By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
select concat_ws('-',
substr(mobile_number, 1, 2),
substr(mobile_number, 3, 2),
substr(mobile_number, 5, 2),
substr(mobile_number, 7, 2)
)
from (select '1234587920' as mobile_number) x
| concat_ws('-', substr(mobile_number, 1, 2), substr(mobile_number, 3, 2), substr(mobile_number, 5, 2), substr(mobile_number, 7, 2) ) |
|---|
| 12-34-58-79 |