Oracle 18c

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create table t (x varchar2(30))
insert into t (x) values ('abcdefghijiklmnopqrstuvwxyz')
1 rows affected

alter table t modify  x varchar2(20)
ORA-01441: cannot decrease column length because some value is too big

update t
    set x = substr(x, 1, 20)
    where length(x) > 20;
1 rows affected

alter table t modify  x varchar2(20)
select * from t


X


abcdefghijiklmnopqrs