how to get index of no zero column in mysql

I have this table in my database.

I want to get the index of column which value equal 1 use select.
If have many one in one row,I want to get first index.
For example,1 for the first row,2 for the fifth row,0 for the eighth row.
this is a sql for init this table.

CREATE TABLE `action` (
  `action_id` BIGINT NOT NULL AUTO_INCREMENT,
  `action_level_0` SMALLINT DEFAULT NULL COMMENT 'L0',
  `action_level_1` SMALLINT DEFAULT NULL COMMENT 'L1',
  `action_level_2` SMALLINT DEFAULT NULL COMMENT 'L2',
  `action_level_3` SMALLINT DEFAULT NULL COMMENT 'L3',
   PRIMARY KEY (`action_id`)
) ENGINE=INNODB AUTO_INCREMENT=2
INSERT INTO `action` 
(`action_level_0`,`action_level_1`,`action_level_2`,`action_level_3`)
VALUES
(1,0,0,0),
(0,1,0,0),
(0,0,1,0),
(0,0,0,1),
(0,0,0,1);

I want to get this reuslt

You can do it using CASE WHEN clause :

select case when action_level_0 = 1 then 'L0'
            when action_level_1 = 1 then 'L1'
            when action_level_2 = 1 then 'L2'
            when action_level_3 = 1 then 'L3' end as Result
from `action`;

Demo here

One way without normalising the table is to do something like

select 
  case when `action_level_0` = 1 then 'L0'
       when `action_level_1` = 1 then 'L1'
       when `action_level_2` = 1 then 'L2'
       when `action_level_3` = 1 then 'L3'
       else 'N/A' end as result
from `action`

See fiddle http://sqlfiddle.com/#!9/baa2f/10 but as pointed out it may be wise to create column as pointed out.