我可以在 Oracle 中对来自 2 个不同表的 2 列的组合进行唯一约束吗？

Question

Column A is the PK of table A. Column B is a column in table B. Column C is a FK is table B that references column A in table A.

Can I define a constraint that says that that a column B AND column C have to be unique? As in, I don't want any repeats of the same combination of values from A and B.

Here's one possibility i'm thinking about:

1. Create a unique ID column.

unique_id varchar2 GENERATED ALWAYS AS (B || '-' || FK that references column A) VIRTUAL

2. set it as unique

CONSTRAINT unique_id UNIQUE

If i go with this solution i'm confused about one thing. The docs say: "Virtual columns are not supported for index-organized, external, object, cluster, or temporary tables." Obviously, because they don't get stored like other columns. Would this be a problem if I wanted to make my tables clustered?

Answer 1

Yes, you can create a UNIQUE constraint on two columns. For example:

create table table_a (
  a number(6) primary key not null
);

create table table_b (
  b number(6) not null,
  c number(6) not null,
  constraint uq1 unique (b, c),
  constraint fk1 foreign key (c) references table_a (a)
);

Then, if you try to insert it will fail for duplicates. For example:

insert into table_a (a) values (1);
insert into table_a (a) values (2);
insert into table_b (b, c) values (10, 1);
insert into table_b (b, c) values (10, 1); -- fails!
insert into table_b (b, c) values (10, 2);

See running example at db<>fiddle .