GEOL 403503 Principles of GIS

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GEOL 403/503 Principles of GIS

• Ted Dunsford
• Bhushan Gokhale
• Fall 2007

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Lecture 9Querying the Database
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Query

• A request that selects features or records from a
  database.
• A query is often written as a statement or
  logical expression.

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Spatial Query

• Spatial query is a search of the data to satisfy
  a given condition
• There are two types of spatial query
• Query of attribute data
• A spatial distribution or an area will be
  searched with respect to a given attribute of
  interest.
• Query of geometric data
• With a given geometric condition for example
  location, shape or intersection, all data that
  satisfy the condition will be searched.
• In the case of a vector data form, to search an
  area which includes a given point, and to find
  all line segments which intersect a given line
  would be a typical query of geometric data.

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Spatial Query (Cont.)

• A spatial query enables you to construct a map
  question, which allows you to find occurrences in
  your chosen layer
• Based on attributes of that layer
• Also based on their spatial relationship with
  objects within a different layer.

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Selection

• Selection operations
• Involve identifying features based on several
  conditions or criteria
• The attributes or geometry of features are
  checked against the conditions or criteria
• You can write the selected features into new
  output data layer
• You can use the selection for other analysis
• Examples

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• Select
• State Arkansas
• States entirely north of Arkansas
• States_areagt84,000 sq. mi.
• States both entirely north of Arkansas and larger
  than 84,000 sq. mi.

Bolstad, 2005
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Selection (Query) Procedures

• On screen query
• By human operation
• Conditional selection
• Set Algebra
• Less than (lt)
• Greater than (gt)
• Equal to ()
• Not equal to (ltgt)
• Boolean Algebra
• Conditions OR, AND, and NOT

Bolstad, 2005
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Examples of Expression in Boolean Algebra
Bolstad, 2005
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Select by Attribute

• Open attribute table of the querying layer
• Use Option Selection gt Select by attributes
• Use Boolean expressions
• Interactive selection methods
• Create new selection
• Add to current selection
• Remove from current selection
• Can select from current selection
• Available under Selection menu

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Select by Location

• Available under Selection menu
• Can select a layer with geometric relation to
  another layer
• Check the bottom image for all the geometric
  options

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Select by Location (Cont.)

• Selecting options
• That Meet
• That Overlap
• That Contains
• That are Contained by
• That are Entirely Contained By
• That are Spatially Equal
• That Touch

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Examples of Selection by Location
States adjacent to Missouri
Bolstad, 2005
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Examples of Selection by Location - cont.
States containing a portion of Mississippi River
or its tributaries are selected
Bolstad, 2005
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Selection by Graphics

• Select features using rectangles, circles, etc.
• Select features partially or completely within
  the box or graphics
• Select features completely within the box or
  graphics
• Select features that the box or graphics are
  completely within
• Can use selection tolerance
• Can use some threshold to select until a limit to
  warn you

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Spatial Query or Selection

• Conducted with
• Query expression
• A type of expression that evaluates to a
  Boolean (true or false) value, which is typically
  used to select those rows in a table in which the
  expression evaluates to true.
• Select fields From table Where expression
• "HUC_NO" 16020309 "HUC_NO" gt 16020309 AND
  "HUC_NO" lt 17010308

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Structured Query Language

• Structured Query Language (SQL) is a powerful
  language we use to set one or more criteria that
  can consist of attributes, operators, and
  calculations.
• For example
• Imagine you have a map of customers and want to
  find those who spent more than 50,000 with you
  last year and whose business type is
  "Restaurant".
• You would select the customers with this
  expression Sales gt 50000 AND Business_type
  'Restaurant'.

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DEMO