MSSQL provider: Implement minimumValue, maximumValue and uniqueValues (fixes #5610)

Author: szekerest

src/providers/mssql/qgsmssqlprovider.cpp

@@ -604,7 +604,10 @@ void QgsMssqlProvider::select( QgsAttributeList fetchAttributes,
 
   if ( !mSqlWhereClause.isEmpty() )
   {
-    mStatement += " and (" + mSqlWhereClause + ")";
+    if ( rect.isEmpty() )
+        mStatement += " where (" + mSqlWhereClause + ")";
+    else
+        mStatement += " and (" + mSqlWhereClause + ")";
   }
 
   mFetchGeom = fetchGeometry;
@@ -626,6 +629,129 @@ void QgsMssqlProvider::select( QgsAttributeList fetchAttributes,
   }
 }
 
+// Returns the minimum value of an attribute
+QVariant QgsMssqlProvider::minimumValue( int index )
+{
+  // get the field name
+  QgsField fld = mAttributeFields[ index ];
+  QString sql = QString( "select min([%1]) from " )
+                .arg( fld.name() );
+
+  if ( !mSchemaName.isEmpty() )
+  sql += "[" + mSchemaName + "].";
+
+  sql += "[" + mTableName + "]";
+
+  if ( !mSqlWhereClause.isEmpty() )
+  {
+    sql += QString( " where (%1)" ).arg( mSqlWhereClause );
+  }
+
+  mQuery = QSqlQuery( mDatabase );
+  mQuery.setForwardOnly( true );
+
+  if ( !mQuery.exec( sql ) )
+  {
+    QString msg = mQuery.lastError().text();
+    QgsDebugMsg( msg );
+  }
+
+  if ( mQuery.isActive() )
+  {
+    if ( mQuery.next() )
+    {
+      return mQuery.value( 0 );
+    }
+  }
+
+  return QVariant( QString::null );
+}
+
+// Returns the maximum value of an attribute
+QVariant QgsMssqlProvider::maximumValue( int index )
+{
+  // get the field name
+  QgsField fld = mAttributeFields[ index ];
+  QString sql = QString( "select max([%1]) from " )
+                .arg( fld.name() );
+
+  if ( !mSchemaName.isEmpty() )
+  sql += "[" + mSchemaName + "].";
+
+  sql += "[" + mTableName + "]";
+
+  if ( !mSqlWhereClause.isEmpty() )
+  {
+    sql += QString( " where (%1)" ).arg( mSqlWhereClause );
+  }
+
+  mQuery = QSqlQuery( mDatabase );
+  mQuery.setForwardOnly( true );
+
+  if ( !mQuery.exec( sql ) )
+  {
+    QString msg = mQuery.lastError().text();
+    QgsDebugMsg( msg );
+  }
+
+  if ( mQuery.isActive() )
+  {
+    if ( mQuery.next() )
+    {
+      return mQuery.value( 0 );
+    }
+  }
+
+  return QVariant( QString::null );
+}
+
+// Returns the list of unique values of an attribute
+void QgsMssqlProvider::uniqueValues( int index, QList<QVariant> &uniqueValues, int limit )
+{
+  uniqueValues.clear();
+
+  // get the field name
+  QgsField fld = mAttributeFields[ index ];
+  QString sql = QString( "select distinct ");
+  
+  if (limit > 0)
+  {
+    sql += QString( " top %1 " ).arg( limit );
+  }
+
+  sql += QString( "[%1] from " )
+                .arg( fld.name() );
+
+  if ( !mSchemaName.isEmpty() )
+  sql += "[" + mSchemaName + "].";
+
+  sql += "[" + mTableName + "]";
+
+  if ( !mSqlWhereClause.isEmpty() )
+  {
+    sql += QString( " where (%1)" ).arg( mSqlWhereClause );
+  }
+
+  mQuery = QSqlQuery( mDatabase );
+  mQuery.setForwardOnly( true );
+
+  if ( !mQuery.exec( sql ) )
+  {
+    QString msg = mQuery.lastError().text();
+    QgsDebugMsg( msg );
+  }
+
+  if ( mQuery.isActive() )
+  {
+    // read all features
+    while ( mQuery.next() )
+    {
+      uniqueValues.append( mQuery.value( 0 ) );
+    }
+  }
+}
+
+
 // update the extent, feature count, wkb type and srid for this layer
 void QgsMssqlProvider::UpdateStatistics( bool estimate )
 {

src/providers/mssql/qgsmssqlprovider.h

@@ -133,6 +133,36 @@ class QgsMssqlProvider : public QgsVectorDataProvider
                          bool fetchGeometry = true,
                          bool useIntersect = false );
 
+    /**
+     * Returns the minimum value of an attribute
+     * @param index the index of the attribute
+     *
+     * Default implementation walks all numeric attributes and caches minimal
+     * and maximal values. If provider has facilities to retrieve minimal
+     * value directly, override this function.
+     */
+    virtual QVariant minimumValue( int index );
+
+    /**
+     * Returns the maximum value of an attribute
+     * @param index the index of the attribute
+     *
+     * Default implementation walks all numeric attributes and caches minimal
+     * and maximal values. If provider has facilities to retrieve maximal
+     * value directly, override this function.
+     */
+    virtual QVariant maximumValue( int index );
+
+    /**
+     * Return unique values of an attribute
+     * @param index the index of the attribute
+     * @param uniqueValues values reference to the list to fill
+     * @param limit maxmum number of the values to return (added in 1.4)
+     *
+     * Default implementation simply iterates the features
+     */
+    virtual void uniqueValues( int index, QList<QVariant> &uniqueValues, int limit = -1 );
+
     /**
      * Get the next feature resulting from a select operation.
      * @param feature feature which will receive data from the provider