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 process 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 is 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 cursors?
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 casesensitivity,
accent marks, kana character types and character width.
What are different type 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.
How to implement one-to-one, one-to-many and many-to-many relationships while
designing tables?
One-to-One relationship can be implemented as a single table and rarely as two tables with primary
and foreign key relationships.
One-to-Many relationships are implemented by splitting the data into two tables with primary key and
foreign key relationships.
Many-to-Many relationships are implemented using a junction table with the keys from both the tables
forming the composite primary key of the junction table.
What is a NOLOCK?
Using the NOLOCK query optimiser hint is generally considered good practice in order to improve
concurrency on a busy system. When the NOLOCK hint is included in a SELECT statement, no locks are
taken when data is read. The result is a Dirty Read, which means that another process could be
updating the data at the exact time you are reading it. There are no guarantees that your query will
retrieve the most recent data. The advantage to performance is that your reading of data will not block
updates from taking place, and updates will not block your reading of data. SELECT statements take
Shared (Read) locks. This means that multiple SELECT statements are allowed simultaneous access, but
other processes are blocked from modifying the data. The updates will queue until all the reads have
completed, and reads requested after the update will wait for the updates to complete. The result to
your system is delay(blocking).
What is difference between DELETE & TRUNCATE commands?
Delete command removes the rows from a table based on the condition that we provide with a WHERE
clause. Truncate will actually remove all the rows from a table and there will be no data in the table
after we run the truncate command.
TRUNCATE
TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.
TRUNCATE removes the data by deallocating the data pages used to store the table’s data, and only the
page deallocations are recorded in the transaction log.
TRUNCATE removes all rows from a table, but the table structure and its columns, constraints, indexes
and so on remain. The counter used by an identity for new rows is reset to the seed for the column.
You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint.
Because TRUNCATE TABLE is not logged, it cannot activate a trigger.
TRUNCATE can not be Rolled back.
TRUNCATE is DDL Command.
TRUNCATE Resets identity of the table.
DELETE
DELETE removes rows one at a time and records an entry in the transaction log for each deleted row.
If you want to retain the identity counter, use DELETE instead. If you want to remove table definition
and its data, use the DROP TABLE statement.
DELETE Can be used with or without a WHERE clause
DELETE Activates Triggers.
DELETE Can be Rolled back.
DELETE is DML Command.
DELETE does not reset identity of the table.
When is the use of UPDATE_STATISTICS command?
This command is basically used when a large processing of data has occurred. If a large amount of
deletions any modification or Bulk Copy into the tables has occurred, it has to update the indexes to
take these changes into account. UPDATE_STATISTICS updates the indexes on these tables
accordingly.
What types of Joins are possible with Sql Server?
Joins are used in queries to explain how different tables are related. Joins also let you select data from
a table depending upon data from another table.
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT
OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS.
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?
Specifies a search condition for a group or an aggregate. HAVING can be used only with the SELECT
statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING
behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a
query. WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.
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 result
set. 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.
Properties of Sub-Query
A subquery must be enclosed in the parenthesis.
A subquery must be put in the right hand of the comparison operator, and
A subquery cannot contain a ORDER-BY clause.
A query can contain more than one sub-queries.
What are types of sub-queries?
Single-row subquery, where the subquery returns only one row.
Multiple-row subquery, where the subquery returns multiple rows,.and
Multiple column subquery, where the subquery returns multiple columns.
What is SQL Profiler?
SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of
Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to
analyze later. For example, you can monitor a production environment to see which stored procedures
are hampering performance by executing too slowly.
Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too
large, you can filter them based on the information you want, so that only a subset of the event data is
collected. Monitoring too many events adds overhead to the server and the monitoring process and can
cause the trace file or trace table to grow very large, especially when the monitoring process takes
place over a long period of time.
What is User Defined Functions?
User-Defined Functions allow to define its own T-SQL functions that can accept 0 or more parameters
and return a single scalar data value or a table data type.
What kind of User-Defined Functions can be created?
There are three types of User-Defined functions in SQL Server 2000 and they are Scalar, Inline Table-
Valued and Multi-statement Table-valued.
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.
Which TCP/IP port does SQL Server run on? How can it be changed?
SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties –> Port
number.both on client and the server.
What are the authentication modes in SQL Server? How can it be changed?
Windows mode and mixed mode (SQL & Windows).
To change authentication mode in SQL Server click Start, Programs, Microsoft SQL Server and click SQL
Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group.
Select the server then from the Tools menu select SQL Server Configuration Properties, and choose the Security page.
Where are SQL server users names and passwords are stored in sql server?
They get stored in master db in the sysxlogins table.
Which command using Query Analyzer will give you the version of SQL server and operating
system?
SELECT SERVERPROPERTY('productversion'), SERVERPROPERTY ('productlevel'), SERVERPROPERTY('edition')
What is SQL server agent?
SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the
implementation of tasks for the DBA, with its full-function scheduling engine, which allows you to
schedule your own jobs and scripts.
Can a stored procedure call itself or recursive stored procedure? How many level SP nesting
possible?
Yes. Because Transact-SQL supports recursion, you can write stored procedures that call themselves.
Recursion can be defined as a method of problem solving wherein the solution is arrived at by
repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps.
Stored procedures are nested when one stored procedure calls another or executes managed code by
referencing a CLR routine, type, or aggregate. You can nest stored procedures and managed code
references up to 32 levels.
What is the difference between a local and a global variable?
A local temporary table exists only for the duration of a connection or, if defined inside a compound
statement, for the duration of the compound statement.
A global temporary table remains in the database permanently, but the rows exist only within a given
connection. When connection are closed, the data in the global temporary table disappears. However,
the table definition remains with the database for access when database is opened next time.
What are the OS services that the SQL Server installation adds?
MS SQL SERVER SERVICE, SQL AGENT SERVICE, DTC (Distribution transac co-ordinator)
What are three SQL keywords used to change or set someone’s permissions?
GRANT, DENY, and REVOKE.
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 data integrity? Explain constraints?
Data integrity is an important feature in SQL Server. When used properly, it ensures that data is
accurate, correct, and valid. It also acts as a trap for otherwise undetectable bugs within applications.
A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should
have a primary key constraint to uniquely identify each row and only one primary key constraint can be
created for each table. The primary key constraints are used to enforce entity integrity.
A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values
are entered. The unique key constraints are used to enforce entity integrity as the primary key
constraints.
A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the
corresponding data values. A foreign key in one table points to a primary key in another table. Foreign
keys prevent actions that would leave rows with foreign key values when there are no primary keys
with that value. The foreign key constraints are used to enforce referential integrity.
A CHECK constraint is used to limit the values that can be placed in a column. The check constraints
are used to enforce domain integrity.
A NOT NULL constraint enforces that the column will not accept null values. The not null constraints
are used to enforce domain integrity, as the check constraints.
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 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 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 controled. Identity/GUID columns do not need to be indexed.
What is a Scheduled Jobs or What is a Scheduled Tasks?
Scheduled tasks let user automate processes that run on regular or predictable cycles. User can
schedule administrative tasks, such as cube processing, to run during times of slow business activity.
User can also determine the order in which tasks run by creating job steps within a SQL Server Agent
job. E.g. Back up database, Update Stats of Tables. Job steps give user control over flow of execution.
If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.
What is a table called, if it does not have neither Cluster nor Non-cluster Index? What is it
used for?
Unindexed table or Heap. Microsoft Press Books and Book On Line (BOL) refers it as Heap.
A heap is a table that does not have a clustered index and, therefore, the pages are not linked by
pointers. The IAM pages are the only structures that link the pages in a table together.
Unindexed tables are good for fast storing of data. Many times it is better to drop all indexes from table and than do bulk of inserts and to restore those indexes after that.
What is BCP? When does it used?
BulkCopy is a tool used to copy huge amount of data from tables and views. BCP does not copy the
structures same as source to destination.
How do you load large data to the SQL server database?
BulkCopy is a tool used to copy huge amount of data from tables. BULK INSERT command helps to
Imports a data file into a database table or view in a user-specified format.
Can we rewrite subqueries into simple select statements or with joins?
Subqueries can often be re-written to use a standard outer join, resulting in faster performance. As we
may know, an outer join uses the plus sign (+) operator to tell the database to return all non-matching
rows with NULL values. Hence we combine the outer join with a NULL test in the WHERE clause to
reproduce the result set without using a sub-query.
Can SQL Servers linked to other servers like Oracle?
SQL Server can be lined to any server provided it has OLE-DB provider from Microsoft to allow a link.
E.g. Oracle has a OLE-DB provider for oracle that Microsoft provides to add it as linked server to SQL Server group.
How to know which index a table is using?
SELECT table_name,index_name FROM user_constraints
How to copy the tables, schema and views from one SQL server to another?
Microsoft SQL Server 2000 Data Transformation Services (DTS) is a set of graphical tools and
programmable objects that lets user extract, transform, and consolidate data from disparate sources
into single or multiple destinations.
What is 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 have a hierarchical reporting structure whereby one member of staff reports to another.
What is 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.
Which virtual table does a trigger use?
Inserted and Deleted.
List few advantages of Stored Procedure.
· Stored procedure can reduced network traffic and latency, boosting application performance.
· Stored procedure execution plans can be reused, staying cached in SQL Server's memory,reducing server overhead.
· Stored procedures help promote code reuse.
· Stored procedures can encapsulate logic. You can change stored procedure code without affecting clients.
· Stored procedures provide better security to your data.
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.
What is OLTP(OnLine Transaction Processing)?
In OLTP - online transaction processing systems relational database design use the discipline of data
modeling and generally follow the Codd rules of data normalization in order to ensure absolute data
integrity. Using these rules complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.
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