Home / Interview / SQL :: General Questions

Interview :: SQL

1) What is SQL?

SQL stands for the Structured Query Language. 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. It is a database language used for database creation, deletion, fetching rows and modifying rows, etc. sometimes it is pronounced as 'sequel.'

2) When SQL appeared?

It appeared in 1974. SQL is one of the often used languages for maintaining the relational database. SQL. In 1986 SQL become the standard of American National Standards Institute (ANSI) and ISO(International Organization for Standardization) in 1987.

3) 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.
4) Does SQL support programming?

SQL refers to the Standard Query Language, which is not actually the programming language. SQL doesn't have a loop, Conditional statement, logical operations, it can not be used for anything other than data manipulation. It is used like commanding (Query) language to access databases. The primary purpose of SQL is to retrieve, manipulate, update and perform complex operations like joins on the data present in the database.

5) 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.
6) What is a Data Definition Language?

Data definition language (DDL) is the subset of the database which defines the data structure of the database in the initial stage when the database is about to be created. It consists of the following commands: CREATE, ALTER and DELETE database objects such as schema, tables, view, sequence, etc.

7) What is a Data Manipulation Language?

Data manipulation language makes the user able to retrieve and manipulate data. It is used to perform the following operations.

  • Insert data into database through INSERT command.
  • Retrieve data from the database through SELECT command.
  • Update data in the database through UPDATE command.
  • Delete data from the database through DELETE command.

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 :  It enables system administrators to assign privileges and roles to the specific user accounts to perform specific tasks on the database.

REVOKE : It enables system administrators to revoke privileges and roles from the user accounts so that they cannot use the previously assigned permission on the database.

9) What are tables and fields in the database?

A table is a set of organized data. It has rows and columns. Rows here refers to the tuples which represent the simple data item and columns are the attribute of the data items present in particular row. Columns can categorize as vertical, and Rows are horizontal.

A table contains a specified number of the column called fields but can have any number of rows which is known as the record. So, the columns in the table of the database are known as the fields and they represent the attribute or characteristics of the entity in the record.

10) What is a primary key?

A primary key is a field or the combination of fields which uniquely specify a row. The Primary key is a special kind of unique key. Primary key values cannot be NULL. For example, the Social Security Number can be treated as the primary key for any individual.