远离错误：没有唯一约束匹配引用表的给定键

Question

Been at this for some time now. I'm getting this psql error when attepmting to create a joins table (product_type_combo):

There is no unique constraint matching given keys for referenced table product

I'm receiving this because the field (customer) I'm referencing from the table product does not have a UNIQUE constraint.

CREATE TABLE product 
(
    id SERIAL PRIMARY KEY,
    name VARCHAR(128) NOT NULL,
    customer VARCHAR,
    CONSTRAINT product_c01 UNIQUE (name)
);

CREATE TABLE product_type_combo 
(
    customer VARCHAR REFERENCES product(customer),
    type_id INT REFERENCES type(id),
    PRIMARY KEY (customer, type_id),
    CONSTRAINT product_type_combo_c01 UNIQUE (customer, type_id)
);

I don't want to have a UNIQUE constraint on the customer field on the product table since I want duplicates to appear there. I only want the customer & type combo in the joins table to have a constraint.

Is there a way around this?

Answer 1

This is silly:

customer VARCHAR REFERENCES product(customer),

You have a primary key on the table, an id column. You should be using that.

You could define customer to be unique -- the lack of a unique constraint on a column used for a foreign key reference is the cause of the error. However, I strongly, strongly recommend that you use the primary key.

Answer 2

The error is coming from the Foreign Key ( REFERENCES ) constraint, for reasons listed in the PostgreSQL manual :

A foreign key must reference columns that either are a primary key or form a unique constraint.

Since your "customer" column is not unique, it cannot be the target of a foreign key reference. It also suggests a problem in your data model:

• If the relationship is between a product and a type , then referencing the "id" from the products table would make sense.
• If the relationship is between a customer and a type , then there should be a table listing all the possible customers, and you can create foreign keys referencing that from both "product" and "product_type_combo" (which would presumably be better named "customer_type_combo").