mysql select statement bring back column if column is empty

Question

Is there a way to select col1 if col2 is empty else use col2? col1 will never be empty.

select 
ug.permissions,
g.name,
g.display,
g.permissions
from system.users_groups as ug
inner join system.groups as g on ug.group_id = g.id
where ug.user_id = ?

The above statement brings back both permissions columns, I would like to bring back only one permissions column.

if ug.permissions = '' then use g.permissions

Answer 1

Using COALESCE and NULLIF:

select 
COALESCE(NULLIF(ug.permissions,''),g.permissions) as permissions ,
g.name,
g.display
from system.users_groups as ug
inner join system.groups as g on ug.group_id = g.id
where ug.user_id = ?

Using CASE:

select 
CASE WHEN ug.permissions = '' OR ug.permissions IS NULL
THEN
     g.permissions
ELSE
     ug.permissions 
END as permissions ,
g.name,
g.display
from system.users_groups as ug
inner join system.groups as g on ug.group_id = g.id
where ug.user_id = ?