MySQLは省略記号でテキストを切り捨てます

select case when length(message) > 7 
then concat(substring(message, 1, 7), '...')
else message end as adapted_message
from ...

テスト/確認するには：

SELECT CASE WHEN LENGTH('1234567890') > 7 
THEN CONCAT(SUBSTRING('1234567890', 1, 7), '...') 
ELSE '1234567890' END AS adapted_message

UNION  

SELECT CASE WHEN LENGTH('12345') > 7 
THEN CONCAT(SUBSTRING('12345', 1, 7), '...') 
ELSE '12345' END AS adapted_message