SQL - Introduction, Versions and History

A query is a more powerful type of filter that can gather information from multiple tables in a relational database. Some database systems provide special windows or forms for creating queries.

A query is a more powerful type of filter that can gather information from multiple tables in a relational database. Some database systems provide special windows or forms for creating queries. 

Because of similar structure of almost all database, a common type of query language was developed. In the mid-1970s, a language called structured English Query Language (SEQUEL) was developed by mainframe database developers.

Later SEQUEL was changed to SQL with some improvements in it. SQL is the English-like query language that allows the user to query a database without much knowing about the underlying database. SQL can be used as a data definition language (DDL) and as data manipulation language (DML). File extension of SQL is .sql. 

SQL Versions List

Year
Name
1986
SQL-86
1989
SQL-89
1992
SQL-92
1999
SQL-1999
2003
SQL-2003
2006
SQL-2006
2008
SQL-2009
2011
SQL-2011

Important Terms of SQL 

Database

  • A database is an organized collection of related information or data, which are shared and used for multiple purposes. 
  • A database can be a flat file database, relational Database, Distributed Database etc. The organized information or database serves as a base from which desired information can be retrieved; many meaningful conclusion can be drawn, or decision made, by further reorganizing or processing these data. 
  • A telephone directory is one of the most common examples of a database where the names of telephone subscribers are given in alphabetical order. The dictionary is another example of a database in which words are arranged alphabetically. 

Tables

  • In relational databases and flat file databases, a table is an organized set of data elements (values) using a model of vertical columns (which are identified by their name) and horizontal rows, the cell being the unit where a row and column intersect. 
  • A table has a specified number of columns but can have any number of rows. Each row is identified by the values appearing in a particular column subset which has been identified as a unique key index.

Fields

  • Fields are the attributes of the entity which is also known as a table, the here table name is the entity and all properties of the entities are known as the fields or attributes.

Record

  • A group of related fields in a table. Every record of a database contains the same fields.

Field Name

  • Each field in a table has a field name. Field names are used to reference the values in a record.

Field Value


  • The value in a field is known as the field value or attribute value is also known as Field value.

Query

  • You can use a query to select certain records from one or more tables. A query can also be used as a source of records for forms and reports.

Form


  • A form is used to enter or view the information of one record.

Report


  • A document produced from the data in a database.

Macro


  • The sequence of instructions or actions you can specify. Macros allow you to write programs that control user interaction and objects in your database. (Macros are an advanced topic not covered in this course.)

Module


  • A collection of Visual Basic for Applications declarations and procedures are called modules.

DML - Data Manipulation Language

The data management system also provides a data manipulation language to access and use the stored data in applications. A data manipulation language DML is a language that enables users to access or maintain data as organized by the appropriate database system, Data manipulation means
  1. Insertion.
  2. Deletion
  3. Retrieval.
  4. Modification.
In general, the DML is very flexible in that it may be used by itself to create, read, update, and delete records; or its commands may be called from a separate host programming language such as visual basic or Java.
A data manipulation language is a language that enables users to access or manipulate data as organized by the appropriate data model, i.e, DML is used to update the database by adding new data or modify or delete the existing data. 
Some or the manipulation commands are, 
Insert into student values (12, 'Hari', 'Kathmandu',23232);
Delete from student 
where address='kathmandu';

DDL - Data Definition Language

A database schema is specified by a set of definitions expressed by a special language called data-definition language DDL. The result of compilation of DDL statement is a set of tables that is stored in a special file called data dictionary. A data dictionary is a file that contains metadata that is data about data.

DDL is a special language, which supports the definition or declaration of database objects, i.e. it is used to specify the data in the database. (It allows users to define the structure of data in the database. This language is not a procedure language. DDL includes statements for creating, manipulating, altering and deleting the tables and views. Some of the data definition commands are,
Create Table, Create View, Alter Table, Drop Table 
Create Table student(
roll int,
address varchar(100),
name varchar(100), phone int
)

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