SELECT count(*) INTO @EXIST
FROM INFORMATION_SCHEMA.COLUMNS
WHERE TABLE_SCHEMA = 'mydatabase'
  AND COLUMN_NAME = 'mycolumn'
  AND TABLE_NAME = 'mytable' LIMIT 1;

SET @query = IF (@exist <= 0, 'ALTER TABLE mydatabase.`mytable`  ADD COLUMN `mycolumn` MEDIUMTEXT NULL',
    'select \' COLUMN EXISTS\' status');
PREPARE stmt FROM @query;
EXECUTE stmt;