Sql Server Interview Questions with Answers

Friends,

In this post I am gonna add stuff related to Interview questions from SQL SERVER with answers. Hope this one will help you in cracking the Interviews on SQL SERVER.

•  What are Constraints  or Define Constraints ?

• Different types of Constraints ?

There are THREE Types of Constraints.

1. Domain
2. Entity
3. Referential

Domain has the following constraints types –

1. Not Null
2. Check

Entity has the following constraint types –

1. Primary Key
2. Unique Key

Referential has the following constraint types –

1. Foreign Key

• What is the difference between Primary Key and Unique Key ?

1. By default PK defines Clustered Index in the column where as UK defines Non Clustered Index.
2. PK doesn’t allow NULL Value where as UK allow ONLY ONE NULL.
3. You can have only one PK per table where as UK can be more than one per table.
4. PK can be used in Foreign Key relationships where as UK cannot be used.

• What is the difference between Delete and Truncate ?

1. Truncate is Faster where as Delete is Slow process.
2. Truncate doesn’t log where as Delete logs an entry for every record deleted in Transaction Log.
3. We can rollback the Deleted data where as Truncated data cannot be rolled back.
4. Truncate resets the Identity column where as Delete doesn’t.
5. We can have WHERE Clause for delete where as for Truncate we cannot have WHERE Clause.
6. Delete Activates TRIGGER where as TRUNCATE Cannot.
7. Truncate is a DDL statement where as Delete is DML statement.

• What are Indexes or Indices ?

• Types of Indices in SQL ?

• What is Transaction in SQL Server ? 

• Types of Transactions ?

1. Implicit – Specifies any Single Insert,Update or Delete statement as Transaction Unit.  No need to specify Explicitly.
2. Explicit – A group of T-Sql statements with the beginning and ending marked with Begin Transaction,Commit and RollBack. PFB an Example for Explicit transactions.

BEGIN TRANSACTION

Update Employee Set Emp_ID = 54321 where Emp_ID = 12345

If(@@Error <>0)

ROLLBACK

Update LEave_Details Set Emp_ID = 54321 where Emp_ID = 12345

If(@@Error <>0)

ROLLBACK

COMMIT

In the above example we are trying to update an EMPLOYEE ID from 12345 to 54321 in both the master table “Employee” and Transaction table “Leave_Details”. In this case either BOTH the tables will be updated with new EMPID or NONE.

• What is the Max size and Max number of columns for a row in a table ?

• What is Normalization and Explain different normal forms.

Database normalization is a process of data design and organization which applies to data structures based on rules that help building relational databases.
1. Organizing data to minimize redundancy.
2. 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.

1NF: Eliminate Repeating Groups

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

2NF: Eliminate Redundant Data

1. Table must be in 1NF.
2. All fields are dependent on the whole of the primary key, or a relation is in 2NF if it is in 1NF and every non-key attribute is fully dependent on each candidate key of the relation. 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

1. The table must be in 2NF.
2. Transitive dependencies must be eliminated. All attributes must rely only on the primary 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
for every one of its non-trivial functional dependencies X → Y, X is a superkey—that is, X is either a candidate key or a superset thereof. 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.
For example, if you can have two phone numbers values and two email address values, then you should not have them in the same table.
5NF: Isolate Semantically Related Multiple Relationships
A 4NF table is said to be in the 5NF if and only if every join dependency in it is implied by the candidate keys. There may be practical constrains on information that justify separating logically related many‐to‐many relationships.

• What is Denormalization ?

For optimizing the performance of a database by adding redundant data or by grouping data is called de-normalization.
It is sometimes necessary because current DBMSs implement the relational model poorly.
In some cases, de-normalization helps cover up the inefficiencies inherent in relational database software. A relational normalized database imposes a heavy access load over physical storage of data even if it is well tuned for high performance.
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.

• Query to Pull ONLY duplicate records from table ?

There are many ways of doing the same and let me explain one here. We can acheive this by using the keywords GROUP and HAVING. The following query will extract duplicate records from a specific column of a particular table.

Select specificColumn
FROM particluarTable
GROUP BY specificColumn
HAVING COUNT(*) > 1

This will list all the records that are repeated in the column specified by “specificColumn” of a “particlarTable”.

• Types of Joins in SQL SERVER ?

1. Inner Join
2. Outer Join
3. Cross Join

1. Right Outer Join
2. Left Outer Join
3. Full Outer Join.

• What is Table Expressions in Sql Server ?

1. Derived tables
2. Common Table Expressions.

• What is CTE or Common Table Expression ?

1. Recursive
2. Non Recursive

• Difference between SmallDateTime and DateTime datatypes in Sql server ?

1. DateTime occupies 4 Bytes of data where as SmallDateTime occupies only 2 Bytes.
2. DateTime ranges from 01/01/1753 to 12/31/9999 where as SmallDateTime ranges from 01/01/1900 to 06/06/2079.

• What is SQL_VARIANT Datatype ? 

The SQL_VARIANT data type can be used to store values of various data types at the same time, such as numeric values, strings, and date values. (The only types of values that cannot be stored are TIMESTAMP values.) Each value of an SQL_VARIANT column has two parts: the data value and the information that describes the value. (This information contains all properties of the actual data type of the value, such as length, scale, and precision.)

• What is Temporary table ? 

A temporary table is a database object that is temporarily stored and managed by the database system. There are two types of Temp tables.

1. Local
2. Global

• What are the differences between Local Temp table and Global Temp table ? 

1. Both are stored in tempdb database.
2. Both will be cleared once the connection,which is used to create the table, is closed.
3. Both are meant to store data temporarily.

1. Local temp table is prefixed with # where as Global temp table with ##.
2. Local temp table is valid for the current connection i.e the connection where it is created where as Global temp table is valid for all the connection.
3.  Local temp table cannot be shared between multiple users where as Global temp table can be shared.

• Whar are the differences between Temp table and Table variable ?

1. Table variables are Transaction neutral where as Temp tables are Transaction bound. For example if we declare and load data into a temp table and table variable in a transaction and if the transaction is ROLLEDBACK, still the table variable will have the data loaded where as Temp table will not be available as the transaction is rolled back.
2. Temporary Tables are real tables so you can do things like CREATE INDEXes, etc. If you have large amounts of data for which accessing by index will be faster then temporary tables are a good option.
3. Table variables don’t participate in transactions, logging or locking. This means they’re faster as they don’t require the overhead.
4. You can create a temp table using SELECT INTO, which can be quicker to write (good for ad-hoc querying) and may allow you to deal with changing datatypes over time, since you don’t need to define your temp table structure upfront.

• What is the difference between Char,Varchar and nVarchar datatypes ?

• What are Magic Tables ? 

• Explain about RANK,ROW_NUMBER and DENSE_RANK in Sql server ?

Found a very interesting explanation for the same in the url Click Here . PFB the content of the same here.

Lets take 1 simple example to understand the difference between 3.
First lets create some sample data :

— create table
CREATE TABLE Salaries
(
Names VARCHAR(1),
SalarY INT
)
GO
— insert data
INSERT INTO Salaries SELECT
‘A’,5000 UNION ALL SELECT
‘B’,5000 UNION ALL SELECT
‘C’,3000 UNION ALL SELECT
‘D’,4000 UNION ALL SELECT
‘E’,6000 UNION ALL SELECT
‘F’,10000
GO
— Test the data
SELECT Names, Salary
FROM Salaries

Now lets query the table to get the salaries of all employees with their salary in descending order.
For that I’ll write a query like this :
SELECT names
, salary
,row_number () OVER (ORDER BY salary DESC) as ROW_NUMBER
,rank () OVER (ORDER BY salary DESC) as RANK
,dense_rank () OVER (ORDER BY salary DESC) as DENSE_RANK
FROM salaries

>>Output

Interesting Names in the result are employee A, B and D.  Row_number assign different number to them. Rank and Dense_rank both assign same rank to A and B. But interesting thing is what RANK and DENSE_RANK assign to next row? Rank assign 5 to the next row, while dense_rank assign 4.

The numbers returned by the DENSE_RANK function do not have gaps and always have consecutive ranks.  The RANK function does not always return consecutive integers.  The ORDER BY clause determines the sequence in which the rows are assigned their unique ROW_NUMBER within a specified partition.
So question is which one to use?
Its all depends on your requirement and business rule you are following.
1. Row_number to be used only when you just want to have serial number on result set. It is not as intelligent as RANK and DENSE_RANK.
2. Choice between RANK and DENSE_RANK depends on business rule you are following. Rank leaves the gaps between number when it sees common values in 2 or more rows. DENSE_RANK don’t leave any gaps between ranks.
So while assigning the next rank to the row RANK will consider the total count of rows before that row and DESNE_RANK will just give next rank according to the value.
So If you are selecting employee’s rank according to their salaries you should be using DENSE_RANK and if you are ranking students according to there marks you should be using RANK(Though it is not mandatory, depends on your requirement.)

•  What are the differences between WHERE and HAVING clauses in SQl Server ? 

• Explain Physical Data Model or PDM ?

Physical data model represents how the model will be built in the database. A physical database model shows all table structures, including column name, column data type, column constraints, primary key, foreign key, and relationships between tables. Features of a physical data model include:

1. Specification all tables and columns.
2. Foreign keys are used to identify relationships between tables.
3. Specying Data types.

EG –

Reference from Here

•  Explain Logical Data Model ?

A logical data model describes the data in as much detail as possible, without regard to how they will be physical implemented in the database. Features of a logical data model include:

1. Includes all entities and relationships among them.
2. All attributes for each entity are specified.
3. The primary key for each entity is specified.
4. Foreign keys (keys identifying the relationship between different entities) are specified.
5. Normalization occurs at this level.

Reference from Here

• Explain Conceptual Data Model ?

A conceptual data model identifies the highest-level relationships between the different entities. Features of conceptual data model include:

1. Includes the important entities and the relationships among them.
2. No attribute is specified.
3. No primary key is specified.

Reference from Here

• What is Log Shipping ?

Msg 544, Level 16, State 1, Line 3
Cannot insert explicit value for identity column in table 'tablename' when IDENTITY_INSERT is set to OFF.

USE master;
GO
ALTER DATABASE databasename
Modify Name = newname ;
GO