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The function SMWSQLStore::getSQLConditions(), when it does pattern matching on values, always replaces spaces in the substring with underlines. This is fine for property names, and property values when the property is a relation, but not when the property is an attribute - there, spaces should be preserved. Thankfully, there's a simple fix - around line 1476 of /includes/storage/SMW_SQLStore.php, replace the line:

                                        $string = str_replace(array('_', ' '), array('\_', '\_'), $strcond->string);

with:

                                        if ($labelcol == 'value_xsd')
                                                $string = str_replace('_', '\_', $strcond->string);
                                        else
                                                $string = str_replace(array('_', ' '), array('\_', '\_'), $strcond->string);