Database (DBMS) Basic Concepts

 Questions : 1      What is database or database management systems (DBMS)? and - What’s the difference between file and database? Can files qualify as a database?

Answers : 1        

Database provides a systematic and organized way of storing, managing and retrieving from collection of logically related information.

Secondly the information has to be persistent, that means even after the application is closed the information should be persisted.

Finally it should provide an independent way of accessing data and should not be dependent on the application to access the information.

Main difference between a simple file and database that database has independent way (SQL) of accessing information while simple files do not File meets the storing, managing and retrieving part of a database but not the independent way of accessing data. Many experienced programmers think that the main difference is that file can not provide multi-user capabilities which a DBMS provides. But if we look at some old COBOL and C programs where file where the only means of storing data, we can see functionalities like locking, multi-user etc provided very efficiently. So it’s a matter of debate if some interviewers think this as a main difference between files and database accept it… going in to debate is probably loosing a job.

           

Questions : 2      What is SQL ?

Answers : 2        

SQL stands for Structured Query Language.SQL is an ANSI (American National Standards Institute) standard computer language for accessing and manipulating database systems. SQL statements are used to retrieve and update data in a database.

           

Questions : 3      What’s difference between DBMS and RDBMS ?

Answers : 3        

DBMS provides a systematic and organized way of storing, managing and retrieving from collection of logically related information. RDBMS also provides what DBMS provides but above that it provides relationship integrity. So in short we can say

RDBMS = DBMS + REFERENTIAL INTEGRITY

These relations are defined by using “Foreign Keys” in any RDBMS.Many DBMS companies claimed there DBMS product was a RDBMS compliant, but according to industry rules and regulations if the DBMS fulfills the twelve CODD rules it’s truly a RDBMS. Almost all DBMS (SQL SERVER, ORACLE etc) fulfills all the twelve CODD rules and are considered as truly RDBMS.

           

Questions : 4      What are CODD rules?

Answers : 4        

In 1969 Dr. E. F. Codd laid down some 12 rules which a DBMS should adhere in order to get the logo of a true RDBMS.

Rule 1: Information Rule.

"All information in a relational data base is represented explicitly at the logical level and in exactly one way - by values in tables."

Rule 2: Guaranteed access Rule.

"Each and every datum (atomic value) in a relational data base is guaranteed to be logically accessible by resorting to a combination of table name, primary key value and column name."

In flat files we have to parse and know exact location of field values. But if a DBMS is truly RDBMS you can access the value by specifying the table name, field name, for instance Customers.Fields [‘Customer Name’].

Rule 3: Systematic treatment of null values.

"Null values (distinct from the empty character string or a string of blank characters and distinct from zero or any other number) are supported in fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of data type.".

Rule 4: Dynamic on-line catalog based on the relational model.

"The data base description is represented at the logical level in the same way as ordinary data, so that authorized users can apply the same relational language to its interrogation as they apply to the regular data."The Data Dictionary is held within the RDBMS, thus there is no-need for off-line volumes to tell you the structure of the database.

Rule 5: Comprehensive data sub-language Rule.

"A relational system may support several languages and various modes of terminal use (for example, the fill-in-the-blanks mode). However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings and that is comprehensive in supporting all the following items

Data Definition

View Definition

Data Manipulation (Interactive and by program).

Integrity Constraints

Authorization.

Transaction boundaries ( Begin , commit and rollback)

Rule 6: .View updating Rule

"All views that are theoretically updatable are also updatable by the system."

Rule 7: High-level insert, update and delete.

"The capability of handling a base relation or a derived relation as a single operand applies not only to the retrieval of data but also to the insertion, update and deletion of data."

Rule 8: Physical data independence.

"Application programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representations or access methods."

Rule 9: Logical data independence.

"Application programs and terminal activities remain logically unimpaired when information-preserving changes of any kind that theoretically permit un-impairment are made to the base tables."

Rule 10: Integrity independence.

"Integrity constraints specific to a particular relational data base must be definable in the relational data sub-language and storable in the catalog, not in the application programs." Rule 11: Distribution independence.

"A relational DBMS has distribution independence."

Rule 12: Non-subversion Rule.

"If a relational system has a low-level (single-record-at-a-time) language, that low level cannot be used to subvert or bypass the integrity Rules and constraints expressed in the higher level relational language (multiple-records-at-a-time)."

           

Questions : 5      What are E-R diagrams?

Answers : 5        

E-R diagram also termed as Entity-Relationship diagram shows relationship between various tables in the database. .

           

Questions : 6      How many types of relationship exist in database designing?

Answers : 6        

There are three major relationship models:-

One-to-one

One-to-many

Many-to-many

           

Questions : 7      7.What is normalization? What are different type of normalization?

Answers : 7        

There is set of rules that has been established to aid in the design of tables that are meant to be connected through relationships. This set of rules is known as Normalization.

Benefits of Normalizing your database include:

=>Avoiding repetitive entries

=>Reducing required storage space

=>Preventing the need to restructure existing tables to accommodate new data.

=>Increased speed and flexibility of queries, sorts, and summaries.

Following are the three normal forms :-

First Normal Form

For a table to be in first normal form, data must be broken up into the smallest un possible.In addition to breaking data up into the smallest meaningful values, tables first normal form should not contain repetitions groups of fields.

Second Normal form

The second normal form states that each field in a multiple field primary keytable must be directly related to the entire primary key. Or in other words,each non-key field should be a fact about all the fields in the primary key.

Third normal form

A non-key field should not depend on other Non-key field.

           

Questions : 8      What is denormalization ?

Answers : 8        

Denormalization is the process of putting one fact in numerous places (its vice-versa of normalization).Only one valid reason exists for denormalizing a relational design - to enhance performance.The sacrifice to performance is that you increase redundancy in database.

           

Questions : 9      Can you explain Fourth Normal Form and Fifth Normal Form ?

Answers : 9        

In fourth normal form it should not contain two or more independent multi-v about an entity and it should satisfy “Third Normal form”.

Fifth normal form deals with reconstructing information from smaller pieces of information. These smaller pieces of information can be maintained with less redundancy.

           

Questions : 10    Have you heard about sixth normal form?

Answers : 10      

If we want relational system in conjunction with time we use sixth normal form. At this moment SQL Server does not supports it directly.

           

Questions : 11    What are DML and DDL statements?

Answers : 11      

DML stands for Data Manipulation Statements. They update data values in table. Below are the most important DDL statements:-

=>SELECT - gets data from a database table

=> UPDATE - updates data in a table

=> DELETE - deletes data from a database table

=> INSERT INTO - inserts new data into a database table

DDL stands for Data definition Language. They change structure of the database objects like table, index etc. Most important DDL statements are as shown below:-

=>CREATE TABLE - creates a new table in the database.

=>ALTER TABLE – changes table structure in database.

=>DROP TABLE - deletes a table from database

=> CREATE INDEX - creates an index

=> DROP INDEX - deletes an index

           

Questions : 12    How do we select distinct values from a table?

Answers : 12      

DISTINCT keyword is used to return only distinct values. Below is syntax:- Column age and Table pcdsEmp

SELECT DISTINCT age FROM pcdsEmp

           

Questions : 13    What is Like operator for and what are wild cards?

Answers : 13      

LIKE operator is used to match patterns. A "%" sign is used to define the pattern.

Below SQL statement will return all words with letter "S"

SELECT * FROM pcdsEmployee WHERE EmpName LIKE 'S%'

Below SQL statement will return all words which end with letter "S"

SELECT * FROM pcdsEmployee WHERE EmpName LIKE '%S'

Below SQL statement will return all words having letter "S" in between

SELECT * FROM pcdsEmployee WHERE EmpName LIKE '%S%'

"_" operator (we can read as “Underscore Operator”). “_” operator is the character defined at that point. In the below sample fired a query Select name from pcdsEmployee where name like '_s%' So all name where second letter is “s” is returned.

           

Questions : 14    Can you explain Insert, Update and Delete query?

Answers : 14      

Insert statement is used to insert new rows in to table. Update to update existing data in the table. Delete statement to delete a record from the table. Below code snippet for Insert, Update and Delete :-

INSERT INTO pcdsEmployee SET name='rohit',age='24';

UPDATE pcdsEmployee SET age='25' where name='rohit';

DELETE FROM pcdsEmployee WHERE name = 'sonia';

           

Questions : 15    What is order by clause?

Answers : 15      

ORDER BY clause helps to sort the data in either ascending order to descending order.

Ascending order sort query

SELECT name,age FROM pcdsEmployee ORDER BY age ASC

Descending order sort query

SELECT name FROM pcdsEmployee ORDER BY age DESC

           

Questions : 16    What is the SQL " IN " clause?

Answers : 16      

SQL IN operator is used to see if the value exists in a group of values. For instance the below SQL checks if the Name is either 'rohit' or 'Anuradha' SELECT * FROM pcdsEmployee WHERE name IN ('Rohit','Anuradha') Also you can specify a not clause with the same. SELECT * FROM pcdsEmployee WHERE age NOT IN (17,16)

           

Questions : 17    Can you explain the between clause?

Answers : 17       Below SQL selects employees born between '01/01/1975' AND '01/01/1978' as per mysql

SELECT * FROM pcdsEmployee WHERE DOB BETWEEN '1975-01-01' AND '2011-09-28'

           

Questions : 18    we have an employee salary table how do we find the second highest from it?

Answers : 18      

below Sql Query find the second highest salary

SELECT * FROM pcdsEmployeeSalary a WHERE (2=(SELECT COUNT(DISTINCT(b.salary)) FROM pcdsEmployeeSalary b WHERE b.salary>=a.salary))