1sql(1) GRASS GIS User's Manual sql(1)
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6 Vector points, lines and areas usually have attribute data that are
7 stored in DBMS. The attributes are linked to each vector object using a
8 category number (attribute ID, usually the "cat" integer column). The
9 category numbers are stored both in the vector geometry and the attri‐
10 bute table.
11 GRASS GIS supports various RDBMS (Relational database management sys‐
13 tem) and embedded databases. SQL (Structured Query Language) queries
14 are directly passed to the underlying database system. The set of sup‐
15 ported SQL commands depends on the RDMBS and database driver selected.
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18 The default database driver used by GRASS GIS 8 is SQLite. GRASS GIS
19 handles multiattribute vector data by default. The db.* set of commands
20 provides basic SQL support for attribute management, while the v.db.*
21 set of commands operates on vector maps.
22 Note: The list of available database drivers can vary in various binary
24 distributions of GRASS GIS:
25 sqlite Data storage in SQLite database files (default DB backend) http://sqlite.org/
27 dbf Data storage in DBF files http://shapelib.maptools.org/dbf_api.html
29 pg Data storage in PostgreSQL RDBMS http://postgresql.org/
31 mysql Data storage in MySQL RDBMS http://mysql.org/
33 odbc Data storage via UnixODBC (PostgreSQL, Oracle, etc.) http://www.unixodbc.org/
35 ogr Data storage in OGR files http://gdal.org/
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40 Database table name restrictions
41 • No dots are allowed as SQL does not support ’.’ (dots) in table
42 names.
43 • Supported table name characters are only:
45 [A-Za-z][A-Za-z0-9_]*
46 • A table name must start with a character, not a number.
48 • Text-string matching requires the text part to be ’single
50 quoted’. When run from the command line multiple queries
51 should be contained in "double quotes". e.g.
52 d.vect map where="individual=’juvenile’ and area=’beach’"
53 • Attempts to use a reserved SQL word (depends on database back‐
55 end) as column or table name will cause a "SQL syntax error".
56 • An error message such as "dbmi: Protocol error" either indi‐
58 cates an invalid column name or an unsupported column type
59 (then the GRASS SQL parser needs to be extended).
60 • DBF column names are limited to 10 characters (DBF API defini‐
62 tion).
63 Database table column types
65 The supported types of columns depend on the database backend. However,
66 all backends should support VARCHAR, INT, DOUBLE PRECISION and DATE.
67
69 Display of vector feature selected by attribute query
70 Display all vector points except for LAMAR valley and extensive trap‐
71 ping (brackets are superfluous in this example):
72 g.region vector=schools_wake -p
73 d.mon wx0
74 d.vect roadsmajor
75 # all schools
76 d.vect schools_wake fcol=black icon=basic/diamond col=white size=13
77 # numerical selection: show schools with capacity of above 1000 kids:
78 d.vect schools_wake fcol=blue icon=basic/diamond col=white size=13 \
79 where="CAPACITYTO > 1000"
80 # string selection: all schools outside of Raleigh
81 # along with higher level schools in Raleigh
82 d.vect schools_wake fcol=red icon=basic/diamond col=white size=13 \
83 where="ADDRCITY <> ’Raleigh’ OR (ADDRCITY = ’Raleigh’ AND GLEVEL = ’H’)"
84 Select all attributes from table where CORECAPACI column values are
86 smaller than 200 (children):
87 # must be run from the mapset which contains the table
88 echo "SELECT * FROM schools_wake WHERE CORECAPACI < 200" | db.select input=-
89 Example of subquery expressions from a list (not supported for DBF
91 driver):
92 v.db.select schools_wake where="ADDRCITY IN (’Apex’, ’Wendell’)"
93 Example of pattern matching
95 # field contains string:
96 # for DBF driver:
97 v.extract schools_wake out=elementary_schools where="NAMELONG LIKE ’ELEM’"
98 # for SQLite driver:
99 v.extract schools_wake out=rivers_noce where="DES LIKE ’%NOCE%’"
100 v.extract schools_wake out=elementary_schools where="NAMELONG LIKE ’%ELEM%’"
101 # match exactly number of characters (here: 2), does not work for DBF driver:
102 v.db.select mysites where="id LIKE ’P__’"
103 #define wildcard:
104 v.db.select mysites where="id LIKE ’P%’"
105 Example of null handling
107 v.db.addcolumn map=roads col="nulltest int"
108 v.db.update map=roads col=nulltest value=1 where="cat > 2"
109 d.vect roads where="nulltest is null"
110 v.db.update map=roads col=nulltest value=2 where="cat <= 2"
111 Update of attributes
113 Examples of complex expressions in updates (using v.db.* modules):
114 v.db.addcolumn map=roads column="exprtest double precision"
115 v.db.update map=roads column=exprtest value="cat/nulltest"
116 v.db.update map=roads column=exprtest value="cat/nulltest+cat" where="cat=1"
117 # using data from another column
118 v.db.update map=roads column=exprtest qcolumn="(cat*100.)/SHAPE_LEN."
119 Examples of more complex expressions in updates (using db.* modules):
121 echo "UPDATE roads SET exprtest=null"
122 echo "UPDATE roads SET exprtest=cat/2" | db.execute
123 echo "UPDATE roads SET exprtest=cat/2+cat/3" | db.execute
124 echo "UPDATE roads SET exprtest=NULL WHERE cat>2" | db.execute
125 echo "UPDATE roads SET exprtest=cat/3*(cat+1) WHERE exprtest IS NULL" | db.execute"
126 Instead of creating and updating new columns with an expression, you
128 can use the expression directly in a command:
129 d.vect roads where="(cat/3*(cat+1))>8"
130 d.vect roads where="cat>exprtest"
131 Example of changing a SQL type (type casting)
133 Note: not supported for DBF driver.
134 North Carolina data set: convert string column to double precision:
136 # first copy map into current mapset
138 g.copy vect=geodetic_pts,mygeodetic_pts
139 v.db.addcolumn mygeodetic_pts col="zval double precision"
140 # the ’z_value’ col contains ’N/A’ strings, not to be converted
141 v.db.update mygeodetic_pts col=zval \
142 qcol="CAST(z_value AS double precision)" \
143 where="z_value <> ’N/A’"
144 Example of concatenation of fields
146 Note: not supported for DBF driver.
147 v.db.update vectormap column=column3 qcolumn="column1 || column2"
148 Example of conditions
150 Conditions (like if statements) are usually written as CASE statement
151 in SQL:
152 v.db.update vectormap column=species qcolumn="CASE WHEN col1 >= 12 THEN cat else NULL end"
153 # a more complex example with nested conditions
154 v.db.update vectormap column=species qcolumn="CASE WHEN col1 >= 1 THEN cat WHEN row = 13 then 0 ELSE NULL end"
155
157 db.connect, db.select, db.execute, v.db.connect, v.db.select, v.db.up‐
158 date
159 Database management in GRASS GIS, Help pages for database modules
161
163 Radmin Blazek
164 Main index | Topics index | Keywords index | Graphical index | Full in‐
166 dex
167 © 2003-2023 GRASS Development Team, GRASS GIS 8.2.1 Reference Manual
169GRASS 8.2.1 sql(1)