In the modern GIS industry, programming skill is an essential asset and one of the programming languages the is very popular within the industry is Structure Query Language (SQL) as you will later find out how it is been used in few moments.
Most of the query expressions used in ArcGIS or QGIS desktop software are derived from SQL. SQL is a standard language for storing, manipulating and retrieving data in databases.
Both ArcGIS and QGIS support the following common database engines: SQLite, MySQL, SQL Server, MS Access, Oracle, Sybase, Informix, PostgreSQL, and other database systems. When connected to any of them, you can take full advantage of SQL in GIS.
For small GIS projects where database isn't required, we make use of traditional GIS files type such as "Shapefile" which allow us to use query expressions that adhere to standard SQL expressions.
What is an expressions?
An expressions is a combination of "Constants, Variables, Operators and Functions" organized in an ordered statement that returns an output value. Expressions are unique to the computer language they are created in. An example of SQL expressions is: "SELECT * FROM <Layer_name> WHERE <Field_name> <Operator> <Value or String>".
If you data resides in a shapefiles or any of these (coverages, INFO tables, and dBASE tables), this part of the SQL expression (SELECT * FROM <Layer_name> WHERE) is automatically supplied for you, so you only provide this part (<Field_name> <Operator> <Value or String>) to query your data.
Since GIS data is made up of Spatial and Attribute, it is worth noting that 'Attribute Query' is similar to the standard SQL queries found in traditional database systems and this will be our focus in this article. On the other hand, 'Spatial Query' which allow operations such as "Contains, Equals, Intersects, Is Disjoint, Overlaps, Touches, Within and Crosses" requires some extension installed on traditional database systems to make them work.
Building SQL Expressions for Attribute Query
There are many places you can build expressions within both ArcGIS and QGIS software, some of the common places are listed below:-
ArcGIS Tools:
~ Select By Attribute
~ Definition Query
~ Field Calculator
~ Label Expression Dialog Box
~ Add Query Layer
QGIS Tools:
~ Select By Expression
~ Filter Query Builder
~ Field Calculator
~ Label Expression Dialog Box
~ Layer Property Display
~ Database Manager
Most of the query expressions used in ArcGIS or QGIS desktop software are derived from SQL. SQL is a standard language for storing, manipulating and retrieving data in databases.
Both ArcGIS and QGIS support the following common database engines: SQLite, MySQL, SQL Server, MS Access, Oracle, Sybase, Informix, PostgreSQL, and other database systems. When connected to any of them, you can take full advantage of SQL in GIS.
For small GIS projects where database isn't required, we make use of traditional GIS files type such as "Shapefile" which allow us to use query expressions that adhere to standard SQL expressions.
What is an expressions?
An expressions is a combination of "Constants, Variables, Operators and Functions" organized in an ordered statement that returns an output value. Expressions are unique to the computer language they are created in. An example of SQL expressions is: "SELECT * FROM <Layer_name> WHERE <Field_name> <Operator> <Value or String>".
If you data resides in a shapefiles or any of these (coverages, INFO tables, and dBASE tables), this part of the SQL expression (SELECT * FROM <Layer_name> WHERE) is automatically supplied for you, so you only provide this part (<Field_name> <Operator> <Value or String>) to query your data.
Since GIS data is made up of Spatial and Attribute, it is worth noting that 'Attribute Query' is similar to the standard SQL queries found in traditional database systems and this will be our focus in this article. On the other hand, 'Spatial Query' which allow operations such as "Contains, Equals, Intersects, Is Disjoint, Overlaps, Touches, Within and Crosses" requires some extension installed on traditional database systems to make them work.
Building SQL Expressions for Attribute Query
There are many places you can build expressions within both ArcGIS and QGIS software, some of the common places are listed below:-
ArcGIS Tools:
~ Select By Attribute
~ Definition Query
~ Field Calculator
~ Label Expression Dialog Box
~ Add Query Layer
QGIS Tools:
~ Select By Expression
~ Filter Query Builder
~ Field Calculator
~ Label Expression Dialog Box
~ Layer Property Display
~ Database Manager
