Add nullable boolean attribute to column type

src/main/resources/tap2VOSI.xsl

@@ -38,7 +38,7 @@
    </xsl:template>
     <xsl:template match="table/columns/column">
         <column>
-            <xsl:apply-templates select="(column_name,description,unit,ucd,utype,datatype,indexed,principal)"/>
+            <xsl:apply-templates select="(column_name,description,unit,ucd,utype,datatype,indexed,principal,nullable)"/>
         </column>
     </xsl:template>
     <xsl:template match="indexed">
@@ -47,6 +47,11 @@
     <xsl:template match="principal">
         <flag>primary</flag>
     </xsl:template>
+    <xsl:template match="nullable[text()='true']">
+        <flag>nullable</flag>
+    </xsl:template>
+    <!-- suppress output for nullable=false -->
+    <xsl:template match="nullable"/>
     <xsl:template match="datatype">
         <xsl:element name="dataType" >
             <xsl:attribute name="type" namespace="http://www.w3.org/2001/XMLSchema-instance">vs:VOTableType</xsl:attribute>

src/main/vo-dml/tapschema.vo-dml.xml

@@ -431,6 +431,18 @@
                    <maxOccurs>1</maxOccurs>
                  </multiplicity>
                </attribute>
+               <attribute>
+                 <vodml-id>column.nullable</vodml-id>
+                 <name>nullable</name>
+                 <description>indicates whether the column may have null values</description>	    
+                 <datatype>
+                    <vodml-ref>ivoa:boolean</vodml-ref>
+                 </datatype>
+                 <multiplicity>
+                   <minOccurs>0</minOccurs>
+                   <maxOccurs>1</maxOccurs>
+                 </multiplicity>
+               </attribute>
                <attribute>
                  <vodml-id>column.column_index</vodml-id>
                  <name>column_index</name>

src/main/vodsl/tapschema.vodsl

@@ -69,6 +69,7 @@ query. In cases where the service selects the columns to return (such as a
 query language without an explicit output selection), the principal column
 indicates those columns that are returned by default";
 	std: ivoa:boolean "is the column defined by a standard";
+	nullable: ivoa:boolean @? "indicates whether the column may have null values";
 	column_index: ivoa:integer @? "used to recommend table ordering for clients. Clients
 may order by index (ascending) so lower index items would appear
 earlier in a listing.";