SQL Interview questions

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 is normalization?
Database normalization is a data design and organization processes applied to
data structures based on rules that help build relational databases. In
relational database design, the process of organizing data to minimize
redundancy. 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.

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 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; the 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.

Clustered indexes define the physical sorting of a database table’s rows
in the storage media. For this reason, each database table may have only one
clustered index.
Non-clustered indexes are created outside of the database table and
contain a sorted list of references to the table itself.

What are the difference between clustered and a non-clustered index?A clustered index is a special type of index that reorders the way
records in the table are physically stored. Therefore table can have only one
clustered index. The leaf nodes of a clustered index contain the data pages.

A nonclustered index is a special type of index in which the logical
order of the index does not match the physical stored order of the rows on
disk. The leaf node of a nonclustered index does not consist of the data pages.
Instead, the leaf nodes contain index rows.

What are the different index configurations a table can have?
A table can have one of the following index configurations:



No indexes
A clustered index
A clustered index and many nonclustered indexes
A nonclustered index
Many nonclustered indexes

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 the use of DBCC commands?DBCC stands for database consistency checker. We use these commands to
check the consistency of the databases, i.e., maintenance, validation task and
status checks.
E.g. DBCC CHECKDB - Ensures that tables in the db and the indexes are correctly
linked.
DBCC CHECKALLOC - To check that all pages in a db are correctly allocated.
DBCC CHECKFILEGROUP - Checks all tables file group for any damage.

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.
Storped Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new
Linked Server.

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 are different types of Collation Sensitivity?Case sensitivity
A and a, B and b, etc.

Accent sensitivity
a and á, o and ó, etc.

Kana Sensitivity
When Japanese kana characters Hiragana and Katakana are treated differently, it
is called Kana sensitive.

Width sensitivity
when a single-byte character (half-width) and the same character when
represented as a double-byte character (full-width) are treated differently
then it is width sensitive.

What’s the difference between a primary key and a unique key?Both primary key and unique enforce uniqueness of the column on which they
are defined. But by default primary key creates a clustered index on the
column, where are unique creates a nonclustered index by default. Another major
difference is that, primary key doesn’t allow NULLs, but unique key allows one
NULL only.