DBMS Interview Questions and Answers

QUESTION 1:
What is database?

ANSWER:
A database is a logically coherent collection of data with some inherent meaning, representing some aspect of real world and which is designed, built and populated with data for a specific purpose.

QUESTION 2:
What is DBMS?

ANSWER:
? Redundancy is controlled.
? Unauthorised access is restricted.
? Providing multiple user interfaces.
? Enforcing integrity constraints.
? Providing backup and recovery.

QUESTION 4:
What is a Database system?

ANSWER:
The database and DBMS software together is called as Database system.

QUESTION 5:
Disadvantage in File Processing System?

ANSWER:
? Data redundancy & inconsistency.
? Difficult in accessing data.
? Data isolation.
? Data integrity.
? Concurrent access is not possible.
? Security Problems. .

QUESTION 6:
Describe the three levels of data abstraction?

ANSWER:
The are three levels of abstraction:

? Physical level: The lowest level of abstraction describes how data are stored.

? Logical level: The next higher level of abstraction, describes what data are stored in database and what relationship among those data.

? View level: The highest level of abstraction describes only part of entire database.

QUESTION 7:
Define the "integrity rules"

ANSWER:
There are two Integrity rules.

? Entity Integrity: States that ?Primary key cannot have NULL value?

? Referential Integrity: States that ?Foreign Key can be either a NULL value or should be Primary Key value of other relation.

QUESTION 8:
What is extension and intension?

ANSWER:
Extension -It is the number of tuples present in a table at any instance. This is time dependent.

Intension - It is a constant value that gives the name, structure of table and the constraints laid on it.

QUESTION 9:
What is System R? What are its two major subsystems?

ANSWER:
System R was designed and developed over a period of 1974-79 at IBM San Jose Research Center . It is a prototype and its purpose was to demonstrate that it is possible to build a Relational System that can be used in a real life environment to solve real life problems, with performance at least comparable to that of existing system.

Its two subsystems are

? Research Storage

? System Relational Data System.

QUESTION 10:
How is the data structure of System R different from the relational structure?

ANSWER:
Unlike Relational systems in System R

? Domains are not supported

? Enforcement of candidate key uniqueness is optional

? Enforcement of entity integrity is optional

? Referential integrity is not enforced

QUESTION 11:
What is Data Independence?

ANSWER:
Data independence means that ?the application is independent of the storage structure and access strategy of data?. In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.

Two types of Data Independence:
? Physical Data Independence : Modification in physical level should not affect the logical level.
? Logical Data Independence : Modification in logical level should affect the view level.

NOTE: Logical Data Independence is more difficult to achieve

QUESTION 12:
What is a view? How it is related to data independence?

ANSWER:
A view may be thought of as a virtual table, that is, a table that does not really exist in its own right but is instead derived from one or more underlying base table. In other words, there is no stored file that direct represents the view instead a definition of view is stored in data dictionary.
Growth and restructuring of base tables is not reflected in views. Thus the view can insulate users from the effects of restructuring and growth in the database. Hence accounts for logical data independence. .

QUESTION 13:
What is Data Model?

ANSWER:
A collection of conceptual tools for describing data, data relationships data semantics and constraints.

QUESTION 14:
What is E-R model?

ANSWER:
This data model is based on real world that consists of basic objects called entities and of relationship among these objects. Entities are described in a database by a set of attributes.

QUESTION 15:
What is Object Oriented model?

ANSWER:
This model is based on collection of objects. An object contains values stored in instance variables with in the object. An object also contains bodies of code that operate on the object. These bodies of code are called methods. Objects that contain same types of values and the same methods are grouped together into classes.

QUESTION 16:
What is an Entity?

ANSWER:
It is a 'thing' in the real world with an independent existence.

QUESTION 17:
What is an Entity type?

ANSWER:
It is a collection (set) of entities that have same attributes.

QUESTION 18:
What is an Entity set?

ANSWER:
It is a collection of all entities of particular entity type in the database.

QUESTION 19:
What is an Extension of entity type?

ANSWER:
The collections of entities of a particular entity type are grouped together into an entity set.

QUESTION 20:
What is Weak Entity set?

ANSWER:
An entity set may not have sufficient attributes to form a primary key, and its primary key compromises of its partial key and primary key of its parent entity, then it is said to be Weak Entity set.

QUESTION 21:
What is an attribute?

ANSWER:
It is a particular property, which describes the entity.

QUESTION 22:
What is a Relation Schema and a Relation?

ANSWER:
A relation Schema denoted by R(A1, A2, ?, An) is made up of the relation name R and the list of attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which contains set tuples (t1, t2, t3, ..., tn). Each tuple is an ordered list of n-values t=(v1,v2, ..., vn).

QUESTION 23:
What is degree of a Relation?

ANSWER:
It is the number of attribute of its relation schema.

QUESTION 24:
What is Relationship?

ANSWER:
It is an association among two or more entities.

QUESTION 25:
What is Relationship set?

ANSWER:
The collection (or set) of similar relationships.

QUESTION 26:
What is Relationship type?

ANSWER:
Relationship type defines a set of associations or a relationship set among a given set of entity types.

QUESTION 27:
What is degree of Relationship type?

ANSWER:
It is the number of entity type participating.

QUESTION 28:
What is Data Storage - Definition Language?

ANSWER:
The storage structures and access methods used by database system are specified by a set of definition in a special type of DDL called data storage-definition language.

QUESTION 29:
What is DML (Data Manipulation Language)?

ANSWER:
This language that enable user to access or manipulate data as organised by appropriate data model.

? Procedural DML or Low level: DML requires a user to specify what data are needed and how to get those data.

? Non-Procedural DML or High level: DML requires a user to specify what data are needed without specifying how to get those data.

QUESTION 30:
What is VDL (View Definition Language)?

ANSWER:
It specifies user views and their mappings to the conceptual schema.

QUESTION 31:
What is DML Compiler?

ANSWER:
It translates DML statements in a query language into low-level instruction that the query evaluation engine can understand.

QUESTION 32:
What is Query evaluation engine?

ANSWER:
It executes low-level instruction generated by compiler.

QUESTION 33:
What is DDL Interpreter?

ANSWER:
It interprets DDL statements and record them in tables containing metadata.

QUESTION 34:
What is Record-at-a-time?

ANSWER:
The Low level or Procedural DML can specify and retrieve each record from a set of records. This retrieve of a record is said to be Record-at-a-time.

QUESTION 35:
What is Set-at-a-time or Set-oriented?

ANSWER:
The High level or Non-procedural DML can specify and retrieve many records in a single DML statement. This retrieve of a record is said to be Set-at-a-time or Set-oriented.

QUESTION 36:
What is Relational Algebra?

ANSWER:
It is procedural query language. It consists of a set of operations that take one or two relations as input and produce a new relation.

QUESTION 37:
What is Relational Calculus?

ANSWER:
It is an applied predicate calculus specifically tailored for relational databases proposed by E.F. Codd. E.g. of languages based on it are DSL ALPHA, QUEL.

QUESTION 38:
How does Tuple-oriented relational calculus differ from domain-oriented relational calculus

ANSWER:
The tuple-oriented calculus uses a tuple variables i.e., variable whose only permitted values are tuples of that relation. E.g. QUEL

The domain-oriented calculus has domain variables i.e., variables that range over the underlying domains instead of over relation. E.g. ILL, DEDUCE.

QUESTION 39:
What is normalization?

ANSWER:
It is a process of analysing the given relation schemas based on their Functional Dependencies (FDs) and primary key to achieve the properties

? Minimizing redundancy

? Minimizing insertion, deletion and update anomalies.

QUESTION 40:
What is Functional Dependency?

ANSWER:
A Functional dependency is denoted by X Y between two sets of attributes X and Y that are subsets of R specifies a constraint on the possible tuple that can form a relation state r of R. The constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This means the value of X component of a tuple uniquely determines the value of component Y.

QUESTION 41:
When is a functional dependency F said to be minimal?

ANSWER:
? Every dependency in F has a single attribute for its right hand side.

? We cannot replace any dependency X A in F with a dependency Y A where Y is a proper subset of X and still have a set of dependency that is equivalent to F.

? We cannot remove any dependency from F and still have set of dependency that is equivalent to F.

QUESTION 42:
What is Multivalued dependency?

ANSWER:
Multivalued dependency denoted by X Y specified on relation schema R, where X and Y are both subsets of R, specifies the following constraint on any relation r of R: if two tuples t1 and t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also exist in r with the following properties

? t3[x] = t4[X] = t1[X] = t2[X]

? t3[Y] = t1[Y] and t4[Y] = t2[Y]

? t3[Z] = t2[Z] and t4[Z] = t1[Z]

where [Z = (R-(X U Y)) ]

QUESTION 43:
What is Lossless join property?

ANSWER:
It guarantees that the spurious tuple generation does not occur with respect to relation schemas after decomposition.

QUESTION 44:
What is 1 NF (Normal Form)?

ANSWER:
The domain of attribute must include only atomic (simple, indivisible) values.

QUESTION 45:
What is Fully Functional dependency?

ANSWER:
It is based on concept of full functional dependency. A functional dependency X Y is full functional dependency if removal of any attribute A from X means that the dependency does not hold any more.

QUESTION 46:
What is 2NF?

ANSWER:
A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R is fully functionally dependent on primary key.

QUESTION 47:
What is 3NF?

ANSWER:
A relation schema R is in 3NF if it is in 2NF and for every FD X A either of the following is true

? X is a Super-key of R.

? A is a prime attribute of R.

In other words, if every non prime attribute is non-transitively dependent on primary key.

QUESTION 48:
What is BCNF (Boyce-Codd Normal Form)?

ANSWER:
A relation schema R is in BCNF if it is in 3NF and satisfies an additional constraint that for every FD X A, X must be a candidate key.

QUESTION 49:
What is 4NF?

ANSWER:
A relation schema R is said to be in 4NF if for every Multivalued dependency X Y that holds over R, one of following is true

? X is subset or equal to (or) XY = R.

? X is a super key.

QUESTION 50:
What is 5NF?

ANSWER:
A Relation schema R is said to be 5NF if for every join dependency {R1, R2, ..., Rn} that holds R, one the following is true

? Ri = R for some i.

? The join dependency is implied by the set of FD, over R in which the left side is key of R.

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Difference between DBMS and RDBMS?

This has been very important question i've come across...I'll tell you the difference...

A DBMS has to be persistent, that is it should be accessible when the program created the data ceases to exist or even the application that created the data restarted. A DBMS also has to provide some uniform methods independent of a specific application for accessing the information that is stored.

RDBMS is a Relational Data Base Management System Relational DBMS. This adds the additional condition that the system supports a tabular structure for the data, with enforced relationships between the tables. This excludes the databases that don't support a tabular structure or don't enforce relationships between tables.

Many DBA's think that RDBMS is a Client Server Database system but thats not the case with RDBMS.

Yes you can say DBMS does not impose any constraints or security with regard to data manipulation it is user or the programmer responsibility to ensure the ACID PROPERTY of the database whereas the rdbms is more with this regard bcz rdbms define the integrity constraint for the purpose of holding ACID PROPERTY.

I have found many answers on many websites saying that

DBMS are for smaller organizations with small amount of data, where security of the data is not of major concern and RDBMS are designed to take care of large amounts of data and also the security of this data.

and this is completely wrong by definition of RDBMS and DBMS

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Fundamentals of Relational DataBase design

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keys

Key

A key is a single or combination of multiple fields. Its purpose is to access or retrieve data rows from table according to the requirement. The keys are defined in tables to access or sequence the stored data quickly and smoothly. They are also used to create links between different tables.

Types of Keys

The following tables or relations will be used to define different types of keys.

Primary Key

The attribute or combination of attributes that uniquely identifies a row or record in a relation is known as primary key.

Candidate Key or Alternate key

A relation can have only one primary key. It may contain many fields or combination of fields that can be used as primary key. One field or combination of fields is used as primary key. The fields or combination of fields that are not used as primary key are known as candidate key or alternate key.

Composite key or concatenate key

A primary key that consists of two or more attributes is known as composite key.

Foreign Key

A foreign key is an attribute or combination of attribute in a relation whose value match a primary key in another relation. The table in which foreign key is created is called as dependent table. The table to which foreign key is refers is known as parent table.

Database normalization

In the design of a relational database management system (RDBMS), the process of organizing data to minimize redundancy is called normalization. The goal of database normalization is to decompose relations with anomalies in order to produce smaller, well-structured relations. Normalization usually involves dividing large, badly-formed tables into smaller, well-formed tables and defining relationships between them. 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.

The Design Process

The design process consists of the following steps:

1. Determine the purpose of your database - This helps prepare you for the remaining steps.
2. Find and organize the information required - Gather all of the types of information you might want to record in the database, such as product name and order number.
3. Divide the information into tables - Divide your information items into major entities or subjects, such as Products or Orders. Each subject then becomes a table.
4. Turn information items into columns - Decide what information you want to store in each table. Each item becomes a field, and is displayed as a column in the table. For example, an Employees table might include fields such as Last Name and Hire Date.
5. Specify primary keys - Choose each table’s primary key. The primary key is a column that is used to uniquely identify each row. An example might be Product ID or Order ID.
6. Set up the table relationships - Look at each table and decide how the data in one table is related to the data in other tables. Add fields to tables or create new tables to clarify the relationships, as necessary.
7. Refine your design - Analyze your design for errors. Create the tables and add a few records of sample data. See if you can get the results you want from your tables. Make adjustments to the design, as needed.
8. Apply the normalization rules - Apply the data normalization rules to see if your tables are structured correctly. Make adjustments to the tables.

ER Diagram (Entity-relationship model)

Database designs also include ER(Entity-relationship model) diagrams. An ER diagram is a diagram that helps to design databases in an efficient way.
Attributes in ER diagrams are usually modeled as an oval with the name of the attribute, linked to the entity or relationship that contains the attribute.
Within the relational model the final step can generally be broken down into two further steps, that of determining the grouping of information within the system, generally determining what are the basic objects about which information is being stored, and then determining the relationships between these groups of information, or objects. This step is not necessary with an Object database.

Database Design

Database design is the process of producing a detailed data model of a database. This logical data model contains all the needed logical and physical design choices and physical storage parameters needed to generate a design in a Data Definition Language, which can then be used to create a database. A fully attributed data model contains detailed attributes for each entity.
The term database design can be used to describe many different parts of the design of an overall database system. Principally, and most correctly, it can be thought of as the logical design of the base data structures used to store the data. In the relational model these are the tables and views. In an object database the entities and relationships map directly to object classes and named relationships. However, the term database design could also be used to apply to the overall process of designing, not just the base data structures, but also the forms and queries used as part of the overall database application within the database management system .


The process of doing database design generally consists of a number of steps which will be carried out by the database designer. Usually, the designer must:

• Determine the relationships between the different data elements.
• Superimpose a logical structure upon the data on the basis of these relationships.

concurrency control and deadlocks

Concurrency control and locking:

Concurrency control is essential for the correctness of transactions executed concurrently in a DBMS, which is the common execution mode for performance reasons. The main concern and goal of concurrency control is isolation.

Isolation refers to the ability of one transaction to see the results of other transactions. Greater isolation typically reduces performance and/or concurrency, leading DBMSs to provide administrative options to reduce isolation. For example, in a database that analyzes trends rather than looking at low-level detail, increased performance might justify allowing readers to see uncommitted changes ("dirty reads".)

A common way to achieve isolation is by locking. When a transaction modifies a resource, the DBMS stops other transactions from also modifying it, typically by locking it. Locks also provide one method of ensuring that data does not change while a transaction is reading it or even that it doesn't change until a transaction that once read it has completed.

Lock types:

Locks can be shared or exclusive, and can lock out readers and/or writers. Locks can be created implicitly by the DBMS when a transaction performs an operation, or explicitly at the transaction's request.

Shared locks allow multiple transactions to lock the same resource. The lock persists until all such transactions complete. Exclusive locks are held by a single transaction and prevent other transactions from locking the same resource.

Read locks are usually shared, and prevent other transactions from modifying the resource. Write locks are exclusive, and prevent other transactions from modifying the resource. On some systems, write locks also prevent other transactions from reading the resource.

The DBMS implicitly locks data when it is updated, and may also do so when it is read. Transactions explicitly lock data to ensure that they can complete without complications. Explicit locks may be useful for some administrative tasks.

Locking can significantly affect database performance, especially with large and complex transactions in highly concurrent environments.

Lock granularity:

Locks can be coarse, covering an entire database, fine-grained, covering a single data item, or intermediate covering a collection of data such as all the rows in a RDBMS table.

Deadlocks:

Deadlocks occur when two transactions each require data that the other has already locked exclusively. Deadlock detection is performed by the DBMS, which then aborts one of the transactions and allows the other to complete.

Transactions-The ACID rules

As every software system, a DBMS operates in a faulty computing environment and prone to failures of many kinds. A failure can corrupt the respective database unless special measures are taken to prevent this. A DBMS achieves certain levels of fault tolerance by encapsulating in database transactions units of work (executed programs) performed upon the respective database.

Most DBMS provide some form of support for transactions, which allow multiple data items to be updated in a consistent fashion, such that updates that are part of a transaction succeed or fail in unison. The so-called ACID rules, summarized here, characterize this behavior:

• Atomicity: Either all the data changes in a transaction must happen, or none of them. The transaction must be completed, or else it must be undone (rolled back).
• Consistency: Every transaction must preserve the declared consistency rules for the database.
• Isolation: Two concurrent transactions cannot interfere with one another. Intermediate results within one transaction must remain invisible to other transactions. The most extreme form of isolation is serializability, meaning that transactions that take place concurrently could instead be performed in some series, without affecting the ultimate result.
• Durability: Completed transactions cannot be aborted later or their results discarded. They must persist through (for instance) DBMS restarts.

In practice, many DBMSs allow the selective relaxation of these rules to balance perfect behavior with optimum performance.

Various types of databases

Operational database

These databases store detailed data about the operations of an organization. They are typically organized by subject matter, process relatively high volumes of updates using transactions. Essentially every major organization on earth uses such databases. Examples include customer databases that record contact, credit, and demographic information about a business' customers, personnel databases that hold information such as salary, benefits, skills data about employees, Enterprise resource planning that record details about product components, parts inventory, and financial databases that keep track of the organization's money, accounting and financial dealings.

Data warehouse

Data warehouses archive modern data from operational databases and often from external sources such as market research firms. Often operational data undergoes transformation on its way into the warehouse, getting summarized, anonymized, reclassified, etc. The warehouse becomes the central source of data for use by managers and other end-users who may not have access to operational data. For example, sales data might be aggregated to weekly totals and converted from internal product codes to use UPC codes so that it can be compared with ACNielson data.Some basic and essential components of data warehousing include retrieving and analyzing data, transforming,loading and managing data so as to make it available for further use.
Operations in a data warehouse are typically concerned with bulk data manipulation, and as such, it is unusual and inefficient to target individual rows for update, insert or delete. Bulk native loaders for input data and bulk SQL passes for aggregation are the norm.

Analytical database

Analysts may do their work directly against a data warehouse or create a separate analytic database for Online Analytical Processing. For example, a company might extract sales records for analyzing the effectiveness of advertising and other sales promotions at an aggregate level.

Distributed database

These are databases of local work-groups and departments at regional offices, branch offices, manufacturing plants and other work sites. These databases can include segments of both common operational and common user databases, as well as data generated and used only at a user’s own site.

End-user database

These databases consist of data developed by individual end-users. Examples of these are collections of documents in spreadsheets, word processing and downloaded files, even managing their personal baseball card collection.

External database

These databases contain data collected for use across multiple organizations, either freely or via subscription. The Internet Movie Database is one example.

Hypermedia databases

The World wide web can be thought of as a database, albeit one spread across millions of independent computing systems.Web browsers "process" this data one page at a time, while web crawlers and other software provide the equivalent of database indexes to support search and other activities.

Fundamentals of DataBases

A database is a system intended to organize, store, and retrieve large amounts of data easily. It consists of an organized collection of data for one or more uses, typically in digital form. One way of classifying databases involves the type of their contents, for example: bibliographic, document-text, statistical. Digital databases are managed using database management systems, which store database contents, allowing data creation and maintenance, and search and other access.