Oracle ORA-01451: 要修改为 NULL 的列无法修改为 NULL

调试一段程序，遇到如题错误，查资料才发现Oracle中不允许将NULL字段修改为NULL字段。只好在修改之前做判断了。
打开PL/SQL，写如下代码
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; 

运行，又出现错误提示如下
---------------------------------------------------------------------------
ORA-06550: 第 8 行, 第 7 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> <<
continue close current delete fetch lock insert open rollback
savepoint set sql execute commit forall merge pipe purge
---------------------------------------------------------------------------------
仔细一看，原来alter不允许在PL/SQL下直接运行，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; 
运行通过

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