Interview Questions for SQL

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  1. What is SQL?

SQL stands for Structured Query Language and it is used to communicate with the Database. SQL is a standard query language used for maintaining the relational database and perform many different operations of data manipulation on the data. SQL initially was invented in 1970. This is a standard language used to perform tasks such as retrieval, updation, insertion and deletion of data from a database.

  1. What are the usages of SQL?

SQL is responsible for maintaining the relational data and the data structures present in the database.

  • To execute queries against a database
  • To retrieve data from a database
  • To inserts records in a database
  • To updates records in a database
  • To delete records from a database
  • To create new databases
  • To create new tables in a database
  • To create views in a database
  • To perform complex operations on the database.
  1. What is a 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: School Management Database, Bank Management Database.

    4.  What are the subsets of SQL?

There is three significant subset of the SQL:

  1. Data definition language (DDL):DDL is used to define the data structure it consists of the commands like CREATE, ALTER, DROP, etc.
  2. Data manipulation language (DML):DML is used to manipulate already existing data in the database. The commands in this category are SELECT, UPDATE, INSERT, etc.
  3. Data control language (DCL):DCL is used to control access to data in the database and includes commands such as GRANT, REVOKE.

5. What are tables and Fields?

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.


Table: Employee.

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

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

6. What is Data Control Language?

Data control language allows you to control access to the database. DCL is the only subset of the database which decides that what part of the database should be accessed by which user at what point of time. It includes two commands GRANT and REVOKE.

GRANT: to grant the specific user to perform a particular task

REVOKE: to cancel previously denied or granted permissions.

7. 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.

8. 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.

9. What is the difference between primary key and unique key?

Primary key and unique key both are the essential constraints of the SQL, but there is a small difference between them

Primary key carries unique value but the field of the primary key cannot be Null on the other hand unique key also carry unique value but it can have a single Null value field.

  1. 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.

11. What is the primary use of Normalization?

Normalization is mainly used to add, delete or modify a field that can be made in a single table. The primary use of Normalization is to remove redundancy and to remove the insert, delete and update distractions. Normalization breaks the table into small partitions and then link them using different relationships so that it will avoid the chances of redundancy.

  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 systems

  1. 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.

  1. 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.

  1. 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.

16. 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.

17. What are the set operators in SQL?

SQL queries which contain set operations are called compound queries.

Union, Union All, Intersect or minus operators are the set operators used in the SQL.

  1. 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

19. Is it possible to sort a column using a column alias?

Yes. You can use the column alias in the ORDER BY instead of WHERE clause for sorting.

  1. What is sub query?

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

 21. Which are the most commonly used SQL joins?

Most commonly used SQL joins are INNER JOIN and LEFT OUTER JOIN and RIGHT OUTER JOIN.

  1. What is a stored procedure?

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

  1. 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.

  1. 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.

  1. 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.

  1. What is collation?

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.

ASCII value can be used to compare these character data.

  1. What are all different types of collation sensitivity?

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.
  1. Advantages and Disadvantages of Stored Procedure?

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.

  1. What is Online Transaction Processing (OLTP)?

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. Unlike OLAP systems goal of OLTP systems is serving real-time transactions.

Example – Bank Transactions on a daily basis.

  1. What is CLAUSE?

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.

Example – Query that has WHERE condition

  1. What is recursive stored procedure?

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.

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