.TH sql 1grass "" "GRASS 7.8.5" "GRASS GIS User's Manual"
.SH SQL support in GRASS GIS
Vector points, lines and areas usually have attribute data that are
stored in DBMS. The attributes are linked to each vector object using a
category number (attribute ID, usually the \(dqcat\(dq integer column). The
category numbers are stored both in the vector geometry and the
attribute table.
.PP
GRASS GIS supports various RDBMS
(Relational
database management system) and embedded databases. SQL
(Structured Query
Language) queries are directly passed to the underlying database
system. The set of supported SQL commands depends on the RDMBS and
database driver selected.
.SH Database drivers
The default database driver used by GRASS GIS 7 is SQLite. GRASS GIS
handles multiattribute vector data by default. The \fIdb.*\fR set of
commands  provides basic SQL support for attribute management, while the
\fIv.db.*\fR set of commands operates on vector maps.
.PP
Note: The list of available database drivers can vary in various binary
distributions of GRASS GIS:
.PP
.TS
expand;
lw60 lw1 lw60 lw1 lw60.
T{
sqlite
T}	 	T{
Data storage in SQLite database files (default DB backend)
T}	 	T{
http://sqlite.org/
T}
.sp 1
T{
dbf
T}	 	T{
Data storage in DBF files
T}	 	T{
http://shapelib.maptools.org/dbf_api.html
T}
.sp 1
T{
pg
T}	 	T{
Data storage in PostgreSQL RDBMS
T}	 	T{
http://postgresql.org/
T}
.sp 1
T{
mysql
T}	 	T{
Data storage in MySQL RDBMS
T}	 	T{
http://mysql.org/
T}
.sp 1
T{
odbc
T}	 	T{
Data storage via UnixODBC (PostgreSQL, Oracle, etc.)
T}	 	T{
http://www.unixodbc.org/
T}
.sp 1
T{
ogr
T}	 	T{
Data storage in OGR files
T}	 	T{
http://gdal.org/
T}
.sp 1
.TE
.SH NOTES
.SS Database table name restrictions
.RS 4n
.IP \(bu 4n
No dots are allowed as SQL does not support \(cq.\(cq (dots) in table names.
.IP \(bu 4n
Supported table name characters are only:
.br
.br
.nf
\fC
[A\-Za\-z][A\-Za\-z0\-9_]*
\fR
.fi
.IP \(bu 4n
A table name must start with a character, not a number.
.IP \(bu 4n
Text\-string matching requires the text part to be \(cqsingle quoted\(cq.
When run from the command line multiple queries should be contained
in \(dqdouble quotes\(dq. e.g.
.br
.br
.nf
\fC
d.vect map where=\(dqindividual=\(cqjuvenile\(cq and area=\(cqbeach\(cq\(dq
\fR
.fi
.IP \(bu 4n
Attempts to use a reserved SQL word (depends on database backend) as
column or table name will cause a \(dqSQL syntax error\(dq.
.IP \(bu 4n
An error message such as \(dqdbmi: Protocol
error\(dq either indicates an invalid column name or an
unsupported column type (then the GRASS SQL parser needs to be
extended).
.IP \(bu 4n
DBF column names are limited to 10 characters (DBF API definition).
.RE
.SS Database table column types
The supported types of columns depend on the database backend. However, all backends
should support VARCHAR, INT, DOUBLE PRECISION and DATE.
.SH EXAMPLES
.SS Display of vector feature selected by attribute query
Display all vector points except for \fILAMAR\fR valley
and \fIextensive trapping\fR (brackets are superfluous in this
example):
.br
.nf
\fC
g.region vector=schools_wake \-p
d.mon wx0
d.vect roadsmajor
# all schools
d.vect schools_wake fcol=black icon=basic/diamond col=white size=13
# numerical selection: show schools with capacity of above 1000 kids:
d.vect schools_wake fcol=blue icon=basic/diamond col=white size=13 \(rs
    where=\(dqCAPACITYTO > 1000\(dq
# string selection: all schools outside of Raleigh
#   along with higher level schools in Raleigh
d.vect schools_wake fcol=red icon=basic/diamond col=white size=13 \(rs
    where=\(dqADDRCITY <> \(cqRaleigh\(cq OR (ADDRCITY = \(cqRaleigh\(cq AND GLEVEL = \(cqH\(cq)\(dq
\fR
.fi
.PP
Select all attributes from table where \fICORECAPACI\fR column values are
smaller than 200 (children):
.br
.nf
\fC
# must be run from the mapset which contains the table
echo \(dqSELECT * FROM schools_wake WHERE CORECAPACI < 200\(dq | db.select input=\-
\fR
.fi
.PP
.PP
Example of subquery expressions from a list (not supported for DBF driver):
.br
.nf
\fC
v.db.select schools_wake where=\(dqADDRCITY IN (\(cqApex\(cq, \(cqWendell\(cq)\(dq
\fR
.fi
.SS Example of pattern matching
.br
.nf
\fC
# field contains string:
#  for DBF driver:
v.extract schools_wake out=elementary_schools where=\(dqNAMELONG LIKE \(cqELEM\(cq\(dq
#  for SQLite driver:
v.extract schools_wake out=rivers_noce where=\(dqDES LIKE \(cq%NOCE%\(cq\(dq
v.extract schools_wake out=elementary_schools where=\(dqNAMELONG LIKE \(cq%ELEM%\(cq\(dq
# match exactly number of characters (here: 2), does not work for DBF driver:
v.db.select mysites where=\(dqid LIKE \(cqP__\(cq\(dq
#define wildcard:
v.db.select mysites where=\(dqid LIKE \(cqP%\(cq\(dq
\fR
.fi
.SS Example of null handling
.br
.nf
\fC
v.db.addcolumn map=roads col=\(dqnulltest int\(dq
v.db.update map=roads col=nulltest value=1 where=\(dqcat > 2\(dq
d.vect roads where=\(dqnulltest is null\(dq
v.db.update map=roads col=nulltest value=2 where=\(dqcat <= 2\(dq
\fR
.fi
.SS Update of attributes
Examples of complex expressions in updates (using v.db.*
modules):
.br
.nf
\fC
v.db.addcolumn map=roads column=\(dqexprtest double precision\(dq
v.db.update map=roads column=exprtest value=\(dqcat/nulltest\(dq
v.db.update map=roads column=exprtest value=\(dqcat/nulltest+cat\(dq where=\(dqcat=1\(dq
# using data from another column
v.db.update map=roads column=exprtest qcolumn=\(dq(cat*100.)/SHAPE_LEN.\(dq
\fR
.fi
.PP
Examples of more complex expressions in updates (using db.*
modules):
.br
.nf
\fC
echo \(dqUPDATE roads SET exprtest=null\(dq
echo \(dqUPDATE roads SET exprtest=cat/2\(dq | db.execute
echo \(dqUPDATE roads SET exprtest=cat/2+cat/3\(dq | db.execute
echo \(dqUPDATE roads SET exprtest=NULL WHERE cat>2\(dq | db.execute
echo \(dqUPDATE roads SET exprtest=cat/3*(cat+1) WHERE exprtest IS NULL\(dq | db.execute\(dq
\fR
.fi
.PP
Instead of creating and updating new columns with an expression, you
can use the expression directly in a command:
.br
.nf
\fC
d.vect roads where=\(dq(cat/3*(cat+1))>8\(dq
d.vect roads where=\(dqcat>exprtest\(dq
\fR
.fi
.SS Example of changing a SQL type (type casting)
\fINote: not supported for DBF driver.\fR
.PP
North Carolina data set: convert string column to double precision:
.PP
.br
.nf
\fC
# first copy map into current mapset
g.copy vect=geodetic_pts,mygeodetic_pts
v.db.addcolumn mygeodetic_pts col=\(dqzval double precision\(dq
# the \(cqz_value\(cq col contains \(cqN/A\(cq strings, not to be converted
v.db.update mygeodetic_pts col=zval \(rs
            qcol=\(dqCAST(z_value AS double precision)\(dq \(rs
            where=\(dqz_value <> \(cqN/A\(cq\(dq
\fR
.fi
.SS Example of concatenation of fields
\fINote: not supported for DBF driver.\fR
.br
.nf
\fC
v.db.update vectormap column=column3 qcolumn=\(dqcolumn1 || column2\(dq
\fR
.fi
.SS Example of conditions
Conditions (like if statements) are usually written as CASE statement in SQL:
.br
.nf
\fC
v.db.update vectormap column=species qcolumn=\(dqCASE WHEN col1 >= 12 THEN cat else NULL end\(dq
# a more complex example with nested conditions
v.db.update vectormap column=species qcolumn=\(dqCASE WHEN col1 >= 1 THEN cat WHEN row = 13 then 0 ELSE NULL end\(dq
\fR
.fi
.SH SEE ALSO
\fI
db.connect,
db.select,
db.execute,
v.db.connect,
v.db.select,
v.db.update
\fR
.PP
Database management in GRASS GIS,
Help pages for database modules
.PP
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.PP
© 2003\-2020
GRASS Development Team,
GRASS GIS 7.8.5 Reference Manual