SQL Basics

Question 1

What is DBMS?

Answer 1

A Database Management System (DBMS) is a program that controls creation, maintenance and use of a database.

Question 2

What is RDBMS?

Answer 2

RDBMS stands for Relational Database Management System. RDBMS store the data into the collection of tables, which is related by common fields between the columns of the table.

Question 3

What is SQL?

Answer 3

SQL stands for Structured Query Language , and it is used to communicate with the Database. This is a standard language used to perform tasks such as retrieval, updation, insertion and deletion of data from a database.

Question 4

What is a Database?

Answer 4

Database is nothing but an organized form of data for easy access, storing, retrieval and managing of data. This is also known as structured form of data which can be accessed in many ways.

Question 5

What are tables and Fields?

Answer 5

A table is a set of data that are organized in a model with Columns and Rows. Columns can be categorized as vertical, and Rows are horizontal. A table has specified number of column called fields but can have any number of rows which is called record.

Question 6

What is a primary key?

Answer 6

A primary key is a combination of fields which uniquely specify a row. This is a special kind of unique key, and it has implicit NOT NULL constraint. It means, Primary key values cannot be NULL.

Question 7

What is a unique key?

Answer 7

A Unique key constraint uniquely identified each record in the database. This provides uniqueness for the column or set of columns.

Question 8

What is a foreign key?

Answer 8

A foreign key is one table which can be related to the primary key of another table. Relationship needs to be created between two tables by referencing foreign key with the primary key of another table.

Question 9

What is a join?

Answer 9

This is a keyword used to query data from more tables based on the relationship between the fields of the tables. Keys play a major role when JOINs are used.

Question 10

What are the types of join and explain each?

Answer 10

There are various types of join which can be used to retrieve data and it depends on the relationship between tables.
Inner Join - Inner join return rows when there is at least one match of rows between the tables.
Right Join - Right join return rows which are common between the tables and all rows of Right hand side table.
Left Join - Left join return rows which are common between the tables and all rows of Left hand side table.
Full Join - Full join return rows when there are matching rows in any one of the tables.

Question 11

What is normalization?

Answer 11

Normalization is the process of minimizing redundancy and dependency by organizing fields and table of a database.

Question 12

What is Denormalization.

Answer 12

DeNormalization is a technique used to access the data from higher to lower normal forms of database.

Question 13

What are all the different normalizations?

Answer 13

First Normal Form (1NF):
This should remove all the duplicate columns from the table. Creation of tables for the related data and identification of unique columns.

Second Normal Form (2NF):
Meeting all requirements of the first normal form. Placing the subsets of data in separate tables and Creation of relationships between the tables using primary keys.

Third Normal Form (3NF):
This should meet all requirements of 2NF. Removing the columns which are not dependent on primary key constraints.

Fourth Normal Form (4NF):.
Meeting all the requirements of third normal form and it should not have multi- valued dependencies.

Question 14

What is a View?

Answer 14

A view is a virtual table which consists of a subset of data contained in a table. Views are not virtually present, and it takes less space to store. View can have data of one or more tables combined, and it is depending on the relationship.

Question 15

What is an Index?

Answer 15

An index is performance tuning method of allowing faster retrieval of records from the table. An index creates an entry for each value and it will be faster to retrieve data.

Question 16

What are all the different types of indexes?

Answer 16

Unique Index.
This indexing does not allow the field to have duplicate values if the column is unique indexed. Unique index can be applied automatically when primary key is defined.

Clustered Index.
This type of index reorders the physical order of the table and search based on the key values. Each table can have only one clustered index.

NonClustered Index.
NonClustered Index does not alter the physical order of the table and maintains logical order of data. Each table can have 999 nonclustered indexes.

Question 17

What is a Cursor?

Answer 17

A database Cursor is a control which enables traversal over the rows or records in the table. This can be viewed as a pointer to one row in a set of rows. Cursor is very much useful for traversing such as retrieval, addition and removal of database records.

Question 18

What is a relationship and what are they?

Answer 18

Database Relationship is defined as the connection between the tables in a database. There are various data basing relationships, and they are as follows:.

One to One Relationship.
One to Many Relationship.
Many to One Relationship.
Self-Referencing Relationship.

Question 19

What is a query?

Answer 19

A DB query is a code written in order to get the information back from the database. Query can be designed in such a way that it matched with our expectation of the result set. Simply, a question to the Database.

Question 20

What is subquery?

Answer 20

A subquery is a query within another query. The outer query is called as main query, and inner query is called subquery. SubQuery is always executed first, and the result of subquery is passed on to the main query.

Question 21

What are the types of subquery?

Answer 21

There are two types of subquery – Correlated and Non-Correlated.

A correlated subquery cannot be considered as independent query, but it can refer the column in a table listed in the FROM the list of the main query.

A Non-Correlated sub query can be considered as independent query and the output of subquery are substituted in the main query.

Question 22

What is a stored procedure?

Answer 22

Stored Procedure is a function consists of many SQL statement to access the database system. Several SQL statements are consolidated into a stored procedure and execute them whenever and wherever required.

Question 23

What is a trigger?

Answer 23

A DB trigger is a code or programs that automatically execute with response to some event on a table or view in a database. Mainly, trigger helps to maintain the integrity of the database.

Question 24

What is the difference between DELETE and TRUNCATE commands?

Answer 24

DELETE command is used to remove rows from the table, and WHERE clause can be used for conditional set of parameters. Commit and Rollback can be performed after delete statement.

TRUNCATE removes all rows from the table. Truncate operation cannot be rolled back.

Question 25

What are local and global variables and their differences?

Answer 25

Local variables are the variables which can be used or exist inside the function. They are not known to the other functions and those variables cannot be referred or used. Variables can be created whenever that function is called.

Global variables are the variables which can be used or exist throughout the program. Same variable declared in global cannot be used in functions. Global variables cannot be created whenever that function is called.

Question 26

What is a constraint?

Answer 26

Constraint can be used to specify the limit on the data type of table. Constraint can be specified while creating or altering the table statement. Sample of constraint are.

NOT NULL.
CHECK.
DEFAULT.
UNIQUE.
PRIMARY KEY.
FOREIGN KEY.

Question 27

What is data Integrity?

Answer 27

Data Integrity defines the accuracy and consistency of data stored in a database. It can also define integrity constraints to enforce business rules on the data when it is entered into the application or database.

Question 28

What is Auto Increment?

Answer 28

Auto increment keyword allows the user to create a unique number to be generated when a new record is inserted into the table. AUTO INCREMENT keyword can be used in Oracle and IDENTITY keyword can be used in SQL SERVER.

Question 29

What is the difference between Cluster and Non-Cluster Index?

Answer 29

Clustered index is used for easy retrieval of data from the database by altering the way that the records are stored. Database sorts out rows by the column which is set to be clustered index.

A nonclustered index does not alter the way it was stored but creates a complete separate object within the table. It point back to the original table rows after searching.

Question 30

What is Datawarehouse?

Answer 30

Datawarehouse is a central repository of data from multiple sources of information. Those data are consolidated, transformed and made available for the mining and online processing. Warehouse data have a subset of data called Data Marts.

Question 31

What is Self-Join?

Answer 31

Self-join is set to be query used to compare to itself. This is used to compare values in a column with other values in the same column in the same table. ALIAS ES can be used for the same table comparison.

Question 32

What is Cross-Join?

Answer 32

Cross join defines as Cartesian product where number of rows in the first table multiplied by number of rows in the second table. If suppose, WHERE clause is used in cross join then the query will work like an INNER JOIN.

Question 33

What is user defined functions?

Answer 33

User defined functions are the functions written to use that logic whenever required. It is not necessary to write the same logic several times. Instead, function can be called or executed whenever needed.

Question 34

What are all types of user defined functions?

Answer 34

Three types of user defined functions are.

Scalar Functions.
Inline Table valued functions.
Multi statement valued functions.

Question 35

What is collation?

Answer 35

Collation is defined as set of rules that determine how character data can be sorted and compared. This can be used to compare A and, other language characters and also depends on the width of the characters.

Question 36

What are all different types of collation sensitivity?

Answer 36

Following are different types of collation sensitivity -.

Case Sensitivity – A and a and B and b.
Accent Sensitivity.
Kana Sensitivity – Japanese Kana characters.
Width Sensitivity – Single byte character and double byte character.

Question 37

Advantages and Disadvantages of Stored Procedure?

Answer 37

Stored procedure can be used as a modular programming – means create once, store and call for several times whenever required. This supports faster execution instead of executing multiple queries. This reduces network traffic and provides better security to the data.

Disadvantage is that it can be executed only in the Database and utilizes more memory in the database server.

Question 38

What is Online Transaction Processing (OLTP)?

Answer 38

Online Transaction Processing (OLTP) manages transaction based applications which can be used for data entry, data retrieval and data processing. OLTP makes data management simple and efficient.

Question 39

What is CLAUSE?

Answer 39

SQL clause is defined to limit the result set by providing condition to the query. This usually filters some rows from the whole set of records.

Question 40

What is recursive stored procedure?

Answer 40

A stored procedure which calls by itself until it reaches some boundary condition. This recursive function or procedure helps programmers to use the same set of code any number of times.

Question 41

What is Union, minus and Interact commands?

Answer 41

UNION operator is used to combine the results of two tables, and it eliminates duplicate rows from the tables.

MINUS operator is used to return rows from the first query but not from the second query. Matching records of first and second query and other rows from the first query will be displayed as a result set.

INTERSECT operator is used to return rows returned by both the queries.

Question 42

What is an ALIAS command?

Answer 42

ALIAS name can be given to a table or column. This alias name can be referred in WHERE clause to identify the table or column.

Question 43

What is the difference between TRUNCATE and DROP statements?

Answer 43

TRUNCATE removes all the rows from the table, and it cannot be rolled back. DROP command removes a table from the database and operation cannot be rolled back.

Question 44

What are aggregate and scalar functions?

Answer 44

Aggregate functions are used to evaluate mathematical calculation and return single values. This can be calculated from the columns in a table. Scalar functions return a single value based on the input value.

Question 45

How can you create an empty table from an existing table?

Answer 45

Select * into studentcopy from student where 1=2

Here, we are copying student table to another table with the same structure with no rows copied.

Question 46

How to fetch common records from two tables?

Answer 46

Select studentID from student. <strong>INTERSECT </strong> Select StudentID from Exam

Question 47

How to fetch alternate records from a table?

Answer 47

Even Numbers
Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=0

Odd Numbers
Select studentId from (Select rowno, studentId from student) where mod(rowno,2)=1

Question 48

How to select unique records from a table?

Answer 48

Select DISTINCT StudentID, StudentName from Student.

Question 49

What is the command used to fetch first 5 characters of the string?

Answer 49

Select SUBSTRING(StudentName,1,5) as studentname from student

Select LEFT(Studentname,5) as studentname from student

Question 50

Which operator is used in query for pattern matching?

Answer 50

LIKE operator is used for pattern matching, and it can be used as -.

% - Matches zero or more characters.
_(Underscore) – Matching exactly one character.
Example -.

Select * from Student where studentname like ‘a%’
Select * from Student where studentname like ‘ami_’