Introduction to SQL Databases

SQL Database Interview Questions

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1) What is a Database?

A database is a collection of information in an organized form for faster and better access, storage and manipulation. It can also be defined as a collection of tables, schema, views, and other database objects.
 

2) What is Data warehouse?

Data warehouse refers to a central repository of data from multiple sources of information. Those data are consolidated, transformed and made available for the mining as well as online processing.
 

3) What are HDFS and YARN? 

HDFS (Hadoop Distributed File System) is the storage unit of Hadoop. It is responsible for storing different kinds of data as blocks in a distributed environment. It follows master and slave topology.

YARN (Yet Another Resource Negotiator) is the processing framework in Hadoop, which manages resources and provides an execution environment to the processes.

A table is a database object used to store records in a field in the form of columns and rows that holds data.
 

4) What is a Field in a Database?

A field in a Database table is a space allocated to store a particular record within a table.
 

5) What is a Record in a Database?

A record (also called a row of data) is an ordered set of related data in a table.
 

6) What is a column in a Table?

A column is a vertical entity in a table that contains all information associated with a specific field in a table.
 

7) What is DBMS?

Database Management System is a collection of programs that enables a user to store, retrieve, update and delete information from a database.
 

8) What are the types of DBMS?

There are two types of DBMS
1. Relational Database Management System (RDBMS)
2. Non-Relational Database Management System 

9) What is RDBMS?

RDBMS stands for Relational Database Management System. RDBMS is a database management system (DBMS) that is based on the relational model. Data from a relational database can be accessed using Structured Query Language (SQL).
 

10) What are the popular Database Management Systems in the IT Industry?

Oracle, MySQL, Microsoft SQL Server, PostgreSQL, Sybase, MongoDB, DB2, and Microsoft Access etc.
 

11) What is SQL?

SQL Overview: SQL stands for Structured Query Language. It is an American National Standard Institute (ANSI) standard. It is a standard language for accessing and manipulating databases. Using SQL, some of the action we could do are to create databases, tables, stored procedures (SP’s), execute queries, retrieve, insert, update, delete data against a database.
 

12) What is a 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.
 

13) What is a unique key? 

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

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.
 

14) 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.
 

15) 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.
 

16) 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 Right hand side table. Simply, it returns all the rows from the right hand side table even though 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 hand side table. Simply, it returns all the rows from Left hand side table even though there are 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.
 

17) 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.
 

18) What are all the different normalizations? 

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

• 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.
 

19) 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.
 

20) 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.
 

21) 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.
 

22) 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.
 

23) 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

 
24) What is the difference between an inner and outer join? 

An inner join involves joining two tables where a common id/key exists in both. An outer join is the joining of two tables, but where there is no match in the second (or first).
 

25) How do you maintain database integrity where deletions from one table will automatically cause deletions in another table? 

You create a trigger that will automatically delete elements in the second table when elements from the first table are removed.
 

26) What port does SQL server run on? 

1433 is the standard port for the SQL server.
 

27) What are the differences between primary and foreign keys? 

The primary key is the column or set of columns used to uniquely identify the items in a table. A foreign key is used to uniquely identify the items in a different table, allowing join operations to happen.
 

28) What is a query? 

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.
 

29) 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.
 

30) 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.
 

31) 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.
 

32) What is a trigger? 

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. 

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.
 

33) 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.
 

34) What is a constraint? 

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.

 
35) What is data Integrity? 

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.
 

36) What is Auto Increment? 

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.

Mostly this keyword can be used whenever PRIMARY KEY is used.
 

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

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.
 

38) What is Datawarehouse? 

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.
 

39) What is Self-Join? 

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.
 

40) What is Cross-Join? 

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.
 

41) What is user defined functions? 

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.
 

42) What are all types of user defined functions? 

Three types of user defined functions are.

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

Scalar returns unit, variant defined the return clause. Other two types return table as a return.