带有 ON CONFLICT 子句的 Postgres 条件插入

Question

I have the following table:

CREATE TABLE orders(
     order_id text primary key,
     payment_status text not null
)

In which I want to update the payment_status if it's changed:

INSERT INTO orders(
        order_id, 
        payment_status
)
VALUES(
        '101',
        'Paid'
)
ON CONFLICT (order_id) DO UPDATE SET
        payment_status = EXCLUDED.payment_status

There's three possible payment statuses:

• Awaiting payment
• Paid
• Refunded

The ON CONFLICT clause is there to update the order row when it changes from "Paid" to "Refunded". However, the table should only contain rows for orders with status "Paid" or "Refunded", not "Awaiting Payment". If I specify a WHERE condition to the end of the statement as

WHERE EXCLUDED.payment_status != 'Awaiting payment'

This where-condition would only apply when there is a conflict on the primary key, not when a new order_id is inserted, thus letting orders with "Awaiting payment" status to pass on the first insert.

How can I do such an insert that allows only certain values of the payment_status to be inserted on the first insert?

Answer 1

The table should only contain rows for orders with status "Paid" or "Refunded", not "Awaiting Payment".

A CHECK constraint is the proper solution for that, and isn't limited to particular queries either.

CREATE TABLE orders(
     order_id text primary key,
     payment_status text not null CHECK (payment_status IN ('Paid', 'Refunded'))
)