I am student
i doing assignement , do auto increment
How to create an auto increment in Oracle ?
CREATE TABLE mua_thi ( mamuathi varchar2(10) not null, check(mamuathi like 'MT%') ) mamuathi = MT + auto_increment; create or replace trigger tangmuathi before insert or update on mua_thi begin set new.mamuathi := MT + muathitang.nextval from Dual; end; create sequence muathitang start with 1 increment by 1; 
mamuathi = MT + auto_increment; Don't structure your table like that. That's a smart key (a single string with several components concatenated). Smart keys are dumb. If the "MT" is crucial (why have a key with a hardcoded, unchanging element?) make it a separate column.
CREATE TABLE mua_thi ( mamuathi varchar2(2) not null , id number (8) not null , primary key (mamuathi, id ) , check(mamuathi = 'MT') ); Actually there's still some bad practice there. One, name the constraints - it makes life easier:
, constraint mt_pk primary key (mamuathi, id ) , constraint mt_ck check(mamuathi = 'MT') Two, if mamuathi is genuinely a constant it's pointless using it in the key:
, constraint mt_pk primary key ( id ) Three, mamuathi may evolve to several values , so think about whether a foreign key to a look-up table might be better.
Obviously the drawback to splitting a smart key is the need to refrence multiple columns. In 11g we can use the virtual column feature to avoid that inconveience:
CREATE TABLE mua_thi ( mamuathi varchar2(2) not null , id number (8) not null , mamuathi_disp AS mamuathi||lpad(id,8,'0') , primary key (mamuathi, id ) , check(mamuathi = 'MT') ); 