Select a value from a table using table column key

October 25, 2024

I have the following Postures table:

CREATE TABLE users
(
    id bigint NOT NULL,
    created_by character varying(255) COLLATE pg_catalog."default",
    creation_date timestamp(6) without time zone,
    default_travel_method_id character varying(255) COLLATE pg_catalog."default",
    default_shipping_address_id character varying(255) COLLATE pg_catalog."default",
    CONSTRAINT users_pkey PRIMARY KEY (id)
)

When I use this select to select also a column from another table I use this in Oracle:

    SELECT
    created_by,
    (select reference from methods where id = default_travel_method_id) as default_travel_id,
    (select reference from addresses where id = default_shipping_address_id) as defaultShippingAddressId,
    from users;

But in Postures I get this error:

ERROR:  operator does not exist: bigint = character varying
LINE 8: ...reference from methods where id = default_...
                                             ^
HINT:  No operator matches the given name and argument types. You might need to add explicit type casts.

Do you know in Postgres what is the proper way to use a column expression to select a value from another table?

>Solution :

Typically you wouldn’t use character varying for a foreign key. Define the table using traditional integer id fields:

CREATE TABLE users
(
    id bigint NOT NULL,
    created_by character varying(255) COLLATE pg_catalog."default",
    creation_date timestamp(6) without time zone,
    default_travel_method_id int references methods(id),
    default_shipping_address_id int references addresses(id),
    CONSTRAINT users_pkey PRIMARY KEY (id)
)

Then select with a JOIN, assuming your related tables have id and reference columns

SELECT
  created_by,
  methods.reference AS "defaultTravel",
  addresses.reference AS "defaultShippingAddress"
FROM
  users
JOIN methods ON
  methods.id = default_travel_method_id
JOIN addresses ON
  addresses.id = default_shipping_address_id