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GDAL
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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.
Since GDAL/OGR 1.10, an alternate "dialect", the SQLite dialect, can be used instead of the OGRSQL dialect. Refer to the SQLite SQL dialect page for more details.
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.
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.
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:
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.
A much more ambitious SELECT, restricting the features fetched with a WHERE clause, and sorting the results might look like:
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.
The general syntax of a SELECT statement is:
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.
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 a lot of memory may be used for datasets with a large number of distinct values.
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 summarization information on parcel property values:
It is also possible to apply the COUNT() operator to a DISTINCT SELECT to get a count of distinct values, for instance:
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.
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.
or
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.
Starting with OGR 2.0, the hstore_get_value() function can be used to extract a value associate to a key from a HSTORE string, formatted like 'key=>value,other_key=>other_value,...'
OGR SQL supports renaming the fields following the SQL92 specification by using the AS keyword according to the following example:
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:
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:
Currently casting to the following target types are supported:
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].
Starting with GDAL 2.0 (see RFC 52 - Strict OGR SQL quoting), 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 surrounded with double-quote characters. e.g. WHERE "from" = 5.
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 =, !=, <>, <, >, <=, >=, LIKE and ILIKE, BETWEEN and IN. Most of the operators are self explanatory, but it is worth noting that != is the same as <>, the string equality is case insensitive, but the <, >, <= and >= operators are case sensitive. Both the LIKE and ILIKE operators are case insensitive.
The value argument to the LIKE 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.
The IN takes a list of values as its argument and tests the attribute value for membership in the provided set.
The syntax of the BETWEEN 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 IS NULL and IS NOT NULL operators.
Basic field tests can be combined in more complicated predicates using logical operators include AND, OR, and the unary logical NOT. Subexpressions should be bracketed to make precedence clear. Some more complicated predicates are:
The ORDER BY 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:
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.
Starting with GDAL 2.2, the LIMIT clause can be used to limit the number of features returned. For example
The OFFSET clause can be used to skip the first features of the result set. The value after OFFSET is the number of features skipped. For example, to skip the first 3 features from the result set:
Both clauses can be combined:
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 nation_id column that can be used as a reference into a secondary nation table to fetch a nation name. A joined query might look like:
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 ON clause of the join.
Wildcards are also somewhat more involved. All fields from the primary table (city in this case) and the secondary table (nation in this case) may be selected using the usual * 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 name, nation_id, nation.nation_id and nation.name field if the city and nation tables both have the nation_id and name fieldnames.
On the other hand if the nation table had a continent_id 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.
In the above examples, the nation table was found in the same datasource as the city 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.
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:
It is possible to do multiple joins in a single query.
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 mentioned 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.
(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.
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.
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.
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 FID. The field wildcard expansions will not include the feature id, but it may be explicitly included using a syntax like:
Some of the data sources (like MapInfo tab) can handle geometries of different types within the same layer. The OGR_GEOMETRY 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:
The Well Known Text representation of the geometry can also be used as a special field. To select the WKT of the geometry OGR_GEOM_WKT might be included in the select list, like:
Using the OGR_GEOM_WKT and the LIKE operator in the WHERE clause we can get similar effect as using OGR_GEOMETRY:
(Since GDAL 1.7.0)
The OGR_GEOM_AREA 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:
The OGR_STYLE special field represents the style string of the feature returned by OGRFeature::GetStyleString(). By using this field and the LIKE operator the result of the query can be filtered by the style. For example we can select the annotation features as:
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 fieldname = value, which is what is used by the JOIN capability. To create an attribute index on the nation_id field of the nation table a command like this would be used:
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.
(OGR >= 1.9.0)
The following OGR SQL ALTER TABLE commands can be used.
The columntype value follows the syntax of the types supported by the CAST operator described above.
(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.
SQL is executed against an GDALDataset, not against a specific layer. The call looks like this:
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.
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.
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.
All OGR drivers for database systems: MySQL, PostgreSQL and PostGIS (PG), Oracle (OCI), SQLite, ODBC, ESRI Personal Geodatabase (PGeo) and MS SQL Spatial (MSSQLSpatial), 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.
1.8.13.