mysql seq
create table tb_sequence(name varchar(50) not null,current_value int not null,_increment int not null default 1, primary key(name));
insert into tb_sequence values('userid',0,1);
DELIMITER //
create function _nextval(n varchar(50)) returns integer
begin
declare _cur int;
set _cur=(select current_value from tb_sequence where name= n for update);
update tb_sequence set current_value = _cur + _increment where name=n ;
return _cur; end;
//
DELIMITER ;