Oracle ORA-01451: 要修改為 NULL 的列無法修改為 NULL

調試一段程式，遇到如題錯誤，查資料才發現Oracle中不允許將NULL欄位修改為NULL欄位。只好在修改之前做判斷了。
開啟PL/SQL，寫如下代碼
declare
visnull varchar2(4);
begin
select nullable into visnull from user_tab_columns
where table_name = upper('tblStockInspect')
and column_name = upper('FDepartID');
if visnull = 'N' then
alter table tblStockInspect modify FDepartID int null;
end if;
end; 

運行，又出現錯誤提示如下
---------------------------------------------------------------------------
ORA-06550: 第 8 行, 第 7 列:
PLS-00103: 出現符號 "ALTER"在需要下列之一時：
( 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> <<
continue close current delete fetch lock insert open rollback
savepoint set sql execute commit forall merge pipe purge
---------------------------------------------------------------------------------
仔細一看，原來alter不允許在PL/SQL下直接運行，只好更改如下
declare
visnull varchar2(4);
begin
select nullable into visnull from user_tab_columns
where table_name = upper('tblStockInspect')
and column_name = upper('FDepartID');
if visnull = 'N' then
execute immediate 'alter table tblStockInspect modify FDepartID int null‘;
end if;
end; 
運行通過