Променете таблицата с помощта на подизбор

SELECT CONCAT('ALTER TABLE `field_data` ', 
  GROUP_CONCAT(' CHANGE COLUMN `field_id_', field_id, '` ',
    ' `field_id_', field_id, '` DECIMAL NOT NULL')) 
FROM `field_info` 
WHERE `field_type` = 'a_decimal_field'
INTO @sql;

PREPARE stmt FROM @sql;

EXECUTE stmt;