Postgresql 左连接问题

Question

I have two tables

create table sources (
    user_id bigint,
    source varchar,
    created timestamp default now(),
    unique(user_id, source)
)

create table subscriptions (
    user_id bigint unique primary key,
    plan int not null references plans(id),
    starts timestamp default now(),
    ends timestamp default now() + interval '30' day
)

I try to select everything from sources when user has active subscription, I use this query

SELECT src.* FROM sources AS src LEFT JOIN subscriptions as sub ON sub.user_id=src.user_id WHERE now() < sub.ends

However, it does not return all data, the problem became clear when I tried

SELECT * FROM sources AS src LEFT JOIN subscriptions as sub ON sub.user_id=src.user_id

What may be the problem and how itspossible for me to get this info in the other way? PS subscriptions table has rows with other user_id's. Thank you very much!

Answer 1

UPDATE

Your left join is working. If your image is of SELECT * FROM sources AS src LEFT JOIN subscriptions as sub ON sub.user_id=src.user_id then some of your users have no subscriptions.

SELECT * FROM sources AS src LEFT JOIN subscriptions ... only ensures every matching row in sources will be fetched regardless of whether it has a matching row in subscriptions .

---

[I wrote the rest when I thought it was a different problem, but the advice remains.]

This is possible because sources.user_id is not set up as a foreign key so the database cannot enforce referential integrity . As far as the database is concerned, sources.user_id is just some number. If you delete a user, their sources and subscriptions will hang around.

In general...

• Clever primary keys just bring complications. Give everything more complicated than a simple join table id bigserial primary key and be done with it.
• Declare everything not null unless you have a good reason it should be null.
• Use on delete cascade to clean up when a user is deleted, but see below.

create table sources (
    id bigserial primary key,

    -- When a user is deleted, its sources will also be deleted
    user_id bigint not null references users(id) on delete cascade,

    source varchar not null,
    created timestamp not null default now(),

    unique(user_id, source)
)

create table subscriptions (
    id bigserial primary key,

    -- When a user is deleted, its subscriptions will also be deleted
    user_id bigint not null unique references foreign key users(id) on delete cascade,

    -- Make foreign key names consistent, and use a consistent type for ids.
    plan_id bigint not null references plans(id) on delete cascade,

    starts timestamp not null default now(),
    ends timestamp not null default now() + interval '30' day
)

Whether to use on delete cascade when a deleted user still has sources and subscriptions depends on how you want to handle that situation. You might want to prevent deleting a user which still has active sources and subs, in which case leave off on delete cascade and handle cleaning up the user's sources and subs manually.