public function primaryKey ($tableName)
		{
			if (! isset($this->primaryKeys[$tableName])) {
				$sql = "SELECT *
					    FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc
					        JOIN INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE ccu ON tc.CONSTRAINT_NAME = ccu.Constraint_name
					    WHERE tc.CONSTRAINT_TYPE = 'Primary Key'
					        AND tc.TABLE_NAME = " . $this->quote($tableName);

				$keyResult                      = $this->query($sql);
				$key                            = $keyResult->fetchRow();
				$this->primaryKeys[$tableName]  = $key['COLUMN_NAME'];
			}

			return $this->primaryKeys[$tableName];
		}