ultimatepp/bazaar/plugin/gdal/ogr/ogr_sql.dox

/*! \page ogr_sql OGR SQL

The GDALDataset supports executing commands against a datasource via the
GDALDataset::ExecuteSQL() method.  While in theory 
any sort of command could be handled this way, in practice the mechanism is 
used to provide a subset of SQL SELECT capability to applications.  This 
page discusses the generic SQL implementation implemented within OGR, and 
issue with driver specific SQL support.<p>

Since GDAL/OGR 1.10, an alternate "dialect", the SQLite dialect, can be used
instead of the OGRSQL dialect. Refer to the
<b><a href="ogr_sql_sqlite.html">SQLite SQL dialect</a></b> page for more details.<p>

The OGRLayer class also supports applying an attribute query filter to 
features returned using the OGRLayer::SetAttributeFilter() method.  The
syntax for the attribute filter is the same as the WHERE clause in the 
OGR SQL SELECT statement.  So everything here with regard to the WHERE 
clause applies in the context of the SetAttributeFilter() method.<p>

NOTE: OGR SQL has been reimplemented for GDAL/OGR 1.8.0.  Many features
discussed below, notably arithmetic expressions, and expressions in the 
field list, were not support in GDAL/OGR 1.7.x and earlier.  See RFC 28 for
details of the new features in GDAL/OGR 1.8.0.

\section ogr_sql_select SELECT

The SELECT statement is used to fetch layer features (analogous to table
rows in an RDBMS) with the result of the query represented as a temporary layer
of features.   The layers of the datasource are analogous to tables in an 
RDBMS and feature attributes are analogous to column values.  The simplest
form of OGR SQL SELECT statement looks like this:

\code
SELECT * FROM polylayer
\endcode

In this case all features are fetched from the layer named "polylayer", and
all attributes of those features are returned.  This is essentially 
equivalent to accessing the layer directly.  In this example the "*" 
is the list of fields to fetch from the layer, with "*" meaning that all 
fields should be fetched.

This slightly more sophisticated form still pulls all features from the layer
but the schema will only contain the EAS_ID and PROP_VALUE attributes.   Any
other attributes would be discarded.

\code
SELECT eas_id, prop_value FROM polylayer
\endcode

A much more ambitious SELECT, restricting the features fetched with a
WHERE clause, and sorting the results might look like:

\code
SELECT * from polylayer WHERE prop_value > 220000.0 ORDER BY prop_value DESC
\endcode

This select statement will produce a table with just one feature, with one
attribute (named something like "count_eas_id") containing the number of
distinct values of the eas_id attribute. 

\code
SELECT COUNT(DISTINCT eas_id) FROM polylayer
\endcode

\subsection ogr_sql_flist_ops Field List Operators

The field list is a comma separate list of the fields to be carried into
the output features from the source layer.  They will appear on output features
in the order they appear on in the field list, so the field list may be used
to re-order the fields.<p>

A special form of the field list uses the DISTINCT keyword.  This returns a 
list of all the distinct values of the named attribute.  When the DISTINCT 
keyword is used, only one attribute may appear in the field list.  The DISTINCT
keyword may be used against any type of field.  Currently the distinctness
test against a string value is case insensitive in OGR SQL.  The result of
a SELECT with a DISTINCT keyword is a layer with one column (named the same
as the field operated on), and one feature per distinct value.  Geometries
are discarded.  The distinct values are assembled in memory, so alot of 
memory may be used for datasets with a large number of distinct values.

\code
SELECT DISTINCT areacode FROM polylayer
\endcode

There are also several summarization operators that may be applied to columns.
When a summarization operator is applied to any field, then all fields must
have summarization operators applied.   The summarization operators are
COUNT (a count of instances), AVG (numerical average), SUM (numerical sum), 
MIN (lexical or numerical minimum), and MAX (lexical or numerical maximum). 
This example produces a variety of sumarization information on parcel
property values:

\code
SELECT MIN(prop_value), MAX(prop_value), AVG(prop_value), SUM(prop_value), 
       COUNT(prop_value) FROM polylayer WHERE prov_name = 'Ontario'
\endcode

It is also possible to apply the COUNT() operator to a DISTINCT SELECT to get
a count of distinct values, for instance:

\code
SELECT COUNT(DISTINCT areacode) FROM polylayer
\endcode

Note: prior to OGR 1.9.0, null values were counted in COUNT(column_name) or
COUNT(DISTINCT column_name), which was not conformant with the SQL standard. Since
OGR 1.9.0, only non-null values are counted.

As a special case, the COUNT() operator can be given a "*" argument instead
of a field name which is a short form for count all the records.

\code
SELECT COUNT(*) FROM polylayer
\endcode


Field names can also be prefixed by a table name though this is only 
really meaningful when performing joins.  It is further demonstrated in 
the JOIN section.

Field definitions can also be complex expressions using arithmetic, and 
functional operators.   However, the DISTINCT keyword, and summarization 
operators like MIN, MAX, AVG and SUM may not be applied to expression fields.
Starting with GDAL 2.0, boolean resulting expressions (comparisons, logical
operators) can also be used.

\code
SELECT cost+tax from invoice
\endcode

or

\code
SELECT CONCAT(owner_first_name,' ',owner_last_name) from properties
\endcode

\subsubsection ogr_sql_functions Functions

Starting with OGR 1.8.2, the SUBSTR function can be used to extract a substring from a string.
Its syntax is the following one : SUBSTR(string_expr, start_offset [, length]). It extracts a substring of string_expr,
starting at offset start_offset (1 being the first character of string_expr, 2 the second one, etc...).
If start_offset is a negative value, the substring is extracted from the end of the string (-1 is the
last character of the string, -2 the character before the last character, ...).
If length is specified, up to length characters are extracted from the string. Otherwise the
remainder of the string is extracted.

Note: for the time being, the character as considered to be equivalent to bytes, which may not be
appropriate for multi-byte encodings like UTF-8.

\code
SELECT SUBSTR('abcdef',1,2) FROM xxx   --> 'ab'
SELECT SUBSTR('abcdef',4)   FROM xxx   --> 'def'
SELECT SUBSTR('abcdef',-2)  FROM xxx   --> 'ef'
\endcode

Starting with OGR 2.0, the <i>hstore_get_value()</i> function can be used to extract
a value associate to a key from a HSTORE string, formatted like 'key=>value,other_key=>other_value,...'

\code
SELECT hstore_get_value('a => b, "key with space"=> "value with space"', 'key with space') FROM xxx --> 'value with space'
\endcode

\subsubsection ogr_sql_fname_alias Using the field name alias

OGR SQL supports renaming the fields following the SQL92 specification by
using the AS keyword according to the following example:

\code
SELECT *, OGR_STYLE AS STYLE FROM polylayer
\endcode

The field name alias can be used as the last operation in the column specification.
Therefore we cannot rename the fields inside an operator, but we can
rename whole column expression, like these two:

\code
SELECT COUNT(areacode) AS "count" FROM polylayer
SELECT dollars/100.0 AS cents FROM polylayer
\endcode

\subsubsection ogr_sql_ftype_cast Changing the type of the fields

Starting with GDAL 1.6.0, OGR SQL supports changing the type of the columns by using the SQL92 compliant CAST
operator according to the following example:

\code
SELECT *, CAST(OGR_STYLE AS character(255)) FROM rivers
\endcode

Currently casting to the following target types are supported:

<ol>
<li> boolean (GDAL &gt;= 2.0)
<li> character(field_length). By default, field_length=1.
<li> float(field_length)
<li> numeric(field_length, field_precision)
<li> smallint(field_length) : 16 bit signed integer (GDAL &gt;= 2.0)
<li> integer(field_length)
<li> bigint(field_length), 64 bit integer, extension to SQL92 (GDAL &gt;= 2.0)
<li> date(field_length)
<li> time(field_length)
<li> timestamp(field_length)
<li> geometry, geometry(geometry_type), geometry(geometry_type,epsg_code)
</ol>

Specifying the field_length and/or the field_precision is optional.  An
explicit value of zero can be used as the width for character() to indicate
variable width.  Conversion to the 'integer list', 'double list' 
and 'string list' OGR data types are not supported, which doesn't conform to 
the SQL92 specification.

While the CAST operator can be applied anywhere in an expression, including
in a WHERE clause, the detailed control of output field format is only
supported if the CAST operator is the "outer most" operators on a field
in the field definition list.  In other contexts it is still useful to
convert between numeric, string and date data types.

Starting with OGR 1.11, casting a WKT string to a geometry is allowed.
geometry_type can be POINT[Z], LINESTRING[Z], POLYGON[Z], MULTIPOINT[Z],
MULTILINESTRING[Z], MULTIPOLYGON[Z], GEOMETRYCOLLECTION[Z] or GEOMETRY[Z].

\subsubsection ogr_sql_quot String literals and identifiers quoting

Starting with GDAL 2.0 (see <a href="http://trac.osgeo.org/gdal/wiki/rfc52_strict_sql_quoting">RFC 52 - Strict OGR SQL quoting</a>),
strict SQL92 rules are applied regarding string literals
and identifiers quoting.

String literals (constants) must be surrounded with single-quote characters. e.g.
WHERE a_field = 'a_value'

Identifiers (column names and tables names) can be used unquoted if they don't
contain special characters or are not a SQL reserved keyword. Otherwise they must
be surroundered with double-quote characters. e.g. WHERE "from" = 5

\subsection ogr_sql_where WHERE

The argument to the WHERE clause is a logical expression used select records 
from the source layer.  In addition to its use within the WHERE statement, 
the WHERE clause handling is also used for OGR attribute queries on regular 
layers via OGRLayer::SetAttributeFilter().

In addition to the arithmetic and other functional operators available in
expressions in the field selection clause of the SELECT statement, in the 
WHERE context logical operators are also available and the evaluated value
of the expression should be logical (true or false).

The available logical operators are 
<b>=</b>, 
<b>!=</b>, 
<b>&lt;&gt;</b>, 
<b>&lt;</b>, 
<b>&gt;</b>, 
<b>&lt;=</b>, 
<b>&gt;=</b>, 
<b>LIKE</b> and 
<b>ILIKE</b>,
<b>BETWEEN</b> and
<b>IN</b>. 
Most of the operators are self explanitory, but is is worth nothing that <b>
!=</b> is the same as <b>&lt;&gt;</b>, the string equality is
case insensitive, but the <b>&lt;</b>, <b>&gt;</b>, <b>&lt;=</b> and <b>&gt;=</b> operators <i>are</i> case sensitive.  Both the LIKE and ILIKE operators are case insensitive.

The value argument to the <b>LIKE</b> operator is a pattern against which
the value string is matched.  In this pattern percent (%) matches any number of
characters, and underscore ( _ ) matches any one character. An optional ESCAPE escape_char
clause can be added so that the percent or underscore characters can be searched
as regular characters, by being preceded with the escape_char.

\code
    String             Pattern       Matches?
    ------             -------       --------
    Alberta            ALB%          Yes
    Alberta            _lberta       Yes
    St. Alberta        _lberta       No
    St. Alberta        %lberta       Yes
    Robarts St.        %Robarts%     Yes
    12345              123%45        Yes
    123.45             12?45         No
    N0N 1P0            %N0N%         Yes
    L4C 5E2            %N0N%         No
\endcode

The <b>IN</b> takes a list of values as it's argument and tests the attribute
value for membership in the provided set.  

\code
    Value              Value Set            Matches?
    ------             -------              --------
    321                IN (456,123)         No
    'Ontario'          IN ('Ontario','BC')  Yes
    'Ont'              IN ('Ontario','BC')  No
    1                  IN (0,2,4,6)         No
\endcode

The syntax of the <b>BETWEEN</b> operator is "field_name BETWEEN value1 AND value2"
and it is equivalent to "field_name >= value1 AND field_name <= value2".

In addition to the above binary operators, there are additional operators
for testing if a field is null or not. These are the <b>IS NULL</b>
and <b>IS NOT NULL</b> operators. 

Basic field tests can be combined in more complicated predicates using logical
operators include <b>AND</b>, <b>OR</b>, and the unary logical <b>NOT</b>.  
Subexpressions should be bracketed to make precedence
clear.  Some more complicated predicates are:

\code
SELECT * FROM poly WHERE (prop_value >= 100000) AND (prop_value < 200000)
SELECT * FROM poly WHERE NOT (area_code LIKE 'N0N%')
SELECT * FROM poly WHERE (prop_value IS NOT NULL) AND (prop_value < 100000)
\endcode

\subsection ogr_sql_where_limits WHERE Limitations

<ol>
<li> Fields must all come from the primary table (the one listed in the FROM
clause).
<li> All string comparisons are case insensitive except for <b>&lt;</b>, <b>&gt;</b>, <b>&lt;=</b> and <b>&gt;=</b>.
</ol>

\subsection ogr_sql_order_by ORDER BY

The <b>ORDER BY</b> clause is used force the returned features to be reordered
into sorted order (ascending or descending) on one of the field values.  
Ascending (increasing) order is the default if neither the ASC or DESC keyword
is provided.  For example:

\code
SELECT * FROM property WHERE class_code = 7 ORDER BY prop_value DESC
SELECT * FROM property ORDER BY prop_value 
SELECT * FROM property ORDER BY prop_value ASC
SELECT DISTINCT zip_code FROM property ORDER BY zip_code
\endcode

Note that ORDER BY clauses cause two passes through the feature set.  One to
build an in-memory table of field values corresponded with feature ids, and
a second pass to fetch the features by feature id in the sorted order. For
formats which cannot efficiently randomly read features by feature id this can
be a very expensive operation.  

Sorting of string field values is case sensitive, not case insensitive like in
most other parts of OGR SQL.

\subsection ogr_sql_joins JOINs

OGR SQL supports a limited form of one to one JOIN.  This allows records from
a secondary table to be looked up based on a shared key between it and the
primary table being queried.  For instance, a table of city locations might
include a <em>nation_id</em> column that can be used as a reference into a
secondary <em>nation</em> table to fetch a nation name.  A joined query might
look like:

\code
SELECT city.*, nation.name FROM city 
     LEFT JOIN nation ON city.nation_id = nation.id
\endcode

This query would result in a table with all the fields from the city table,
and an additional "nation.name" field with the nation name pulled from the
nation table by looking for the record in the nation table that has the "id"
field with the same value as the city.nation_id field. 

Joins introduce a number of additional issues.  One is the concept of table
qualifiers on field names.  For instance, referring to city.nation_id instead
of just nation_id to indicate the nation_id field from the city layer.  The
table name qualifiers may only be used in the field list, and within the 
<b>ON</b> clause of the join.  

Wildcards are also somewhat more involved.  All fields from the primary table 
(<em>city</em> in this case) and the secondary table (<em>nation</em> in this
case) may be selected using the usual <b>*</b> wildcard.  But the fields of
just one of the primary or secondary table may be selected by prefixing the
asterix with the table name.  

The field names in the resulting query layer will be qualified by the table
name, if the table name is given as a qualifier in the field list.  In addition
field names will be qualified with a table name if they would conflict with 
earlier fields.  For instance, the following select would result might result
in a results set with a <em>name, nation_id, nation.nation_id</em> and <em>
nation.name</em> field if the city and nation tables both have the 
<em>nation_id</em> and <em>name</em> fieldnames.  

\code
SELECT * FROM city LEFT JOIN nation ON city.nation_id = nation.nation_id
\endcode

On the other hand if the nation table had a <em>continent_id</em> field, but
the city table did not, then that field would not need to be qualified in
the result set.  However, if the selected instead looked like the following
statement, all result fields would be qualified by the table name. 

\code
SELECT city.*, nation.* FROM city 
    LEFT JOIN nation ON city.nation_id = nation.nation_id
\endcode

In the above examples, the <em>nation</em> table was found in the same 
datasource as the <em>city</em> table.   However, the OGR join support
includes the ability to join against a table in a different data source, 
potentially of a different format.  This is indicated by qualifying the 
secondary table name with a datasource name.  In this case the secondary
datasource is opened using normal OGR semantics and utilized to access the
secondary table until the query result is no longer needed.

\code
SELECT * FROM city 
  LEFT JOIN '/usr2/data/nation.dbf'.nation ON city.nation_id = nation.nation_id
\endcode

While not necessarily very useful, it is also possible to introduce table
aliases to simplify some SELECT statements.  This can also be useful to 
disambiguate situations where tables of the same name are being used from 
different data sources.  For instance, if the actual
tables names were messy we might want to do something like:

\code
SELECT c.name, n.name FROM project_615_city c
  LEFT JOIN '/usr2/data/project_615_nation.dbf'.project_615_nation n 
            ON c.nation_id = n.nation_id
\endcode

It is possible to do multiple joins in a single query.

\code
SELECT city.name, prov.name, nation.name FROM city
  LEFT JOIN province ON city.prov_id = province.id
  LEFT JOIN nation ON city.nation_id = nation.id
\endcode

Before GDAL 2.0, the expression after ON should necessarily be of the form
"{primary_table}.{field_name} = {secondary_table}.{field_name}", and in that
order.
Starting with GDAL 2.0, it is possible to use a more complex boolean expression,
involving multiple comparison operators, but with the restrictions mentionned
in the below "JOIN limitations" section. In particular, in case of multiple joins (3 tables
or more) the fields compared in a JOIN must belong to the primary table (the one
after FROM) and the table of the active JOIN.

\subsection ogr_sql_join_limits JOIN Limitations

<ol>
<li> Joins can be very expensive operations if the secondary table is not
indexed on the key field being used. 
<li> Joined fields may not be used in WHERE clauses, or ORDER BY clauses
at this time.  The join is essentially evaluated after all primary table 
subsetting is complete, and after the ORDER BY pass.
<li> Joined fields may not be used as keys in later joins.  So you could not
use the province id in a city to lookup the province record, and then use a
nation id from the province id to lookup the nation record.  This is a sensible
thing to want and could be implemented, but is not currently supported.
<li> Datasource names for joined tables are evaluated relative to the 
current processes working directory, not the path to the primary datasource.
<li> These are not true LEFT or RIGHT joins in the RDBMS sense.  Whether
or not a secondary record exists for the join key or not, one and only one
copy of the primary record is returned in the result set.  If a secondary
record cannot be found, the secondary derived fields will be NULL.  If more
than one matching secondary field is found only the first will be used. 
</ol>

\section ogr_sql_union_all UNION ALL

(OGR >= 1.10.0)

The SQL engine can deal with several SELECT combined with
UNION ALL. The effect of UNION ALL is to concatenate the rows returned by the right SELECT
statement to the rows returned by the left SELECT statement.

\code
[(] SELECT field_list FROM first_layer [WHERE where_expr] [)]
UNION ALL [(] SELECT field_list FROM second_layer [WHERE where_expr] [)]
[UNION ALL [(] SELECT field_list FROM third_layer [WHERE where_expr] [)]]*
\endcode

\subsection ogr_sql_union_all_restrictions UNION ALL restrictions

The processing of UNION ALL in OGR differs from the SQL standard, in which it accepts
that the columns from the various SELECT are not identical. In that case, it will return
a super-set of all the fields from each SELECT statement.

There is also a restriction : ORDER BY can only be specified for each SELECT, and
not at the level of the result of the union.

\section ogr_sql_special_fields SPECIAL FIELDS

The OGR SQL query processor treats some of the attributes of the features as
built-in special fields can be used in the SQL statements likewise the
other fields. These fields can be placed in the select list, the WHERE clause 
and the ORDER BY clause respectively. The special field will not be included
in the result by default but it may be explicitly included by adding it to
the select list.
When accessing the field values the special fields will take precedence over
the other fields with the same names in the data source. 

\subsection ogr_sql_fid FID

Normally the feature id is a special property of a feature and not treated
as an attribute of the feature.  In some cases it is convenient to be able to 
utilize the feature id in queries and result sets as a regular field.  To do
so use the name <b>FID</b>.  The field wildcard expansions will not include
the feature id, but it may be explicitly included using a syntax like:

\code
SELECT FID, * FROM nation
\endcode

\subsection ogr_sql_geometry OGR_GEOMETRY

Some of the data sources (like MapInfo tab) can handle geometries of different
types within the same layer. The <b>OGR_GEOMETRY</b> special field represents
the geometry type returned by OGRGeometry::getGeometryName() and can be used to
distinguish the various types. By using this field one can select particular 
types of the geometries like:

\code
SELECT * FROM nation WHERE OGR_GEOMETRY='POINT' OR OGR_GEOMETRY='POLYGON'
\endcode

\subsection ogr_sql_geom_wkt OGR_GEOM_WKT

The Well Known Text representation of the geometry can also be used as
a special field. To select the WKT of the geometry <b>OGR_GEOM_WKT</b>
might be included in the select list, like:

\code
SELECT OGR_GEOM_WKT, * FROM nation
\endcode

Using the <b>OGR_GEOM_WKT</b> and the <b>LIKE</b> operator in the WHERE
clause we can get similar effect as using OGR_GEOMETRY:

\code
SELECT OGR_GEOM_WKT, * FROM nation WHERE OGR_GEOM_WKT
   LIKE 'POINT%' OR OGR_GEOM_WKT LIKE 'POLYGON%'
\endcode 

\subsection ogr_sql_geom_area OGR_GEOM_AREA

(Since GDAL 1.7.0)

The <b>OGR_GEOM_AREA</b> special field returns the area of the feature's 
geometry computed by the OGRSurface::get_Area() method. For 
OGRGeometryCollection and OGRMultiPolygon the value is the sum of the 
areas of its members. For non-surface geometries the returned area is 0.0.

For example, to select only polygon features larger than a given area:

\code
SELECT * FROM nation WHERE OGR_GEOM_AREA > 10000000
\endcode 

\subsection ogr_sql_style OGR_STYLE

The <b>OGR_STYLE</b> special field represents the style string of the feature
returned by OGRFeature::GetStyleString(). By using this field and the
<b>LIKE</b> operator the result of the query can be filtered by the style.
For example we can select the annotation features as:

\code
SELECT * FROM nation WHERE OGR_STYLE LIKE 'LABEL%'
\endcode 

\section ogr_sql_create_index CREATE INDEX

Some OGR SQL drivers support creating of attribute indexes.  Currently
this includes the Shapefile driver.  An index accelerates very simple
attribute queries of the form <em>fieldname = value</em>, which is what
is used by the <b>JOIN</b> capability.  To create an attribute index on
the nation_id field of the nation table a command like this would be used:

\code
CREATE INDEX ON nation USING nation_id
\endcode

\subsection ogr_sql_index_limits Index Limitations

<ol>
<li> Indexes are not maintained dynamically when new features are added to or
removed from a layer.
<li> Very long strings (longer than 256 characters?) cannot currently be
indexed.
<li> To recreate an index it is necessary to drop all indexes on a layer and
then recreate all the indexes. 
<li> Indexes are not used in any complex queries.   Currently the only
query the will accelerate is a simple "field = value" query.
</ol>

\section ogr_sql_drop_index DROP INDEX

The OGR SQL DROP INDEX command can be used to drop all indexes on a particular
table, or just the index for a particular column. 

\code
DROP INDEX ON nation USING nation_id
DROP INDEX ON nation
\endcode

\section ogr_sql_alter_table ALTER TABLE

(OGR >= 1.9.0)

The following OGR SQL ALTER TABLE commands can be used.

<ol>
<li>"ALTER TABLE tablename ADD [COLUMN] columnname columntype" to add a new field. Supported if the layer declares the OLCCreateField capability.
<li>"ALTER TABLE tablename RENAME [COLUMN] oldcolumnname TO newcolumnname" to rename an existing field. Supported if the layer declares the OLCAlterFieldDefn capability.
<li>"ALTER TABLE tablename ALTER [COLUMN] columnname TYPE columntype" to change the type of an existing field. Supported if the layer declares the OLCAlterFieldDefn capability.
<li>"ALTER TABLE tablename DROP [COLUMN] columnname" to delete an existing field. Supported if the layer declares the OLCDeleteField capability.
</ol>

The columntype value follows the syntax of the types supported by the CAST operator described above.

\code
ALTER TABLE nation ADD COLUMN myfield integer
ALTER TABLE nation RENAME COLUMN myfield TO myfield2
ALTER TABLE nation ALTER COLUMN myfield2 TYPE character(15)
ALTER TABLE nation DROP COLUMN myfield2
\endcode

\section ogr_sql_drop_table DROP TABLE

(OGR >= 1.9.0)

The OGR SQL DROP TABLE command can be used to delete a table. This is only
supported on datasources that declare the ODsCDeleteLayer capability.

\code
DROP TABLE nation
\endcode

\section ogr_sql_exec_sql ExecuteSQL()

SQL is executed against an GDALDataset, not against a specific layer.  The
call looks like this:

\code
OGRLayer * GDALDataset::ExecuteSQL( const char *pszSQLCommand,
				      OGRGeometry *poSpatialFilter,
                                      const char *pszDialect );
\endcode

The pszDialect argument is in theory intended to allow for support of
different command languages against a provider, but for now applications
should always pass an empty (not NULL) string to get the default dialect.<p>

The poSpatialFilter argument is a geometry used to select a bounding rectangle
for features to be returned in a manner similar to the 
OGRLayer::SetSpatialFilter() method.  It may be NULL for no special spatial
restriction.<p>

The result of an ExecuteSQL() call is usually a temporary OGRLayer representing
the results set from the statement.  This is the case for a SELECT statement
for instance.  The returned temporary layer should be released with 
GDALDataset::ReleaseResultsSet() method when no longer needed.  Failure
to release it before the datasource is destroyed may result in a crash.<p>

\section ogr_sql_non_ogr_sql Non-OGR SQL

All OGR drivers for database systems: <a href="/ogr/drv_mysql.html">MySQL</a>, 
PostgreSQL and PostGIS (<a href="/ogr/drv_pg.html">PG</a>), 
Oracle (<a href="/ogr/drv_oci.html">OCI</a>), 
<a href="/ogr/drv_sqlite.html">SQLite</a>, 
<a href="/ogr/drv_odbc.html">ODBC</a>,
ESRI Personal Geodatabase (<a href="/ogr/drv_pgeo.html">PGeo</a>) and
MS SQL Spatial (<a href="/ogr/drv_mssqlspatial.html">MSSQLSpatial</a>),
override the GDALDataset::ExecuteSQL() function with dedicated implementation 
and, by default, pass the SQL statements directly to the underlying RDBMS. 
In these cases the SQL syntax varies in some particulars from OGR SQL.
Also, anything possible in SQL can then be accomplished for these particular
databases.  Only the result of SQL WHERE statements will be returned as
layers. <p>

*/