UNION ALL并使用NULL填充date列

Question

DB-Fiddle

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

INSERT INTO sales
(last_order, customer)
VALUES 
('2020-04-10', 'user_01'),
('2020-06-15', 'user_02'),
('2020-08-26', 'user_03');



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

INSERT INTO customers
(first_order, customer)
VALUES 
('2020-03-10', 'user_01'),
('2020-05-15', 'user_02'),
('2020-01-17', 'user_03');

预期结果：

last_order   |   first_order     |
-------------|-------------------|----
2020-04-10   |                   |
2020-06-15   |                   |
2020-08-26   |                   |
             |    2020-03-10     |
             |    2020-05-15     |
             |    2020-01-17     |
             |                   |

在表sales中，我有last_order的日期，在表customers中，我有first_order的日期。

现在，我希望两个表都是UNION ALL，因此需要填充列last_order和--分别填充列first_order和NULL，以保持结构相等。

在postgresSQL中，我使用此查询，该查询工作正常，没有任何问题：

SELECT
s.last_order AS last_order,
NULL AS first_order
FROM sales s

UNION ALL

SELECT
NULL AS last_order,
c.first_order AS first_order
FROM customers c

但是，当我将此查询应用于amazon-redshift时，会出现以下错误：

Invalid operation: UNION types date and text cannot be matched

如何修改amazon-redshift中的查询以达到预期的结果？

Answer 1

一种有趣的方法是：

select s.last_order, c.first_order
from sales s full join
     customers c
     on 1 = 0;  -- never true!