Where’s My Data?
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Transcript of Where’s My Data?
COLIN MACKAY
WHERE’S MY DATA?
Spatial queries in SQL Server 2008
SQL Bits III – 13th September 2008
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WHAT’S COVERED IN THIS TALK New data types geometry and
geography Spatial references Spatial operations Spatial indexes Case study
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WHY SHOULD I CARE ABOUT SPATIAL DATA?
80-90% of all data has a spatial element Where are your customers? Where are your assets? Where are potential customers? Where are the flood risks? Where are your complaints coming
from? Where are the accident black-spots? Where are crimes happening?
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COULD FUDGE THE QUERIES Postcodes in
Glasgow Zoned Historical reasons G5 adjacent to G42 G40 in an island Postcodes
designed for delivering letters
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WHAT DOES SQL SERVER 2008 PROVIDE? The data analysis engine No useful rendering engine
Virtual Earth Map Point Other GIS systems
OGC Standards compliance Plus some “extension” methods of their
own
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GEOMETRY X/Y coordinate on a
planar grid British National Grid Works well to
~750,000km2 Different projections
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GEOGRAPHY Geodetic
coordinates Covers larger areas International
datasets Approximation
Earth actually flattened sphere (oblate spheroid)
Different models Airy 1830 (used by
OS) WGS84 (used by
GPS)
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SRID Spatial Reference Identifier All spatial data has an SRID SRIDs must match for spatial
operations Null returned if SRIDs don’t match
Geometry can have an SRID of 0 Not Geography.
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TYPES OF SPATIAL DATA Point LineString Polygon GeomCollection MultiPolygon MultiLineString MultiPoint
From BOL
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FINDING DISTANCESSELECT a.Name AS StartVenue, b.Name AS EndVenue, a.Location.STDistance(b.Location) / 1000.0 As DistanceFROM Venue AS aINNER JOIN Venue AS b ON a.Id < b.IdORDER BY a.Id, b.Id
GCU 60Dundee U
62 102
MS TVP 520 537 572MS Edin’
6 66 58 521
HBOS GCU Dundee U
MS TVP
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LINESTRING A linestring is a series of coordinates 1 dimension Defines a linear object
Road Railway line River
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CREATING LINESTRINGS Can use
STGeomFromText STGeomFromWKB STLineFromText STLineFromWKB Parse
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GEOMETRY LENGTHS & DISTANCES Geography uses SI Units Geometry uses the units of the planar
system
The square of the hypotenuse is equal to the sum of the square of the other two sides
Not to scale
3 units
4 units
? unitsDistance from A to B:√(32+42) = 5
A
B
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POLYGON A series of coordinates in a closed ring
First and last coordinate are the same 2 dimensions Defines an area
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GEOGRAPHY POLYGON ORIENTATION Interior is
everything inside an anti-clockwise ring Everything on the
left-hand side of the perimeter line.
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GEOGRAPHY POLYGON ORIENTATIONThe specified input does not represent a valid geography instance because it exceeds a single hemisphere. Each geography instance must fit inside a single hemisphere. A common reason for this error is that a polygon has the wrong ring orientation.
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CREATING POLYGONS Can use
STGeomFromText STGeomFromWKB STPolygonFromText STPolygonFromWKB Parse
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FICTIONAL CASE STUDY Estate Agent
Filter by price, # bedrooms, type – EASY! Filter by location?
Until now very vague
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COMMON SPATIAL QUESTIONS
Near a railwa
y statio
n
Near my work
Near a motorway junctionNear a good
school Inside the cityOutside the city
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THE DATA Railway data
Stations Routes
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DOESN’T ALWAYS WORK Edinburgh - Glenrothes
(via Kirkcaldy)Name DistKM-------------------------- ----------------Edinburgh Waverley Station 0Haymarket 1.89298770395887South Gyle 6.95446540329848Burntisland 12.086984317701Dalmeny 12.49585147351Kinghorn 13.1439998546632Aberdour 13.3392361220632North Queensferry 14.3833962761344Dalgety Bay 15.0082794626365Inverkeithing 15.7316327831032Kirkcaldy 17.9484646860063Glenrothes With Thornton 23.7022936117453
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DISPLAY THE PROPERTIES
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SPATIAL INDEXING Decomposes space
into 4 levels of grid Level 1 is the top Cells are uniform in
a level A level can be a
4x4, 8x8 or 16x16 grid 8x8 by default
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RESTRICTIONS Table must have a primary key Primary key cannot subsequently be
changed. Not on views Maximum of 249 Spatial indexes per
column
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WHY HAVE MULTIPLE INDEXES ON ONE COLUMN
Where geometry/ geography sizes vary
e.g. Rail routes Small: Suburban
lines Large: Intercity lines
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SPATIAL INDEXING Supports
STIntersects STEquals STDistance
One Geography must be a point Both sides of the spatial operation must
have the same SRID
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SPATIAL INDEXING ON A GEOMETRY Must specify boundary of spatial area Additional methods supported
STContains STOverlaps STTouches STWithin
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AND THERE’S MORE! CodePlex project
More spatial methods Aggregations Scripts
http://www.codeplex.com/sqlspatialtools
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WHAT I’VE NOT MENTIONED GML
Import and export M and Z
Can store Cannot operate.
Other spatial Operations Geometry has more!
Visualisation .NET application
integration Data Importing
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GET MORE INFORMATION Slide Deck on my website
http://www.colinmackay.net Blog posts on Spatial Data
http://blog.colinmackay.net
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QUESTIONS
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