================== GeoDjango Tutorial ================== Introduction ============ GeoDjango is an add-on for Django that turns it into a world-class geographic Web framework. GeoDjango strives to make it as simple as possible to create geographic Web applications, like location-based services. Some features include: * Django model fields for `OGC`_ geometries. * Extensions to Django's ORM for the querying and manipulation of spatial data. * Loosely-coupled, high-level Python interfaces for GIS geometry operations and data formats. * Editing of geometry fields inside the admin. This tutorial assumes a familiarity with Django; thus, if you're brand new to Django please read through the :doc:`regular tutorial ` to introduce yourself with basic Django concepts. .. note:: GeoDjango has special prerequisites overwhat is required by Django -- please consult the :ref:`installation documentation ` for more details. This tutorial will guide you through the creation of a geographic Web application for viewing the `world borders`_. [#]_ Some of the code used in this tutorial is taken from and/or inspired by the `GeoDjango basic apps`_ project. [#]_ .. note:: Proceed through the tutorial sections sequentially for step-by-step instructions. .. _OGC: http://www.opengeospatial.org/ .. _world borders: http://thematicmapping.org/downloads/world_borders.php .. _GeoDjango basic apps: http://code.google.com/p/geodjango-basic-apps/ Setting Up ========== Create a Spatial Database ------------------------- .. note:: MySQL and Oracle users can skip this section because spatial types are already built into the database. First, a spatial database needs to be created for our project. If using PostgreSQL and PostGIS, then the following commands will create the database from a :ref:`spatial database template `: .. code-block:: bash $ createdb -T template_postgis geodjango .. note:: This command must be issued by a database user that has permissions to create a database. Here is an example set of commands to create such a user: .. code-block:: bash $ sudo su - postgres $ createuser --createdb geo $ exit Replace ``geo`` with the system login user name that will be connecting to the database. For example, ``johndoe`` if that is the system user that will be running GeoDjango. Users of SQLite and SpatiaLite should consult the instructions on how to create a :ref:`SpatiaLite database `. Create GeoDjango Project ------------------------ Use the ``django-admin.py`` script like normal to create a ``geodjango`` project: .. code-block:: bash $ django-admin.py startproject geodjango With the project initialized, now create a ``world`` Django application within the ``geodjango`` project: .. code-block:: bash $ cd geodjango $ python manage.py startapp world Configure ``settings.py`` ------------------------- The ``geodjango`` project settings are stored in the ``geodjango/settings.py`` file. Edit the database connection settings appropriately:: DATABASES = { 'default': { 'ENGINE': 'django.contrib.gis.db.backends.postgis', 'NAME': 'geodjango', 'USER': 'geo', } } In addition, modify the :setting:`INSTALLED_APPS` setting to include :mod:`django.contrib.admin`, :mod:`django.contrib.gis`, and ``world`` (our newly created application):: INSTALLED_APPS = ( 'django.contrib.auth', 'django.contrib.contenttypes', 'django.contrib.sessions', 'django.contrib.sites', 'django.contrib.messages', 'django.contrib.staticfiles', 'django.contrib.admin', 'django.contrib.gis', 'world' ) Geographic Data =============== .. _worldborders: World Borders ------------- The world borders data is available in this `zip file`__. Create a data directory in the ``world`` application, download the world borders data, and unzip. On GNU/Linux platforms the following commands should do it: .. code-block:: bash $ mkdir world/data $ cd world/data $ wget http://thematicmapping.org/downloads/TM_WORLD_BORDERS-0.3.zip $ unzip TM_WORLD_BORDERS-0.3.zip $ cd ../.. The world borders ZIP file contains a set of data files collectively known as an `ESRI Shapefile`__, one of the most popular geospatial data formats. When unzipped the world borders data set includes files with the following extensions: * ``.shp``: Holds the vector data for the world borders geometries. * ``.shx``: Spatial index file for geometries stored in the ``.shp``. * ``.dbf``: Database file for holding non-geometric attribute data (e.g., integer and character fields). * ``.prj``: Contains the spatial reference information for the geographic data stored in the shapefile. __ http://thematicmapping.org/downloads/TM_WORLD_BORDERS-0.3.zip __ http://en.wikipedia.org/wiki/Shapefile Use ``ogrinfo`` to examine spatial data --------------------------------------- The GDAL ``ogrinfo`` utility is excellent for examining metadata about shapefiles (or other vector data sources): .. code-block:: bash $ ogrinfo world/data/TM_WORLD_BORDERS-0.3.shp INFO: Open of `world/data/TM_WORLD_BORDERS-0.3.shp' using driver `ESRI Shapefile' successful. 1: TM_WORLD_BORDERS-0.3 (Polygon) Here ``ogrinfo`` is telling us that the shapefile has one layer, and that such layer contains polygon data. To find out more we'll specify the layer name and use the ``-so`` option to get only important summary information: .. code-block:: bash $ ogrinfo -so world/data/TM_WORLD_BORDERS-0.3.shp TM_WORLD_BORDERS-0.3 INFO: Open of `world/data/TM_WORLD_BORDERS-0.3.shp' using driver `ESRI Shapefile' successful. Layer name: TM_WORLD_BORDERS-0.3 Geometry: Polygon Feature Count: 246 Extent: (-180.000000, -90.000000) - (180.000000, 83.623596) Layer SRS WKT: GEOGCS["GCS_WGS_1984", DATUM["WGS_1984", SPHEROID["WGS_1984",6378137.0,298.257223563]], PRIMEM["Greenwich",0.0], UNIT["Degree",0.0174532925199433]] FIPS: String (2.0) ISO2: String (2.0) ISO3: String (3.0) UN: Integer (3.0) NAME: String (50.0) AREA: Integer (7.0) POP2005: Integer (10.0) REGION: Integer (3.0) SUBREGION: Integer (3.0) LON: Real (8.3) LAT: Real (7.3) This detailed summary information tells us the number of features in the layer (246), the geographical extent, the spatial reference system ("SRS WKT"), as well as detailed information for each attribute field. For example, ``FIPS: String (2.0)`` indicates that there's a ``FIPS`` character field with a maximum length of 2; similarly, ``LON: Real (8.3)`` is a floating-point field that holds a maximum of 8 digits up to three decimal places. Although this information may be found right on the `world borders`_ Web site, this shows you how to determine this information yourself when such metadata is not provided. Geographic Models ================= Defining a Geographic Model --------------------------- Now that we've examined our world borders data set using ``ogrinfo``, we can create a GeoDjango model to represent this data:: from django.contrib.gis.db import models class WorldBorder(models.Model): # Regular Django fields corresponding to the attributes in the # world borders shapefile. name = models.CharField(max_length=50) area = models.IntegerField() pop2005 = models.IntegerField('Population 2005') fips = models.CharField('FIPS Code', max_length=2) iso2 = models.CharField('2 Digit ISO', max_length=2) iso3 = models.CharField('3 Digit ISO', max_length=3) un = models.IntegerField('United Nations Code') region = models.IntegerField('Region Code') subregion = models.IntegerField('Sub-Region Code') lon = models.FloatField() lat = models.FloatField() # GeoDjango-specific: a geometry field (MultiPolygonField), and # overriding the default manager with a GeoManager instance. mpoly = models.MultiPolygonField() objects = models.GeoManager() # Returns the string representation of the model. def __unicode__(self): return self.name Two important things to note: 1. The ``models`` module is imported from :mod:`django.contrib.gis.db`. 2. The model overrides its default manager with :class:`~django.contrib.gis.db.models.GeoManager`; this is *required* to perform spatial queries. When declaring a geometry field on your model the default spatial reference system is WGS84 (meaning the `SRID`__ is 4326) -- in other words, the field coordinates are in longitude/latitude pairs in units of degrees. If you want the coordinate system to be different, then SRID of the geometry field may be customized by setting the ``srid`` with an integer corresponding to the coordinate system of your choice. __ http://en.wikipedia.org/wiki/SRID Run ``syncdb`` -------------- After you've defined your model, it needs to be synced with the spatial database. First, let's look at the SQL that will generate the table for the ``WorldBorder`` model:: $ python manage.py sqlall world This management command should produce the following output: .. code-block:: sql BEGIN; CREATE TABLE "world_worldborder" ( "id" serial NOT NULL PRIMARY KEY, "name" varchar(50) NOT NULL, "area" integer NOT NULL, "pop2005" integer NOT NULL, "fips" varchar(2) NOT NULL, "iso2" varchar(2) NOT NULL, "iso3" varchar(3) NOT NULL, "un" integer NOT NULL, "region" integer NOT NULL, "subregion" integer NOT NULL, "lon" double precision NOT NULL, "lat" double precision NOT NULL ) ; SELECT AddGeometryColumn('world_worldborder', 'mpoly', 4326, 'MULTIPOLYGON', 2); ALTER TABLE "world_worldborder" ALTER "mpoly" SET NOT NULL; CREATE INDEX "world_worldborder_mpoly_id" ON "world_worldborder" USING GIST ( "mpoly" GIST_GEOMETRY_OPS ); COMMIT; If satisfied, you may then create this table in the database by running the ``syncdb`` management command:: $ python manage.py syncdb Creating table world_worldborder Installing custom SQL for world.WorldBorder model The ``syncdb`` command may also prompt you to create an admin user; go ahead and do so (not required now, may be done at any point in the future using the ``createsuperuser`` management command). Importing Spatial Data ====================== This section will show you how to take the data from the world borders shapefile and import it into GeoDjango models using the :ref:`ref-layermapping`. There are many different ways to import data in to a spatial database -- besides the tools included within GeoDjango, you may also use the following to populate your spatial database: * `ogr2ogr`_: Command-line utility, included with GDAL, that supports loading a multitude of vector data formats into the PostGIS, MySQL, and Oracle spatial databases. * `shp2pgsql`_: This utility is included with PostGIS and only supports ESRI shapefiles. .. _ogr2ogr: http://www.gdal.org/ogr2ogr.html .. _shp2pgsql: http://postgis.refractions.net/documentation/manual-1.5/ch04.html#shp2pgsql_usage .. _gdalinterface: GDAL Interface -------------- Earlier we used the ``ogrinfo`` to explore the contents of the world borders shapefile. Included within GeoDjango is an interface to GDAL's powerful OGR library -- in other words, you'll be able explore all the vector data sources that OGR supports via a Pythonic API. First, invoke the Django shell: .. code-block:: bash $ python manage.py shell If the :ref:`worldborders` data was downloaded like earlier in the tutorial, then we can determine the path using Python's built-in ``os`` module:: >>> import os >>> import world >>> world_shp = os.path.abspath(os.path.join(os.path.dirname(world.__file__), ... 'data/TM_WORLD_BORDERS-0.3.shp')) Now, the world borders shapefile may be opened using GeoDjango's :class:`~django.contrib.gis.gdal.DataSource` interface:: >>> from django.contrib.gis.gdal import DataSource >>> ds = DataSource(world_shp) >>> print ds / ... /geodjango/world/data/TM_WORLD_BORDERS-0.3.shp (ESRI Shapefile) Data source objects can have different layers of geospatial features; however, shapefiles are only allowed to have one layer:: >>> print len(ds) 1 >>> lyr = ds[0] >>> print lyr TM_WORLD_BORDERS-0.3 You can see what the geometry type of the layer is and how many features it contains:: >>> print lyr.geom_type Polygon >>> print len(lyr) 246 .. note:: Unfortunately the shapefile data format does not allow for greater specificity with regards to geometry types. This shapefile, like many others, actually includes ``MultiPolygon`` geometries in its features. You need to watch out for this when creating your models as a GeoDjango ``PolygonField`` will not accept a ``MultiPolygon`` type geometry -- thus a ``MultiPolygonField`` is used in our model's definition instead. The :class:`~django.contrib.gis.gdal.Layer` may also have a spatial reference system associated with it -- if it does, the ``srs`` attribute will return a :class:`~django.contrib.gis.gdal.SpatialReference` object:: >>> srs = lyr.srs >>> print srs GEOGCS["GCS_WGS_1984", DATUM["WGS_1984", SPHEROID["WGS_1984",6378137.0,298.257223563]], PRIMEM["Greenwich",0.0], UNIT["Degree",0.0174532925199433]] >>> srs.proj4 # PROJ.4 representation '+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs ' Here we've noticed that the shapefile is in the popular WGS84 spatial reference system -- in other words, the data uses units of degrees longitude and latitude. In addition, shapefiles also support attribute fields that may contain additional data. Here are the fields on the World Borders layer: >>> print lyr.fields ['FIPS', 'ISO2', 'ISO3', 'UN', 'NAME', 'AREA', 'POP2005', 'REGION', 'SUBREGION', 'LON', 'LAT'] Here we are examining the OGR types (e.g., whether a field is an integer or a string) associated with each of the fields: >>> [fld.__name__ for fld in lyr.field_types] ['OFTString', 'OFTString', 'OFTString', 'OFTInteger', 'OFTString', 'OFTInteger', 'OFTInteger', 'OFTInteger', 'OFTInteger', 'OFTReal', 'OFTReal'] You can iterate over each feature in the layer and extract information from both the feature's geometry (accessed via the ``geom`` attribute) as well as the feature's attribute fields (whose **values** are accessed via ``get()`` method):: >>> for feat in lyr: ... print feat.get('NAME'), feat.geom.num_points ... Guernsey 18 Jersey 26 South Georgia South Sandwich Islands 338 Taiwan 363 :class:`~django.contrib.gis.gdal.Layer` objects may be sliced:: >>> lyr[0:2] [, ] And individual features may be retrieved by their feature ID:: >>> feat = lyr[234] >>> print feat.get('NAME') San Marino Here the boundary geometry for San Marino is extracted and looking exported to WKT and GeoJSON:: >>> geom = feat.geom >>> print geom.wkt POLYGON ((12.415798 43.957954,12.450554 ... >>> print geom.json { "type": "Polygon", "coordinates": [ [ [ 12.415798, 43.957954 ], [ 12.450554, 43.979721 ], ... ``LayerMapping`` ---------------- We're going to dive right in -- create a file called ``load.py`` inside the ``world`` application, and insert the following:: import os from django.contrib.gis.utils import LayerMapping from models import WorldBorder world_mapping = { 'fips' : 'FIPS', 'iso2' : 'ISO2', 'iso3' : 'ISO3', 'un' : 'UN', 'name' : 'NAME', 'area' : 'AREA', 'pop2005' : 'POP2005', 'region' : 'REGION', 'subregion' : 'SUBREGION', 'lon' : 'LON', 'lat' : 'LAT', 'mpoly' : 'MULTIPOLYGON', } world_shp = os.path.abspath(os.path.join(os.path.dirname(__file__), 'data/TM_WORLD_BORDERS-0.3.shp')) def run(verbose=True): lm = LayerMapping(WorldBorder, world_shp, world_mapping, transform=False, encoding='iso-8859-1') lm.save(strict=True, verbose=verbose) A few notes about what's going on: * Each key in the ``world_mapping`` dictionary corresponds to a field in the ``WorldBorder`` model, and the value is the name of the shapefile field that data will be loaded from. * The key ``mpoly`` for the geometry field is ``MULTIPOLYGON``, the geometry type we wish to import as. Even if simple polygons are encountered in the shapefile they will automatically be converted into collections prior to insertion into the database. * The path to the shapefile is not absolute -- in other words, if you move the ``world`` application (with ``data`` subdirectory) to a different location, then the script will still work. * The ``transform`` keyword is set to ``False`` because the data in the shapefile does not need to be converted -- it's already in WGS84 (SRID=4326). * The ``encoding`` keyword is set to the character encoding of string values in the shapefile. This ensures that string values are read and saved correctly from their original encoding system. Afterwards, invoke the Django shell from the ``geodjango`` project directory: .. code-block:: bash $ python manage.py shell Next, import the ``load`` module, call the ``run`` routine, and watch ``LayerMapping`` do the work:: >>> from world import load >>> load.run() .. _ogrinspect-intro: Try ``ogrinspect`` ------------------ Now that you've seen how to define geographic models and import data with the :ref:`ref-layermapping`, it's possible to further automate this process with use of the :djadmin:`ogrinspect` management command. The :djadmin:`ogrinspect` command introspects a GDAL-supported vector data source (e.g., a shapefile) and generates a model definition and ``LayerMapping`` dictionary automatically. The general usage of the command goes as follows: .. code-block:: bash $ python manage.py ogrinspect [options] [options] Where ``data_source`` is the path to the GDAL-supported data source and ``model_name`` is the name to use for the model. Command-line options may be used to further define how the model is generated. For example, the following command nearly reproduces the ``WorldBorder`` model and mapping dictionary created above, automatically: .. code-block:: bash $ python manage.py ogrinspect world/data/TM_WORLD_BORDERS-0.3.shp WorldBorder \ --srid=4326 --mapping --multi A few notes about the command-line options given above: * The ``--srid=4326`` option sets the SRID for the geographic field. * The ``--mapping`` option tells ``ogrinspect`` to also generate a mapping dictionary for use with :class:`~django.contrib.gis.utils.LayerMapping`. * The ``--multi`` option is specified so that the geographic field is a :class:`~django.contrib.gis.db.models.MultiPolygonField` instead of just a :class:`~django.contrib.gis.db.models.PolygonField`. The command produces the following output, which may be copied directly into the ``models.py`` of a GeoDjango application:: # This is an auto-generated Django model module created by ogrinspect. from django.contrib.gis.db import models class WorldBorder(models.Model): fips = models.CharField(max_length=2) iso2 = models.CharField(max_length=2) iso3 = models.CharField(max_length=3) un = models.IntegerField() name = models.CharField(max_length=50) area = models.IntegerField() pop2005 = models.IntegerField() region = models.IntegerField() subregion = models.IntegerField() lon = models.FloatField() lat = models.FloatField() geom = models.MultiPolygonField(srid=4326) objects = models.GeoManager() # Auto-generated `LayerMapping` dictionary for WorldBorder model worldborders_mapping = { 'fips' : 'FIPS', 'iso2' : 'ISO2', 'iso3' : 'ISO3', 'un' : 'UN', 'name' : 'NAME', 'area' : 'AREA', 'pop2005' : 'POP2005', 'region' : 'REGION', 'subregion' : 'SUBREGION', 'lon' : 'LON', 'lat' : 'LAT', 'geom' : 'MULTIPOLYGON', } Spatial Queries =============== Spatial Lookups --------------- GeoDjango extends the Django ORM and allows the use of spatial lookups. Let's do an example where we find the ``WorldBorder`` model that contains a point. First, fire up the management shell: .. code-block:: bash $ python manage.py shell Now, define a point of interest [#]_:: >>> pnt_wkt = 'POINT(-95.3385 29.7245)' The ``pnt_wkt`` string represents the point at -95.3385 degrees longitude, and 29.7245 degrees latitude. The geometry is in a format known as Well Known Text (WKT), an open standard issued by the Open Geospatial Consortium (OGC). [#]_ Import the ``WorldBorder`` model, and perform a ``contains`` lookup using the ``pnt_wkt`` as the parameter:: >>> from world.models import WorldBorder >>> qs = WorldBorder.objects.filter(mpoly__contains=pnt_wkt) >>> qs [] Here we retrieved a ``GeoQuerySet`` that has only one model: the one for the United States (which is what we would expect). Similarly, a :ref:`GEOS geometry object ` may also be used -- here the ``intersects`` spatial lookup is combined with the ``get`` method to retrieve only the ``WorldBorder`` instance for San Marino instead of a queryset:: >>> from django.contrib.gis.geos import Point >>> pnt = Point(12.4604, 43.9420) >>> sm = WorldBorder.objects.get(mpoly__intersects=pnt) >>> sm The ``contains`` and ``intersects`` lookups are just a subset of what's available -- the :ref:`ref-gis-db-api` documentation has more. Automatic Spatial Transformations --------------------------------- When querying the spatial database GeoDjango automatically transforms geometries if they're in a different coordinate system. In the following example, the coordinate will be expressed in terms of `EPSG SRID 32140`__, a coordinate system specific to south Texas **only** and in units of **meters** and not degrees:: >>> from django.contrib.gis.geos import Point, GEOSGeometry >>> pnt = Point(954158.1, 4215137.1, srid=32140) Note that ``pnt`` may also be constructed with EWKT, an "extended" form of WKT that includes the SRID:: >>> pnt = GEOSGeometry('SRID=32140;POINT(954158.1 4215137.1)') When using GeoDjango's ORM, it will automatically wrap geometry values in transformation SQL, allowing the developer to work at a higher level of abstraction:: >>> qs = WorldBorder.objects.filter(mpoly__intersects=pnt) >>> print qs.query # Generating the SQL SELECT "world_worldborder"."id", "world_worldborder"."name", "world_worldborder"."area", "world_worldborder"."pop2005", "world_worldborder"."fips", "world_worldborder"."iso2", "world_worldborder"."iso3", "world_worldborder"."un", "world_worldborder"."region", "world_worldborder"."subregion", "world_worldborder"."lon", "world_worldborder"."lat", "world_worldborder"."mpoly" FROM "world_worldborder" WHERE ST_Intersects("world_worldborder"."mpoly", ST_Transform(%s, 4326)) >>> qs # printing evaluates the queryset [] __ http://spatialreference.org/ref/epsg/32140/ Lazy Geometries --------------- Geometries come to GeoDjango in a standardized textual representation. Upon access of the geometry field, GeoDjango creates a `GEOS geometry object `, exposing powerful functionality, such as serialization properties for popular geospatial formats:: >>> sm = WorldBorder.objects.get(name='San Marino') >>> sm.mpoly >>> sm.mpoly.wkt # WKT MULTIPOLYGON (((12.4157980000000006 43.9579540000000009, 12.4505540000000003 43.9797209999999978, ... >>> sm.mpoly.wkb # WKB (as Python binary buffer) >>> sm.mpoly.geojson # GeoJSON (requires GDAL) '{ "type": "MultiPolygon", "coordinates": [ [ [ [ 12.415798, 43.957954 ], [ 12.450554, 43.979721 ], ... This includes access to all of the advanced geometric operations provided by the GEOS library:: >>> pnt = Point(12.4604, 43.9420) >>> sm.mpoly.contains(pnt) True >>> pnt.contains(sm.mpoly) False ``GeoQuerySet`` Methods ----------------------- Putting your data on the map ============================ Google ------ Geographic Admin ---------------- GeoDjango extends :doc:`Django's admin application ` to enable support for editing geometry fields. Basics ^^^^^^ GeoDjango also supplements the Django admin by allowing users to create and modify geometries on a JavaScript slippy map (powered by `OpenLayers`_). Let's dive in again -- create a file called ``admin.py`` inside the ``world`` application, and insert the following:: from django.contrib.gis import admin from models import WorldBorder admin.site.register(WorldBorder, admin.GeoModelAdmin) Next, edit your ``urls.py`` in the ``geodjango`` application folder to look as follows:: from django.conf.urls import patterns, url, include from django.contrib.gis import admin admin.autodiscover() urlpatterns = patterns('', url(r'^admin/', include(admin.site.urls)), ) Start up the Django development server: .. code-block:: bash $ python manage.py runserver Finally, browse to ``http://localhost:8000/admin/``, and log in with the admin user created after running ``syncdb``. Browse to any of the ``WorldBorder`` entries -- the borders may be edited by clicking on a polygon and dragging the vertexes to the desired position. .. _OpenLayers: http://openlayers.org/ .. _Open Street Map: http://openstreetmap.org/ .. _Vector Map Level 0: http://earth-info.nga.mil/publications/vmap0.html .. _OSGeo: http://www.osgeo.org .. _osmgeoadmin-intro: ``OSMGeoAdmin`` ^^^^^^^^^^^^^^^ With the :class:`~django.contrib.gis.admin.OSMGeoAdmin`, GeoDjango uses a `Open Street Map`_ layer in the admin. This provides more context (including street and thoroughfare details) than available with the :class:`~django.contrib.gis.admin.GeoModelAdmin` (which uses the `Vector Map Level 0`_ WMS data set hosted at `OSGeo`_). First, there are some important requirements and limitations: * :class:`~django.contrib.gis.admin.OSMGeoAdmin` requires that the :ref:`spherical mercator projection be added ` to the ``spatial_ref_sys`` table (PostGIS 1.3 and below, only). * The PROJ.4 datum shifting files must be installed (see the :ref:`PROJ.4 installation instructions ` for more details). If you meet these requirements, then just substitute in the ``OSMGeoAdmin`` option class in your ``admin.py`` file:: admin.site.register(WorldBorder, admin.OSMGeoAdmin) .. rubric:: Footnotes .. [#] Special thanks to Bjørn Sandvik of `thematicmapping.org `_ for providing and maintaining this data set. .. [#] GeoDjango basic apps was written by Dane Springmeyer, Josh Livni, and Christopher Schmidt. .. [#] Here the point is for the `University of Houston Law Center `_. .. [#] Open Geospatial Consortium, Inc., `OpenGIS Simple Feature Specification For SQL `_, Document 99-049.