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Sun Aug 28 22:40:12 2016

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QGIS Planet

Geometry generator symbology

Say hello to geometry generators, a new way to use expression syntax to generate a geometry on the fly during the rendering process.
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Queries in QGIS pt 1 – Attribute Queries

Quick Select Tools

The Select Flyout button on the Main toolbar contains tools used to quickly select features:

QGIS Quick Select Tools

QGIS Quick Select Tools

Attribute Queries

One method to select features in a layer is to select features using an attribute query:-

To query a layer by its attribute data,

  1. Right click on the layer’s name in the Layer Panel
  2. Select Query from the short cut menu to open the Query Builder box
QGIS Select by attributes box

QGIS Select by attributes box

  1. It is possible to type the SQL query into the SQL where clause box. Alternatively:-
    1. Click the Field that contains the attributes you wish to base the query on
    2. Select the operator
    3. Press the All under the values box to list all the available values
    4. Double click the value that you wish to select
    5. You will see the query being built in the SQL where clauses box.

Boolean operations (And, Or, Not)

  • And: narrows the search by requiring all the criteria to be present. It is most commonly used when the criteria are in different columns. E.g. Select Business_type = “bank” And City = “London” will select banks with London addresses.
  • Or: broadens the search criteria by requiring just one of several criteria to be true. It is most commonly used to give different options to the same column e.g. select Business_type = “bank” Or Business_type = “shop” will select all the banks and shops within the data set.
  • Not: returns records that do NOT match the criteria e.g. select Business_type = “bank” AND City NOT “London” would return all the banks that do not have a London address. Note that QGIS uses != for not equal to

It is important to be clear about when to use And Or operators.

If the first example was worded Select Business_type = “bank” OR City = “London” this would return all the banks and any property with a London address

If the second example was worded select Business_type = “bank” AND Business_type = “shop” this would return only those business which had bank and shop entered as business type. This would probably be none at all as businesses tend to either be a shop or a bank but rarely both!

Mathematical Operations

It is possible to use mathematical formulae to select from columns that are formatted in a numeric format (e.g. integer):-

>Greater than e.g. Business_type = “shop” AND floor_space > 100

< Less than e.g. Business_type = “shop” AND floor_space < 100

>= greater than or equal to

<= Less than or equal to

!= Does not equal


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