Monday, November 2, 2015

Data retrieval query (SQL)

Data retrieval involves the capability to easily select data for graphic or attribute editing, updating, querying, analysis and/or display.
Retrieval functions allow the selective search and manipulation of data without the need to create new entities.
Querying is the capability to retrieve data, usually a data subset, based on some user defined formula. These data subsets are often referred to as logical views. Often the querying is closely linked to the data manipulation and analysis subsystem. Several GIS software vendors have attempted to standardize their querying capability by use of a Standard Query Language (SQL).

Structured Query Language (SQL) is a data query and manipulation language designed for relational databases. It was developed by IBM in the 1970s. A new development has been to extend SQL to Object-oriented database management systems and to spatial data.

In order to use SQL to access a database, the syntax of the query language must be used. The basic syntax of SQL is:

select <Attribute list> from <Relation> where <Condition>
Here, 'select','from' and 'where' are the keywords and Attribute list is the field from the database, Relation refers to a table and Condition refers to criteria for data query.