SQL tables for postal codes

Question

Good,

I am creating a BD for a project I am doing and in this project are included Addresses (Customers, Suppliers, etc.), addresses that will contain Postal Code.

My question is this::

I have 3 tables - Districts; Municipalities; Postal Codes;

CREATE TABLE distritos(
    cd VARCHAR(2),  --Código Distrito
    nome_dist VARCHAR(50) not null,  --Nome Distrito
    PRIMARY KEY (cd),
);

CREATE TABLE concelhos(
    cd VARCHAR(2) not null, --Código Distrito
    cc VARCHAR(2), --Código Concelho
    nome_conc VARCHAR(50) not null, --Nome Concelho
    PRIMARY KEY (cc),
    FOREIGN KEY (cd) REFERENCES distritos(cd),
);

CREATE TABLE cp(
    cp_id INT IDENTITY(1, 1),
    cp_cc VARCHAR(2) not null, --Código Concelho
    cp_localidade VARCHAR(50) not null, --Nome Localidade
    cp_cp4 VARCHAR(4) not null, --CP4 Nº Código Postal
    cp_cp3 VARCHAR(3) not null, --CP3 Extensão Nº Código Postal
    cp_dp VARCHAR(50) not null, --Designação Postal
    PRIMARY KEY (cp_id),
    FOREIGN KEY (cp_cc) REFERENCES concelhos(cc),
);

What I intend to do is the relationship between these three tables, but I have a problem and I can’t think of how to solve it. The list of the table Concelhos with the table of Códigos Postais is not possible since there are several councils with the same code and this is not unique.

I don’t know if I explained myself well but anything comment and I try to explain better.