联合两个表中的所有客户并从第二个表中分配日期值，以防客户在第一个表中不存在

Question

DB-Fiddle

CREATE TABLE sales 
(
    id SERIAL PRIMARY KEY,
    last_order DATE,
    customer VARCHAR(255)
);

INSERT INTO sales (last_order, customer)
VALUES 
('2020-09-10', 'user_01'),
('2020-10-15', 'user_02'),
('2020-11-26', 'user_03');


CREATE TABLE customers 
(
    id SERIAL PRIMARY KEY,
    first_order DATE,
    customer VARCHAR(255)
);

INSERT INTO customers (first_order, customer)
VALUES 
('2020-08-10', 'user_01'),
('2020-09-15', 'user_02'),
('2020-10-17', 'user_03'),
('2020-05-03', 'user_04'),
('2020-04-12', 'user_05');

Expected result:

customer   |   used_date    |
-----------|----------------|------
user_01    |   2020-09-10   |
user_02    |   2020-10-15   |
user_03    |   2020-11-26   |
user_04    |   2020-05-03   |
user_05    |   2020-04-12   |

I have two tables and I want to query all customers in both tables.

Additionally, in case the customer does not exist in the table sales I want to apply the first_order from table customers as used_date .

So far I came up with this query:

SELECT
    t1.customer,
    (CASE WHEN t1.last_order IS NULL THEN t1.first_order ELSE t1.last_order END) AS used_date
FROM
   (SELECT
        s.customer,
        s.last_order AS last_order,
        NULL::date AS first_order
    FROM 
        sales s
    GROUP BY 
        1, 2, 3

    UNION ALL
 
    SELECT
        c.customer,
        NULL::date AS last_order,
        c.first_order AS first_order
    FROM 
        customers c
    GROUP BY 
        1, 2, 3) t1
GROUP BY 
    1, 2
ORDER BY 
    1;

How do I need to modify the CASE WHEN statement to get the expected result?

Answer 1

This looks like a left join :

select c.*, 
       coalesce(s.last_order, c.first_order) as used_date
from customers c left join
     sales s
     on c.customer = s.customer;