我可以对SQL中的列中的不同值的计数进行约束吗？

Question

表:亲戚

• emp_id
• dep_id(复合主键)

我们必须将一名员工限制为三名家属.

Answer 1

仅使用检查约束无法完成此操作,但有一种方法可以使用物化视图和检查约束,如我在博客中演示的那样.对于你的例子,这将是:

create materialized view emp_dep_mv
build immediate
refresh complete on commit as
select emp_id, count(*) cnt
from relatives
group by emp_id;

alter table emp_dep_mv
add constraint emp_dep_mv_chk
check (cnt <= 3)
deferrable;

但是,这种方法在大型繁忙的生产数据库中可能无法实现,在这种情况下,您可以使用触发器和检查约束的方法,以及employees表上的额外列:

alter table employees add num_relatives number(1,0) default 0 not null;

-- Populate for existing data
update employees
set num_relatives = (select count(*) from relatives r
                     where r.emp_id = e.emp_id)
where exists (select * from relatives r
              where r.emp_id = e.emp_id);

alter table employees add constraint emp_relatives_chk
check (num_relatives <= 3);

create trigger relatives_trg
after insert or update or delete on relatives
for each row
begin
   if inserting or updating then
      update employees
      set    num_relatives = num_relatives + 1
      where  emp_id = :new.emp_id;
   end if;
   if deleting or updating then
      update employees
      set    num_relatives = num_relatives - 1
      where  emp_id = :old.emp_id;
   end if;
end;