PostgreSQL 自定义 CHECK 列

Question

I have the following table:

DROP TABLE IF EXISTS employees;
CREATE TABLE cars (
    model VARCHAR (50),
    brand VARCHAR (50),
    price INTEGER
);

Which looks like the following:

model    brand        price
Clio     Renault      3000
Clio     Renault      2700
Polo     Volkswagen   4400
Golf     Volkswagen   3400

I want to perform a CHECK (or other) operation to guarantee that a model cannot have multiple different brands . I am not sure how to insert information from another column when performing the check statement for model.

Answer 1

You can enforce your constraint with one additional step of normalization :

CREATE TABLE car_model (
  model text PRIMARY KEY
, brand text NOT NULL
);

CREATE TABLE car (
  car_id integer GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY
, model  text NOT NULL REFERENCES car_model
, price  integer
);

Now, the PRIMARY KEY constraint of car_model enforces a single entry per model. And the FOREIGN KEY constraint (with short notation REFERENCES ... ) enforces the same combination of model and brand every time.

To get your original representation, you might add a VIEW like:

CREATE VIEW AS
SELECT c.car_id, c.model, m.brand, c.price
FROM   car c
JOIN   car_model m USING (model);

You might do more. Like collect brands in a separate table ...