oracle 字段非空时独一的触发器

oracle 字段非空时唯一的触发器
如题我建了一个简单的表

create table test(id number not null,name varchar2(64),code varchar2(18))

creater or replace trigger tg_testbefore update of code on test  for each rowdeclare  cnt number;exp_err exception;beginselect conut(*) into from test where (code is not null) and code＝:new.code;if cnt!＝0 thenraise exp_err;end if;exceptionwhen exp_err thenraise _application_error(-22222,'重复');end;