How to alter column's default value

Hi experts,

I'm trying to alter column's default value,I can set the default value to number,string,or system date function.

But when try to set to an expression,error encountered,as below.

alter table t_default alter(str nvarchar(100) default upper('abc'));

alter table t_default alter(str nvarchar(100) default 'abc'||'c');

alter table t_default alter(num bigint default 100*100);

Would like to know whether hana support define expression or reference other columns in default value definition.

I'm using SPS09.

thx.

Kelvin