如何在两列上强制执行“唯一”组合检查约束，但在PostgreSQL中仍允许重复？

Question

I have a table, say:

CREATE TABLE test (
    id SERIAL UNIQUE,
    first integer NOT NULL,
    second integer NOT NULL,
    data text NOT NULL
);

How do I enforce a check such that the second column is 'unique' with respect to the first column, but duplicate values are allowed? In other words, only allow one value in the 'second' for each of the 'first' column values. Once a value has been inserted into column 'first', only additional values that match the existing (first, second) column combination can be inserted again.

For example, given:

INSERT INTO test (first, second, data) VALUES (1, 2, 'test');

A constraint violation for:

INSERT INTO test (first, second, data) VALUES (1, 3, 'testing');

would occur, but

INSERT INTO test (first, second, data) VALUES (1, 2, 'testing');

or

INSERT INTO test (first, second, data) VALUES (2, 1, 'testing');

or

INSERT INTO test (first, second, data) VALUES (2, 2, 'testing');

would succeed.

Answer 1

The problem is that you need two tables. You are trying to carry information from one row to another -- but that is a bad idea. I'm guessing you really have two entities and just don't want to admit it.

So:

CREATE TABLE firstSecond (
    firstSecondId SERIAL PRIMARY KEY,
    first integer NOT NULL,
    second integer NOT NULL,
    constraint unq_firstSecond_first_second unique (first, second)
);

CREATE TABLE test (
    testId SERIAL PRIMARY KEY,
    firstSecondId int not null references firstSecond(firstSecondId),
    data text NOT NULL
);

These tables have the semantics you want.

Of course, you can also use a composite primary key for the first table:

CREATE TABLE firstSecond (
    first integer NOT NULL,
    second integer NOT NULL,
    primary key (first, second)
);

CREATE TABLE test (
    testId SERIAL PRIMARY KEY,
    first int not null,
    second int not null,
    data text NOT NULL,
    foreign key (first, second) references firstSecond(first, second)
);

Answer 2

Composite Primary Key if you are looking for combination.

Definition :

Combination of two or more columns in a table that can be used to uniquely identify each row in the table.

The demo = Demo