MariaDB 10.2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table mytable (ConstantValue varchar(100));
insert into mytable values
('1252.36'),('abc'),('55969'),('def'),('66955663'),('A8879'),('-41.25'),('+636');
alter table mytable
    add column NewConstantValue double default(0);
update mytable
set    ConstantValue = '0'
where  ConstantValue not regexp '^(-|\\+)?([0-9]+\\.[0-9]*|[0-9]*\\.[0-9]+|[0-9]+)$';
update mytable
set    NewConstantValue = cast(ConstantValue as double);
select * from mytable;


ConstantValue
NewConstantValue


1252.36
1252.36

0
0

55969
55969

0
0

66955663
66955663

0
0

-41.25
-41.25

+636
636


alter table mytable
    drop column ConstantValue;
alter table mytable
    change column NewConstantValue ConstantValue double default(0);
select * from mytable;


ConstantValue


1252.36

0

55969

0

66955663

0

-41.25

636

ConstantValue	NewConstantValue
1252.36	1252.36
0	0
55969	55969
0	0
66955663	66955663
0	0
-41.25	-41.25
+636	636