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 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 modelling in order to speed up database access.
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 fulfil all the criteria of a 2NF and 1NF database.
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.
What is
Cursor?
Cursor is a database object used by applications to
manipulate data in a set on a row-by-row 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
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
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.
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.
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.
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.
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 T-SQL command
unlike the behavior found when using a stored procedure which can also return
record sets.
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.
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.
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