SQL Interview Questions & Answers

1. What is DBMS ?

A Database Management System (DBMS) is a program that controls creation, maintenance and use of a database. DBMS can be termed as file manager that manages data in a database rather than saving it in file system.

2. What is RDBMS ?

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. It also provides relational operators to manipulate the data stored into the tables.

Example : SQL Server.

3. What is SQL ?

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 insert, update, retrieve & delete of data from database.

Standard SQL Command is Select.

4. What is Database ?

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.

Example : Hospital Management Database, Bank Management Database.

5. What are tables, fields and record ?

A table is a set of data are organized in a model with Column and Rows. Column 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.

Example:

Table: Employee.

Field: Emp ID, Emp Name, Date of Birth.

Record: 201456, David, 11/15/1960.

6. What is primary key ?

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.

7. What is a unique key ?

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

uniqueness for the A Primary key constraint has automatic unique constraint defined on it. But not, in the

case of Unique Key. There can be many unique constraint defined per table, but only one Primary key:

constraint defined per table.

8. What is a foreign key ?

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.

9. What is a join ?

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.

10. What are the types of join and explain each?

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 Righthand side table. Simply, it returns all the rows from the right hand side table eventhough there are no matches in the left hand side table.
Left Join. Left join return rows which are common between the tables and all rows of Left handside table. Simply, it returns all the rows from Left hand side table even though thereare no matches in the Right hand side table.
Full Join. Full join return rows when there are matching rows in any one of the tables. This means, it returns all the rows from the left hand side table and all the rows from the right hand side table.

11. What is normalization ?

Normalization is the process of minimizing redundancy and dependency by

organizing fields and table of a database. The main aim of Normalization is to add,

delete or modify field that can be made in a single table.

12. What is Denormalization ?

Denormalization is a technique used to access the data from higher to lower normal forms of database. It is also process of introducing redundancy into a table by incorporating data from the related tables.

13. What are all the different normalizations ?

The normal forms can be divided into 5 forms, and they are explained below

First Normal Form (INF):
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 multivalued dependencies.

14. What is a View ?

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.

15. What is an Index ?

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.

16. What are all the different types of indexes ?

There are three types of indexes –

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.

17. What is a Cursor ?

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.

18. What is a relationship and what are they ?

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.

19. What is a query ?

ADB 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.

20. What is subquery ?

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.

21. What are the types of subquery ?

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.

22. What is a stored procedure ?

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.

23. What is a trigger ?

ADB 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.

Example: When a new student is added to the student database, new records should be created in the related tables like Exam, Score and Attendance tables.

24. What is the difference between DELETE and TRUNCATE commands ?

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.

25. What are local and global variables and their differences ?

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.