错误信息看不懂，请教诸位！

[ningIII]

BEGIN
alter table TEST add t16 number(10) default 11;
comment on column TEST.t16 is '哈哈哈';
END;

在 命令行中执行上面的语句后，错误信息如下：
ORA-06550: 第 2 行, 第 1 列:
PLS-00103: 出现符号 "ALTER"在需要下列之一时：
begin case declare exit
for goto if loop mod null pragma raise return select update
while with <an identifier>
<a double-quoted delimited-identifier> <a bind variable> <<
close current delete fetch lock insert open rollback
savepoint set sql execute commit forall merge
<a single-quoted SQL string> pipe

什么意思？怎么解决？急等！