SQL SERVER Technical Interview Questions 1

 

SQL SERVER Technical Interview Questions 1

 

 

1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. What is 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.

9. 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.

10. 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.

11. 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.

12. What is Cursor?

Cursor is a database object used by applications to manipulate data in a set on a row-byrow

basis, instead of the typical SQL commands that operate on all the rows in the set at

one time.

In order to work with a cursor we need to perform some steps in the following order:

Declare cursor

Open cursor

Fetch row from the cursor

Process fetched row

Close cursor

Deallocate cursor

13. What is Collation?

Collation refers to a set of rules that determine how data is sorted and compared.

Character data is sorted using rules that define the correct character sequence, with

options for specifying case sensitivity, accent marks, kana character types and character

width.

14. What is Difference between Function and Stored Procedure?

UDF can be used in the SQL statements anywhere in the WHERE/HAVING/SELECT

section where as Stored procedures cannot be. UDFs that return tables can be treated as

another rowset. This can be used in JOINs with other tables. Inline UDF's can be thought

of as views that take parameters and can be used in JOINs and other Rowset operations.

15. What is sub-query? Explain properties of sub-query?

Sub-queries are often referred to as sub-selects, as they allow a SELECT statement to be

executed arbitrarily within the body of another SQL statement. A sub-query is executed

by enclosing it in a set of parentheses. Sub-queries are generally used to return a single

row as an atomic value, though they may be used to compare values against multiple

rows with the IN keyword.

A subquery is a SELECT statement that is nested within another T-SQL statement. A

subquery SELECT statement if executed independently of the T-SQL statement, in which

it is nested, will return a resultset. Meaning a subquery SELECT statement can

standalone and is not depended on the statement in which it is nested. A subquery

SELECT statement can return any number of values, and can be found in, the column list

of a SELECT statement, a FROM, GROUP BY, HAVING, and/or ORDER BY clauses

of a T-SQL statement. A Subquery can also be used as a parameter to a function call.

Basically a subquery can be used anywhere an expression can be used.

16. What are different Types of Join?

Cross Join A cross join that does not have a WHERE clause produces the Cartesian

product of the tables involved in the join. The size of a Cartesian product result set is the

number of rows in the first table multiplied by the number of rows in the second table.

The common example is when company wants to combine each product with a pricing

table to analyze each product at each price.

Inner Join A join that displays only the rows that have a match in both joined tables is

known as inner Join. This is the default type of join in the Query and View Designer.

Outer Join A join that includes rows even if they do not have related rows in the joined

table is an Outer Join. You can create three different outer join to specify the unmatched

rows to be included:

Left Outer Join: In Left Outer Join all rows in the first-named table i.e. "left" table, which

appears leftmost in the JOIN clause are included. Unmatched rows in the right table do

not appear.

Right Outer Join: In Right Outer Join all rows in the second-named table i.e. "right" table,

which appears rightmost in the JOIN clause are included. Unmatched rows in the left

table are not included.

Full Outer Join: In Full Outer Join all rows in all joined tables are included, whether they

are matched or not.

Self Join This is a particular case when one table joins to itself, with one or two aliases to

avoid confusion. A self join can be of any type, as long as the joined tables are the same.

A self join is rather unique in that it involves a relationship with only one table. The

common example is when company has a hierarchal reporting structure whereby one

member of staff reports to another. Self Join can be Outer Join or Inner Join.

17. What are primary keys and foreign keys?

Primary keys are the unique identifiers for each row. They must contain unique values

and cannot be null. Due to their importance in relational databases, Primary keys are the

most fundamental of all keys and constraints. A table can have only one Primary key.

Foreign keys are both a method of ensuring data integrity and a manifestation of the

relationship between tables.

18. What is User Defined Functions? What kind of User-Defined Functions can be

created?

User-Defined Functions allow defining its own T-SQL functions that can accept 0 or

more parameters and return a single scalar data value or a table data type.

Different Kinds of User-Defined Functions created are:

Scalar User-Defined Function A Scalar user-defined function returns one of the scalar

data types. Text, ntext, image and timestamp data types are not supported. These are the

type of user-defined functions that most developers are used to in other programming

languages. You pass in 0 to many parameters and you get a return value.

Inline Table-Value User-Defined Function An Inline Table-Value user-defined function

returns a table data type and is an exceptional alternative to a view as the user-defined

function can pass parameters into a T-SQL select command and in essence provide us

with a parameterized, non-updateable view of the underlying tables.

Multi-statement Table-Value User-Defined Function A Multi-Statement Table-Value

user-defined function returns a table and is also an exceptional alternative to a view as the

function can support multiple T-SQL statements to build the final result where the view is

limited to a single SELECT statement. Also, the ability to pass parameters into a TSQL

select command or a group of them gives us the capability to in essence create a

parameterized, non-updateable view of the data in the underlying tables. Within the

create function command you must define the table structure that is being returned. After

creating this type of user-defined function, It can be used in the FROM clause of a TSQL

command unlike the behavior found when using a stored procedure which can also

return record sets.

19. What is Identity?

Identity (or AutoNumber) is a column that automatically generates numeric values. A

start and increment value can be set, but most DBA leave these at 1. A GUID column

also generates numbers; the value of this cannot be controlled. Identity/GUID columns do

not need to be indexed.

20. What is DataWarehousing?

Subject-oriented, meaning that the data in the database is organized so that all the data

elements relating to the same real-world event or object are linked together;

Time-variant, meaning that the changes to the data in the database are tracked and

recorded so that reports can be produced showing changes over time;

Non-volatile, meaning that data in the database is never over-written or deleted, once

committed, the data is static, read-only, but retained for future reporting.

Integrated, meaning that the database contains data from most or all of an organization's

operational applications, and that this data is made consistent.