inherit from a class in C#? 
Place a colon and then the name of the base class.
Example: class MyNewClass : MyBaseClass
 

2.    Can you prevent your class from being inherited by another class? 
Yes.  The keyword “sealed” will prevent the class from being inherited.
 

3.    Can you allow a class to be inherited, but prevent the method from being over-ridden?
Yes.  Just leave the class public and make the method sealed.
 

4.    What’s an abstract class?
A class that cannot be instantiated.  An abstract class is a class that must be inherited and have the methods overridden.  An abstract class is essentially a blueprint for a class without any implementation.
 

5.    When do you absolutely have to declare a class as abstract?
1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden. 
2.  When at least one of the methods in the class is abstract.
 

6.    What is an interface class?
Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.
 

7.    Why can’t you specify the accessibility modifier for methods inside the interface?
They all must be public, and are therefore public by default.
 

8.    Can you inherit multiple interfaces?
Yes.  .NET does support multiple interfaces.
 

9.    What happens if you inherit multiple interfaces and they have conflicting method names?
It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay. 
To Do: Investigate
 

10.  What’s the difference between an interface and abstract class?
In an interface class, all methods are abstract - there is no implementation.  In an abstract class some methods can be concrete.  In an interface class, no accessibility modifiers are allowed.  An abstract class may have accessibility modifiers.
 

11.  What is the difference between a Struct and a Class?
Structs are value-type variables and are thus saved on the stack, additional overhead but faster retrieval.  Another difference is that structs cannot inherit.
  

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