[英]Can I have a constraint on count of distinct values in a column in SQL?
Table: Relatives 表: 亲戚
emp_id
dep_id
(composite primary key) dep_id
(复合主键) We have to restrict one employee to three dependents. 我们必须将一名员工限制为三名家属。
This cannot be done using a check constraint alone, but there is a way using a materialized view and a check constraint as I demonstrate here on my blog . 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。 For your example this would be: 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;
However, this approach might not be performant in a large, busy production database, in which case you could go for an approach that uses triggers and a check constraint, plus an extra column on the employees table: 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;
添加一个新的整数非空列occurrence
,添加一个检查约束, occurrence BETWEEN 1 AND 3
,在emp_id
和occurrence
的复合上添加一个唯一约束,可选择添加辅助过程来维护occurrence
值。
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