Update the SHPWrite doc

docs/dev/SHPWrite.md

@@ -20,6 +20,13 @@ SHPWrite(VARCHAR path, VARCHAR tableName, VARCHAR fileEncoding);
 
 Writes the contents of table `tableName` to a [shapefile][wiki]
 located at `path`.
+
+`tablename` can be either:
+
+* the name of an existing table,
+* the result of a selection (`SELECT` instruction which has to be written between simple quote `' '`).
+
+
 The default value of `fileEncoding` is `ISO-8859-1`.
 
 <div class="note warning">
@@ -48,18 +55,32 @@ CALL SHPWrite('/home/user/area.shp', 'AREA');
 CALL SHPRead('/home/user/area.shp', 'AREA2');
 SELECT * FROM AREA2;
 -- Answer:
--- |                     THE_GEOM                     | IDAREA |
--- | ------------------------------------------------ | ------ |
--- | MULTIPOLYGON(((-10 109,, 90 9, -10 9, -10 109))) |      1 |
--- | MULTIPOLYGON(((90 109, 190 109, 90 9, 90 109)))  |      2 |
+-- |                     THE_GEOM                     | ID |
+-- | ------------------------------------------------ | -- |
+-- | MULTIPOLYGON(((-10 109,, 90 9, -10 9, -10 109))) |  1 |
+-- | MULTIPOLYGON(((90 109, 190 109, 90 9, 90 109)))  |  2 |
 {% endhighlight %}
 
+#### Case where `tablename` is the result of a selection.
+
+{% highlight mysql %}
+CALL SHPWRITE('/home/user/area.shp', 
+              '(SELECT * FROM AREA WHERE ID<2 )');
+
+-- Read it back
+CALL SHPRead('/home/user/area.shp', 'AREA2');
+SELECT * FROM AREA2;
+-- Answer:
+-- |                     THE_GEOM                     | ID |
+-- | ------------------------------------------------ | -- |
+-- | MULTIPOLYGON(((-10 109,, 90 9, -10 9, -10 109))) |  1 |
+{% endhighlight %}
 
 ### Export the .prj file
 
 If you want to export your shapefile with it's projection, stored in a .prj file, you must assume that the table contains a SRID constraint value  greater than 0. 
 
-If not the SRID must be enforced using the following commands:
+If not, the SRID must be enforced using the following commands:
 
 {% highlight mysql %}
 UPDATE mytable SET the_geom = ST_SetSRID(the_geom, EPSG_CODE);